Document zero-member support in the macro API reference

docs/mkdocs/docs/features/arbitrary_types.md already gained a note, but the three api/macros pages are where the parameter contract is actually specified and they still described member as a non-empty list. State that the list may be empty on each page, and add a note showing what the zero-member case generates: an empty JSON object for the plain macros, and base-type-only serialization for the derived ones. Both notes record that the WITH_NAMES variants do not support this. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Name the zero-member macro bodies by intent, not argument count
2026-09-01 14:07:14 +00:00 · 2026-08-20 09:01:11 +02:00 · 2026-08-19 23:05:45 +02:00 · 2026-08-19 21:03:08 +02:00 · 2026-07-12 06:37:49 +00:00 · 2026-07-11 23:55:43 +02:00
141 changed files with 3814 additions and 470 deletions
@@ -4,28 +4,44 @@ updates:
    directory: /
    schedule:
      interval: daily
+    cooldown:
+      default-days: 7
+    groups:
+      codeql-action:
+        patterns:
+          - "github/codeql-action/*"

  - package-ecosystem: pip
    directory: /docs/mkdocs
    schedule:
      interval: daily
+    cooldown:
+      default-days: 7

  - package-ecosystem: pip
    directory: /tools/astyle
    schedule:
      interval: daily
+    cooldown:
+      default-days: 7

  - package-ecosystem: pip
    directory: /tools/generate_natvis
    schedule:
      interval: daily
+    cooldown:
+      default-days: 7

  - package-ecosystem: pip
    directory: /tools/serve_header
    schedule:
      interval: daily
+    cooldown:
+      default-days: 7

  - package-ecosystem: pip
    directory: /cmake/requirements
    schedule:
      interval: daily
+    cooldown:
+      default-days: 7
@@ -11,7 +11,7 @@ jobs:
    runs-on: ubuntu-latest
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -34,7 +34,7 @@ jobs:

    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -9,7 +9,7 @@ jobs:
    runs-on: ubuntu-22.04
    steps:
    - name: Harden Runner
-      uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+      uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
      with:
        egress-policy: audit

@@ -27,7 +27,7 @@ jobs:

    steps:
    - name: Harden Runner
-      uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+      uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
      with:
        egress-policy: audit

@@ -38,14 +38,14 @@ jobs:

    # Initializes the CodeQL tools for scanning.
    - name: Initialize CodeQL
-      uses: github/codeql-action/init@8aad20d150bbac5944a9f9d289da16a4b0d87c1e # v4.36.2
+      uses: github/codeql-action/init@54f647b7e1bb85c95cddabcd46b0c578ec92bc1a # v4.36.3
      with:
        languages: c-cpp

    # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
    # If this step fails, then you should remove it and run the build manually (see below)
    - name: Autobuild
-      uses: github/codeql-action/autobuild@8aad20d150bbac5944a9f9d289da16a4b0d87c1e # v4.36.2
+      uses: github/codeql-action/autobuild@54f647b7e1bb85c95cddabcd46b0c578ec92bc1a # v4.36.3

    - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@8aad20d150bbac5944a9f9d289da16a4b0d87c1e # v4.36.2
+      uses: github/codeql-action/analyze@54f647b7e1bb85c95cddabcd46b0c578ec92bc1a # v4.36.3
@@ -19,7 +19,7 @@ jobs:
      pull-requests: write
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -47,7 +47,10 @@ jobs:

            var hasPatch = artifacts.data.artifacts.some((artifact) => artifact.name == "amalgamation-patch");
            core.setOutput('has_patch', String(hasPatch));
-      - run: unzip pr.zip
+      # Extract the untrusted PR artifact into a dedicated empty directory and
+      # read only the two expected files by fixed path afterwards. This avoids a
+      # malicious archive overwriting workspace files or escaping via ../ paths.
+      - run: unzip -o pr.zip -d ./pr_artifact

      - name: 'Comment on PR'
        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
@@ -55,8 +58,19 @@ jobs:
          github-token: ${{ secrets.GITHUB_TOKEN }}
          script: |
            var fs = require('fs');
-            const author = fs.readFileSync('./author')
-            const issue_number = Number(fs.readFileSync('./number'));
+            // Both values come from a fork-triggered workflow and are therefore
+            // attacker-controlled. Validate them strictly before use to prevent
+            // Markdown/mention injection and bogus REST API filters.
+            const author = fs.readFileSync('./pr_artifact/author', 'utf8').trim();
+            if (!/^[A-Za-z0-9-]{1,39}$/.test(author)) {
+              core.setFailed(`Refusing to proceed: untrusted author value '${author}' is not a valid GitHub username.`);
+              return;
+            }
+            const issue_number = Number(fs.readFileSync('./pr_artifact/number', 'utf8').trim());
+            if (!Number.isInteger(issue_number) || issue_number <= 0) {
+              core.setFailed('Refusing to proceed: untrusted PR number is not a positive integer.');
+              return;
+            }
            const opts = github.rest.issues.listForRepo.endpoint.merge({
              owner: context.repo.owner,
              repo: context.repo.repo,
@@ -17,7 +17,7 @@ jobs:
    runs-on: ubuntu-latest
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -27,7 +27,7 @@ jobs:
      security-events: write
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -17,7 +17,7 @@ jobs:

    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -27,7 +27,7 @@ jobs:
    runs-on: ubuntu-22.04
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -36,7 +36,7 @@ jobs:

    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -32,7 +32,7 @@ jobs:
    runs-on: ubuntu-latest
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -41,12 +41,23 @@ jobs:
        with:
          persist-credentials: false

-      # Scan code using project's configuration on https://semgrep.dev/manage
-      - uses: returntocorp/semgrep-action@713efdd345f3035192eaa63f56867b88e63e4e5d
-        with:
-          publishToken: ${{ secrets.SEMGREP_APP_TOKEN }}
-          publishDeployment: ${{ secrets.SEMGREP_DEPLOYMENT_ID }}
-          generateSarif: "1"
+      # The former returntocorp/semgrep-action is deprecated (the org was renamed
+      # to semgrep/*); the maintained approach is to install the CLI and invoke
+      # it directly. We use `semgrep scan` (not `semgrep ci`, which requires a
+      # login token): with no SEMGREP_APP_TOKEN configured this is exactly what
+      # the old action fell back to, running community rules with no token.
+      # SEMGREP_APP_TOKEN is still passed through so registry auth works if a
+      # token is ever added.
+      - name: Install Semgrep
+        run: python3 -m pip install --user semgrep==1.168.0
+
+      # `semgrep scan --sarif` always exits 0 even with findings; continue-on-error
+      # is a safety net so the SARIF upload still runs if the scan itself errors.
+      - name: Run Semgrep
+        run: semgrep scan --config auto --sarif --output=semgrep.sarif
+        continue-on-error: true
+        env:
+          SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}

      # Upload SARIF file generated in previous step
      - name: Upload SARIF file
@@ -16,7 +16,7 @@ jobs:

    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -33,11 +33,21 @@ jobs:

  ci_infer:
    runs-on: ubuntu-latest
-    container: ghcr.io/nlohmann/json-ci:v2.4.0
    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
+        with:
+          egress-policy: audit
+
+      - name: Install Infer
+        run: |
+          wget -q -O - "https://github.com/facebook/infer/releases/download/v1.3.0/infer-linux-x86_64-v1.3.0.tar.xz" | sudo tar -C /opt -xJ
+          sudo ln -s /opt/infer-linux-x86_64-v1.3.0/bin/infer /usr/local/bin/infer
      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
        with:
          persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@f5b8fbb4d77cec1acc5a5f9f0df4beffaf5d98d9 # v4.3.4
      - name: Run CMake
        run: cmake -S . -B build -DJSON_CI=On
      - name: Build
@@ -50,7 +60,7 @@ jobs:
        target: [ci_test_amalgamation, ci_test_single_header, ci_cppcheck, ci_cpplint, ci_reproducible_tests, ci_non_git_tests, ci_offline_testdata, ci_reuse_compliance, ci_test_valgrind]
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -108,7 +118,7 @@ jobs:
    runs-on: ubuntu-latest
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -143,11 +153,30 @@ jobs:
    strategy:
      matrix:
        compiler: ['4.8', '4.9', '5', '6']
-    container: ghcr.io/nlohmann/json-ci:v2.4.0
+    # official gcc:4.8/4.9/5/6 images fail to check out code (too old for
+    # actions/checkout); install the old compilers on top of official ubuntu:20.04
+    # instead, mirroring what the (now retired) custom json-ci image did.
+    container: ubuntu:20.04
    steps:
+      - name: Install g++-${{ matrix.compiler }}
+        run: |
+          export DEBIAN_FRONTEND=noninteractive
+          apt-get update
+          apt-get install -y --no-install-recommends software-properties-common ca-certificates gnupg make git
+          add-apt-repository -y ppa:ubuntu-toolchain-r/test
+          apt-add-repository -y "deb http://archive.ubuntu.com/ubuntu/ bionic main"
+          apt-add-repository -y "deb http://archive.ubuntu.com/ubuntu/ bionic universe"
+          apt-add-repository -y "deb http://archive.ubuntu.com/ubuntu/ xenial main"
+          apt-add-repository -y "deb http://archive.ubuntu.com/ubuntu/ xenial universe"
+          apt-add-repository -y "deb http://archive.ubuntu.com/ubuntu/ xenial-updates main"
+          apt-add-repository -y "deb http://archive.ubuntu.com/ubuntu/ xenial-updates universe"
+          apt-get update
+          apt-get install -y --no-install-recommends g++-${{ matrix.compiler }}
      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
        with:
          persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@f5b8fbb4d77cec1acc5a5f9f0df4beffaf5d98d9 # v4.3.4
      - name: Run CMake
        run: CXX=g++-${{ matrix.compiler }} cmake -S . -B build -DJSON_CI=On
      - name: Build
@@ -234,11 +263,22 @@ jobs:

  ci_cuda_example:
    runs-on: ubuntu-latest
-    container: ghcr.io/nlohmann/json-ci:v2.4.0
+    strategy:
+      fail-fast: false
+      matrix:
+        # 11.8.0: newest pre-C++20 CUDA release, exercises the C++17 fallback
+        # path (tests/cuda_example/CMakeLists.txt picks the standard per nvcc
+        # version); 12.1.1: permanent regression guard for #3907 (nvcc 12.0/12.1
+        # choke on enable_borrowed_range at C++20, fixed in 12.2); 12.6.3: recent
+        # CUDA/C++20 coverage.
+        cuda: ['11.8.0', '12.1.1', '12.6.3']
+    container: nvidia/cuda:${{ matrix.cuda }}-devel-ubuntu22.04
    steps:
      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
        with:
          persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@f5b8fbb4d77cec1acc5a5f9f0df4beffaf5d98d9 # v4.3.4
      - name: Run CMake
        run: cmake -S . -B build -DJSON_CI=On
      - name: Build
@@ -276,17 +316,22 @@ jobs:

  ci_icpc:
    runs-on: ubuntu-latest
-    container: ghcr.io/nlohmann/json-ci:v2.2.0
+    # Intel discontinued the classic icc/icpc compiler in oneAPI 2024.0; this is
+    # Intel's own last officially published image that still includes it.
+    container: intel/oneapi-hpckit:2023.2.1-devel-ubuntu22.04
    steps:
      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
        with:
          persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@f5b8fbb4d77cec1acc5a5f9f0df4beffaf5d98d9 # v4.3.4
      - name: Run CMake
        run: cmake -S . -B build -DJSON_CI=On
      - name: Build
-        run: |
-          . /opt/intel/oneapi/setvars.sh
-          cmake --build build --target ci_icpc
+        # No need to source setvars.sh here: unlike the old custom image, this
+        # official image already has the oneAPI environment (icc/icpc on PATH)
+        # baked in, and re-sourcing it fails with "already been run" (exit 3).
+        run: cmake --build build --target ci_icpc

  ci_icpx:
    runs-on: ubuntu-latest
@@ -316,7 +361,7 @@ jobs:
    runs-on: ubuntu-latest
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -339,7 +384,7 @@ jobs:
        target: [ci_test_examples, ci_test_build_documentation]
    steps:
      - name: Harden Runner
-        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
        with:
          egress-policy: audit

@@ -11,6 +11,10 @@ Files: include/nlohmann/thirdparty/hedley.hpp
 Copyright: 2016-2021 Evan Nemerson <evan@nemerson.com>
 License: CC0

+Files: include/nlohmann/detail/meta/cpp_future.hpp
+Copyright: 2013-2026 Niels Lohmann <https://nlohmann.me> and 2018 The Abseil Authors
+License: MIT AND Apache-2.0
+
 Files: tests/thirdparty/doctest/*
 Copyright: 2016-2023 Viktor Kirilov
 License: MIT
@@ -669,7 +669,6 @@ add_custom_target(ci_test_compiler_default
 add_custom_target(ci_cuda_example
    COMMAND ${CMAKE_COMMAND}
        -DCMAKE_BUILD_TYPE=Debug -GNinja
-        -DCMAKE_CUDA_HOST_COMPILER=g++-8
        -S${PROJECT_SOURCE_DIR}/tests/cuda_example -B${PROJECT_BINARY_DIR}/build_cuda_example
    COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_cuda_example
 )
@@ -720,6 +719,11 @@ add_custom_target(ci_icpx
 # to zero and does not honor NaN ordering; -Kieee restores strict IEEE 754 behavior
 # (needed for the dtoa/grisu and NaN-comparison code paths).
 #
+# -tp=px pins the target processor to the generic x86-64 baseline (SSE2-only) to avoid
+# a nvc++ 25.5 / LLVM issue: when nvc++ auto-detects -tp from the runner's CPU (e.g. -tp znver4),
+# certain attribute combinations trigger an llc instruction-selection crash on std::ldexp<unsigned>.
+# Pinning to px removes this variability and is robust to future llc/nvc++ updates.
+#
 # The following tests are excluded as they trigger known nvc++ 25.5 defects (not
 # library bugs); see https://github.com/nlohmann/json for tracking. Only the
 # affected language-standard variants are excluded so coverage is otherwise kept:
@@ -733,7 +737,7 @@ add_custom_target(ci_nvhpc
    COMMAND ${CMAKE_COMMAND}
        -DCMAKE_BUILD_TYPE=Debug -GNinja
        -DCMAKE_C_COMPILER=nvc -DCMAKE_CXX_COMPILER=nvc++
-        -DCMAKE_CXX_FLAGS=-Kieee
+        -DCMAKE_CXX_FLAGS="-Kieee;-tp=px"
        -DJSON_BuildTests=ON -DJSON_FastTests=ON
        -S${PROJECT_SOURCE_DIR} -B${PROJECT_BINARY_DIR}/build_nvhpc
    COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_nvhpc
@@ -5,8 +5,11 @@
 # -Wno-extra-semi-stmt            The library uses assert which triggers this warning.
 # -Wno-padded                     We do not care about padding warnings.
 # -Wno-covered-switch-default     All switches list all cases and a default case.
-# -Wno-unsafe-buffer-usage        Otherwise Doctest would not compile.
-# -Wno-missing-noreturn           We found no way to silence this warning otherwise, see PR #4871
+# -Wno-unsafe-buffer-usage        Pervasive: the library's own low-level numeric/buffer code
+#                                 (to_chars, serializer, lexer, binary reader/writer, input
+#                                 adapters, json_pointer) plus vendored Doctest itself (~208
+#                                 distinct sites measured 2026-07-08 on clang trunk) all use
+#                                 raw pointer arithmetic / libc string calls by necessity.

 set(CLANG_CXXFLAGS
    -Werror
@@ -18,5 +21,4 @@ set(CLANG_CXXFLAGS
    -Wno-padded
    -Wno-covered-switch-default
    -Wno-unsafe-buffer-usage
-    -Wno-missing-noreturn
 )
@@ -4,7 +4,7 @@
  "archive": "JSON_for_Modern_C++.tgz",
  "author": {
    "name": "Niels Lohmann",
-    "link": "https://twitter.com/nlohmann"
+    "link": "https://nlohmann.me"
  },
  "aliases": ["nlohmann/json"]
 }
@@ -8,8 +8,8 @@ static bool accept(InputType&& i,
                   const bool ignore_trailing_commas = false);

 // (2)
-template<typename IteratorType>
-static bool accept(IteratorType first, IteratorType last,
+template<typename IteratorType, typename SentinelType = IteratorType>
+static bool accept(IteratorType first, SentinelType last,
                   const bool ignore_comments = false,
                   const bool ignore_trailing_commas = false);
 ```
@@ -17,10 +17,11 @@ static bool accept(IteratorType first, IteratorType last,
 Checks whether the input is valid JSON.

 1. Reads from a compatible input.
-2. Reads from a pair of character iterators
+2. Reads from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)
    
    The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted
-    respectively as UTF-8, UTF-16, and UTF-32.
+    respectively as UTF-8, UTF-16, and UTF-32. If `SentinelType` differs from `IteratorType`, it must be comparable to
+    the iterator type with `operator!=`.
    
 Unlike the [`parse()`](parse.md) function, this function neither throws an exception in case of invalid JSON input
 (i.e., a parse error) nor creates diagnostic information.
@@ -35,7 +36,8 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
    - a C-style array of characters
    - a pointer to a null-terminated string of single byte characters (throws if null)
    - a `std::string`
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)

 `IteratorType`
 :   a compatible iterator type, for instance.
@@ -43,6 +45,12 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
    - a pair of `std::string::iterator` or `std::vector<std::uint8_t>::iterator`
    - a pair of pointers such as `ptr` and `ptr + len`

+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::counted_iterator` with a different sentinel type
+
 ## Parameters

 `i` (in)
@@ -60,7 +68,7 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
 :   iterator to the start of the character range

 `last` (in)
-:   iterator to the end of the character range
+:   iterator to the end of the character range, or a sentinel value that compares equal to the end iterator with `operator!=`

 ## Return value

@@ -109,7 +117,9 @@ A UTF-8 byte order mark is silently ignored.
 - Added in version 3.0.0.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
 - Changed [runtime assertion](../../features/assertions.md) in case of `FILE*` null pointers to exception in version 3.12.0.
- Added `ignore_trailing_commas` in version 3.12.x.
+- Added `ignore_trailing_commas` in version 3.13.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

 !!! warning "Deprecation"

@@ -54,6 +54,7 @@ This function is only needed to express two edge cases that cannot be realized w

 - [`basic_json(initializer_list_t)`](basic_json.md) - create a JSON value from an initializer list
 - [`object`](object.md) - create a JSON object value from an initializer list
+- [Creating JSON values](../../features/creating_values.md) - the article on creating JSON values

 ## Version history

@@ -82,7 +82,13 @@ basic_json(basic_json&& other) noexcept;
 4. This is a constructor for existing `basic_json` types. It does not hijack copy/move constructors, since the parameter
   has different template arguments than the current ones.

-    The constructor tries to convert the internal `m_value` of the parameter.
+    The constructor tries to convert the internal `m_value` of the parameter. Each member value (object, array, string,
+    etc.) is serialized via the corresponding `to_json()` overload. For objects and strings, the conversion requires
+    that the *target* `basic_json` type's `object_t::key_type` (or `string_t`) be directly constructible from the
+    *source* type's corresponding member type via `is_constructible`. If this requirement is not met, the conversion
+    does not fail to compile; instead, it silently falls back to the array-conversion path, which represents objects
+    as arrays of `[key, value]` pairs and strings as arrays of character codes. This is a known limitation tracked in
+    [issue #3425](https://github.com/nlohmann/json/issues/3425).

 5. Creates a JSON value of type array or object from the passed initializer list `init`. In case `type_deduction` is
   `#!cpp true` (default), the type of the JSON value to be created is deducted from the initializer list `init`
@@ -109,7 +115,22 @@ basic_json(basic_json&& other) noexcept;
   
    Function [`array()`](array.md) and [`object()`](object.md) force array and object creation from initializer lists,
    respectively.
-        
+
+    !!! warning "Brace initialization yields arrays"
+
+        Because this constructor takes an `initializer_list_t`, brace-initializing a `json`/`ordered_json` from
+        another `json` value wraps it in a single-element array rather than copying it:
+
+        ```cpp
+        json j1 = "hello";
+        json j2{j1};   // [!] j2 is ["hello"], NOT a copy of j1
+        json j3(j1);   // j3 is "hello" -- parentheses copy as expected
+        ```
+
+        See the FAQ entry on [brace initialization](../../home/faq.md#brace-initialization-yields-arrays) for the
+        full explanation, an opt-in macro to change this behavior, and how to explicitly create a single-element
+        array (`json::array({value})`) if that is what you want.
+
 6. Constructs a JSON array value by creating `cnt` copies of a passed value. In case `cnt` is `0`, an empty array is
   created.

@@ -146,6 +167,11 @@ basic_json(basic_json&& other) noexcept;

    - `BasicJsonType` is a `basic_json` type.
    - `BasicJsonType` has different template arguments than `basic_json_t`.
+    
+    **Note:** For cross-`basic_json` conversions to produce correct results, the target `basic_json`'s
+    `object_t::key_type` and `string_t` must be directly constructible from the source `basic_json`'s
+    corresponding types. See the description of overload (4) above for details on what happens when
+    this requirement is not met.

 `U`:
 :   `uncvref_t<CompatibleType>`
@@ -37,12 +37,52 @@ represent a byte array in modern C++.
 `BinaryType`
 :   container type to store arrays

+    Although not formally expressed as a C++ concept, `BinaryType` must be default-constructible,
+    copy/move-constructible, and support `push_back()`, `.data()`, and `.size()`, because
+    [`byte_container_with_subtype`](../byte_container_with_subtype/index.md) derives directly from it. Its
+    `value_type` must additionally be exactly one byte wide (e.g., `std::uint8_t`/`char`/`std::byte`): the binary
+    serializers (CBOR, MessagePack, BSON, UBJSON) read and write the container's raw bytes via
+    `reinterpret_cast`, which is only correct for byte-sized elements -- a container like
+    `#!cpp std::vector<std::intptr_t>` will not work as `BinaryType`.
+
 ## Notes

 #### Default type

 The default values for `BinaryType` is `#!cpp std::vector<std::uint8_t>`.

+#### Custom BinaryType behavior
+
+When a custom `BinaryType` is configured (other than the default `#!cpp std::vector<std::uint8_t>`), you can assign
+values of that type directly to a `basic_json` instance, and they will automatically be recognized as binary values
+rather than arrays:
+
+```cpp
+using custom_json = nlohmann::basic_json<
+    nlohmann::ordered_map,  // ObjectType
+    std::vector,            // ArrayType
+    std::string,            // StringType
+    bool,                   // BooleanType
+    std::int64_t,           // NumberIntegerType
+    std::uint64_t,          // NumberUnsignedType
+    double,                 // NumberFloatType
+    std::allocator,         // AllocatorType
+    nlohmann::adl_serializer,
+    std::vector<std::byte>  // Custom BinaryType
+>;
+
+std::vector<std::byte> data{std::byte{1}, std::byte{2}, std::byte{3}};
+custom_json j = data;  // Creates a binary value, not an array
+assert(j.is_binary());
+
+// Round-tripping works seamlessly
+auto extracted = j.get<std::vector<std::byte>>();
+assert(extracted == data);
+```
+
+This automatic type detection is a convenience feature that only applies to custom (non-default) `BinaryType` configurations.
+The default `nlohmann::json` continues to treat `#!cpp std::vector<std::uint8_t>` as arrays for backward compatibility.
+
 #### Storage

 Binary Arrays are stored as pointers in a `basic_json` type. That is, for any access to array values, a pointer of the
@@ -75,6 +75,7 @@ Binary values are serialized as an object containing two keys:

 - [to_string](to_string.md) returns a string representation of a JSON value
 - [operator<<](../operator_ltlt.md) serialize to stream
+- [Serialization](../../features/serialization.md) - the serialization article

 ## Version history

@@ -64,6 +64,7 @@ Logarithmic in the size of the container, O(log(`size()`)).

 - [emplace_back](emplace_back.md) add a value to an array
 - [insert](insert.md) add values to an array/object
+- [Modifying values](../../features/modifying_values.md) - the article on modifying values

 ## Version history

@@ -58,6 +58,7 @@ Amortized constant.

 - [operator+=](operator+=.md) add a value to an array/object
 - [push_back](push_back.md) add a value to an array/object
+- [Modifying values](../../features/modifying_values.md) - the article on modifying values

 ## Version history

@@ -206,6 +206,7 @@ Strong exception safety: if an exception occurs, the original value stays intact

 - [clear](clear.md) clears the contents
 - [insert](insert.md) add values to an array/object
+- [Modifying values](../../features/modifying_values.md) - the article on modifying values

 ## Version history

@@ -88,7 +88,8 @@ std::string format_as(const BasicJsonType& j)

 - [dump](dump.md)
 - [std::formatter<basic_json>](std_formatter.md) - the `std::format` (C++20) equivalent
+- [Serialization](../../features/serialization.md) - the serialization article

 ## Version history

- Added in version 3.12.x.
+- Added in version 3.13.0.
@@ -7,8 +7,8 @@ static basic_json from_bjdata(InputType&& i,
                              const bool strict = true,
                              const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
-static basic_json from_bjdata(IteratorType first, IteratorType last,
+template<typename IteratorType, typename SentinelType = IteratorType>
+static basic_json from_bjdata(IteratorType first, SentinelType last,
                              const bool strict = true,
                              const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_bjdata(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the BJData (Binary JData) serialization format.

 1. Reads from a compatible input.
-2. Reads from an iterator range.
+2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

 The exact mapping and its limitations are described on a [dedicated page](../../features/binary_formats/bjdata.md).

@@ -29,11 +29,16 @@ The exact mapping and its limitations are described on a [dedicated page](../../
    - a `FILE` pointer
    - a C-style array of characters
    - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)

 `IteratorType`
 :   a compatible iterator type

+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
+    custom sentinel type for C++20 ranges
+
 ## Parameters

 `i` (in)
@@ -43,7 +48,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input

 `last` (in)
-:   iterator to the end of the input
+:   iterator to the end of the input, or a sentinel value that compares equal to the end iterator with `operator!=`

 `strict` (in)
 :   whether to expect the input to be consumed until EOF (`#!cpp true` by default)
@@ -101,3 +106,5 @@ Linear in the size of the input.
 ## Version history

 - Added in version 3.11.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
@@ -7,8 +7,8 @@ static basic_json from_bson(InputType&& i,
                            const bool strict = true,
                            const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
-static basic_json from_bson(IteratorType first, IteratorType last,
+template<typename IteratorType, typename SentinelType = IteratorType>
+static basic_json from_bson(IteratorType first, SentinelType last,
                            const bool strict = true,
                            const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_bson(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the BSON (Binary JSON) serialization format.

 1. Reads from a compatible input.
-2. Reads from an iterator range.
+2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

 The exact mapping and its limitations are described on a [dedicated page](../../features/binary_formats/bson.md).

@@ -29,11 +29,16 @@ The exact mapping and its limitations are described on a [dedicated page](../../
    - a `FILE` pointer
    - a C-style array of characters
    - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)

 `IteratorType`
 :   a compatible iterator type

+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
+    custom sentinel type for C++20 ranges
+
 ## Parameters

 `i` (in)
@@ -43,7 +48,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input

 `last` (in)
-:   iterator to the end of the input
+:   iterator to the end of the input, or a sentinel value that compares equal to the end iterator with `operator!=`

 `strict` (in)
 :   whether to expect the input to be consumed until EOF (`#!cpp true` by default)
@@ -101,6 +106,8 @@ Linear in the size of the input.
 ## Version history

 - Added in version 3.4.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

 !!! warning "Deprecation"

@@ -9,8 +9,8 @@ static basic_json from_cbor(InputType&& i,
                            const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error);

 // (2)
-template<typename IteratorType>
-static basic_json from_cbor(IteratorType first, IteratorType last,
+template<typename IteratorType, typename SentinelType = IteratorType>
+static basic_json from_cbor(IteratorType first, SentinelType last,
                            const bool strict = true,
                            const bool allow_exceptions = true,
                            const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error);
@@ -19,7 +19,7 @@ static basic_json from_cbor(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the CBOR (Concise Binary Object Representation) serialization format.

 1. Reads from a compatible input.
-2. Reads from an iterator range.
+2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

 The exact mapping and its limitations are described on a [dedicated page](../../features/binary_formats/cbor.md).

@@ -32,11 +32,16 @@ The exact mapping and its limitations are described on a [dedicated page](../../
    - a `FILE` pointer
    - a C-style array of characters
    - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)

 `IteratorType`
 :   a compatible iterator type

+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
+    custom sentinel type for C++20 ranges
+
 ## Parameters

 `i` (in)
@@ -46,7 +51,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input

 `last` (in)
-:   iterator to the end of the input
+:   iterator to the end of the input, or a sentinel value that compares equal to the end iterator with `operator!=`

 `strict` (in)
 :   whether to expect the input to be consumed until EOF (`#!cpp true` by default)
@@ -111,6 +116,8 @@ Linear in the size of the input.
 - Changed to consume input adapters, removed `start_index` parameter, and added `strict` parameter in version 3.0.0.
 - Added `allow_exceptions` parameter in version 3.2.0.
 - Added `tag_handler` parameter in version 3.9.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

 !!! warning "Deprecation"

@@ -7,8 +7,8 @@ static basic_json from_msgpack(InputType&& i,
                               const bool strict = true,
                               const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
-static basic_json from_msgpack(IteratorType first, IteratorType last,
+template<typename IteratorType, typename SentinelType = IteratorType>
+static basic_json from_msgpack(IteratorType first, SentinelType last,
                               const bool strict = true,
                               const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_msgpack(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the MessagePack serialization format.

 1. Reads from a compatible input.
-2. Reads from an iterator range.
+2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

 The exact mapping and its limitations are described on a [dedicated page](../../features/binary_formats/messagepack.md).

@@ -29,11 +29,16 @@ The exact mapping and its limitations are described on a [dedicated page](../../
    - a `FILE` pointer
    - a C-style array of characters
    - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)

 `IteratorType`
 :   a compatible iterator type

+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
+    custom sentinel type for C++20 ranges
+
 ## Parameters

 `i` (in)
@@ -43,7 +48,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input

 `last` (in)
-:   iterator to the end of the input
+:   iterator to the end of the input, or a sentinel value that compares equal to the end iterator with `operator!=`

 `strict` (in)
 :   whether to expect the input to be consumed until EOF (`#!cpp true` by default)
@@ -103,6 +108,8 @@ Linear in the size of the input.
 - Parameter `start_index` since version 2.1.1.
 - Changed to consume input adapters, removed `start_index` parameter, and added `strict` parameter in version 3.0.0.
 - Added `allow_exceptions` parameter in version 3.2.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

 !!! warning "Deprecation"

@@ -7,8 +7,8 @@ static basic_json from_ubjson(InputType&& i,
                              const bool strict = true,
                              const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
-static basic_json from_ubjson(IteratorType first, IteratorType last,
+template<typename IteratorType, typename SentinelType = IteratorType>
+static basic_json from_ubjson(IteratorType first, SentinelType last,
                              const bool strict = true,
                              const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_ubjson(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the UBJSON (Universal Binary JSON) serialization format.

 1. Reads from a compatible input.
-2. Reads from an iterator range.
+2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

 The exact mapping and its limitations are described on a [dedicated page](../../features/binary_formats/ubjson.md).

@@ -29,11 +29,16 @@ The exact mapping and its limitations are described on a [dedicated page](../../
    - a `FILE` pointer
    - a C-style array of characters
    - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)

 `IteratorType`
 :   a compatible iterator type

+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
+    custom sentinel type for C++20 ranges
+
 ## Parameters

 `i` (in)
@@ -43,7 +48,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input

 `last` (in)
-:   iterator to the end of the input
+:   iterator to the end of the input, or a sentinel value that compares equal to the end iterator with `operator!=`

 `strict` (in)
 :   whether to expect the input to be consumed until EOF (`#!cpp true` by default)
@@ -102,6 +107,8 @@ Linear in the size of the input.

 - Added in version 3.1.0.
 - Added `allow_exceptions` parameter in version 3.2.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

 !!! warning "Deprecation"

@@ -114,6 +114,13 @@ overload (3).
    See [Number conversion](../../features/types/number_handling.md#number-conversion)
    for more information.

+!!! note "`std::optional` conversions"
+
+    Prior to version 3.13.0, `#!cpp get<std::optional<T>>()` (and other conversions to `std::optional<T>`) failed to
+    compile in every configuration, due to an internal implementation bug that made the `from_json` overload for
+    `std::optional` unreachable regardless of the [`JSON_USE_IMPLICIT_CONVERSIONS`](../macros/json_use_implicit_conversions.md)
+    setting. This has been fixed.
+
 ## Examples

 ??? example
@@ -155,6 +162,7 @@ overload (3).
 - [get_ptr](get_ptr.md) get a pointer to the stored value
 - [get_ref](get_ref.md) get a reference to the stored value
 - [operator ValueType](operator_ValueType.md) get a value via implicit conversion
+- [Converting values](../../features/conversions.md) - the type conversions article

 ## Version history

@@ -62,6 +62,7 @@ Depends on the `json_serializer<ValueType>::from_json()` implementation.
 - [get](get.md) get a value (explicit conversion)
 - [get_ref](get_ref.md) get a reference to the stored value
 - [get_ptr](get_ptr.md) get a pointer to the stored value
+- [Converting values](../../features/conversions.md) - the type conversions article

 ## Version history

@@ -46,6 +46,17 @@ for (auto& [key, val] : j_object.items())
 }
 ```

+If you need to name the type of the dereferenced element explicitly (e.g., to write a standalone function that
+takes it as a parameter, or to use `items()` with `std::for_each`), use `decltype`:
+
+```cpp
+using element_type = decltype(*j_object.items().begin());
+```
+
+The per-element type (`iteration_proxy_value`) lives in the library's internal `detail` namespace and is
+intentionally unspecified as a stable, named type -- `decltype` is the supported way to obtain it, but its exact
+name/definition may change between versions.
+
 ## Return value

 iteration proxy object wrapping the current value with an interface to use in range-based for loops
@@ -57,6 +57,7 @@ the initializer list constructor `basic_json(initializer_list_t, bool, value_t)`

 - [`basic_json(initializer_list_t)`](basic_json.md) - create a JSON value from an initializer list
 - [`array`](array.md) - create a JSON array value from an initializer list
+- [Creating JSON values](../../features/creating_values.md) - the article on creating JSON values

 ## Version history

@@ -63,7 +63,8 @@ behavior:
  object will agree on the name-value mappings.
 - When the names within an object are not unique, it is unspecified which one of the values for a given key will be
  chosen. For instance, `#!json {"key": 2, "key": 1}` could be equal to either `#!json {"key": 1}` or
-  `#!json {"key": 2}`.
+  `#!json {"key": 2}`. To reject duplicate keys instead of silently resolving them one way or another, see
+  [this parsing recipe](../../features/parsing/parser_callbacks.md#recipe-rejecting-duplicate-object-keys).
 - Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see
  [`dump`](dump.md)) in this order. For instance, `#!json {"b": 1, "a": 2}` and `#!json {"a": 2, "b": 1}` will be stored
  and serialized as `#!json {"a": 2, "b": 1}`.
@@ -93,6 +94,15 @@ alphabetical order as `std::map` with `std::less` is used by default. Please not
 [RFC 8259](https://tools.ietf.org/html/rfc8259), because any order implements the specified "unordered" nature of JSON
 objects.

+#### Cross-`basic_json` conversion requirements
+
+When converting an object from one `basic_json` specialization to another via the
+[converting constructor](basic_json.md#overload-4), the target `object_t`'s `key_type` must be
+directly constructible from the source `basic_json`'s `string_t` type (or more generally, from the
+source object's key type). If this requirement is not met, the conversion does not fail; instead,
+the object is silently converted as an array of key-value pairs, which is incorrect. See
+[issue #3425](https://github.com/nlohmann/json/issues/3425) for details and an example.
+
 ## Examples

 ??? example
@@ -124,6 +124,15 @@ Strong exception safety: if an exception occurs, the original value stays intact
      filled with `#!json null`.
    - The special value `-` is treated as a synonym for the index past the end.

+    !!! note "Creating intermediate levels that don't exist yet"
+
+        When the JSON pointer traverses intermediate levels that don't exist at all yet (not just a missing
+        leaf), each missing level is created as an array or an object depending on whether the corresponding
+        pointer token parses as a non-negative integer: a numeric token creates an array, a non-numeric token
+        creates an object. For example, on an initially `#!json null` value, `/foo/0/0/0` creates nested arrays,
+        while `/foo/one/one/one` creates nested objects. This is not specified by the JSON Pointer RFC; it is
+        this library's own, intentional disambiguation rule. See also [JSON Pointer](../../features/json_pointer.md).
+
 ## Examples

 ??? example "Example: (1) access specified array element"
@@ -251,5 +260,6 @@ Strong exception safety: if an exception occurs, the original value stays intact
 1. Added in version 1.0.0.
 2. Added in version 1.0.0. Added overloads for `T* key` in version 1.1.0. Removed overloads for `T* key` (replaced by 3)
   in version 3.11.0.
-3. Added in version 3.11.0.
+3. Added in version 3.11.0. Fixed in version 3.13.0 to consistently accept `std::string_view`-convertible keys, as
+   already supported by [`at`](at.md), [`value`](value.md), [`find`](find.md), and other lookup functions.
 4. Added in version 2.0.0.
@@ -78,6 +78,7 @@ Linear in the size of the JSON value.
 ## See also

 - [get](get.md) get a value (explicit conversion)
+- [Converting values](../../features/conversions.md) - the type conversions article

 ## Version history

@@ -19,10 +19,8 @@ class basic_json {
 };
 ```

-1. Compares two JSON values for inequality according to the following rules:
-    - The comparison always yields `#!cpp false` if (1) either operand is discarded, or (2) either operand is `NaN` and
-      the other operand is either `NaN` or any other number.
-    - Otherwise, returns the result of `#!cpp !(lhs == rhs)` (until C++20) or `#!cpp !(*this == rhs)` (since C++20).
+1. Compares two JSON values for inequality. Returns `#!cpp !(lhs == rhs)` (until C++20) or `#!cpp !(*this == rhs)` (since C++20).
+    - This means the comparison is simply the logical negation of `operator==`, including for special values like `NaN` and `discarded`.

 2. Compares a JSON value and a scalar or a scalar and a JSON value for inequality by converting the scalar to a JSON
   value and comparing both JSON values according to 1.
@@ -54,13 +52,12 @@ Linear.

 ## Notes

-!!! note "Comparing `NaN`"
+!!! note "Comparing `NaN` and `discarded`"

-    `NaN` values are unordered within the domain of numbers.
-    The following comparisons all yield `#!cpp false`:
-      1. Comparing a `NaN` with itself.
-      2. Comparing a `NaN` with another `NaN`.
-      3. Comparing a `NaN` and any other number.
+    Since `operator!=` is defined as `!(a == b)`, the behavior for special values follows that of `operator==`:
+    
+    - For `NaN` values: `NaN == NaN` yields `#!cpp false`, so `NaN != NaN` yields `#!cpp true`.
+    - For `discarded` values: `discarded == x` yields `#!cpp false` for any `x`, so `discarded != x` yields `#!cpp true`.

 ## Examples

@@ -94,5 +91,7 @@ Linear.

 ## Version history

-1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
-2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
+1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Changed in version 3.13.0 to remove 
+   special-casing for `NaN` and `discarded` values; `operator!=` now consistently means `!(a == b)`.
+2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Changed in version 3.13.0 to remove
+   special-casing for `NaN` and `discarded` values; `operator!=` now consistently means `!(a == b)`.
@@ -10,8 +10,8 @@ static basic_json parse(InputType&& i,
                        const bool ignore_trailing_commas = false);

 // (2)
-template<typename IteratorType>
-static basic_json parse(IteratorType first, IteratorType last,
+template<typename IteratorType, typename SentinelType = IteratorType>
+static basic_json parse(IteratorType first, SentinelType last,
                        const parser_callback_t cb = nullptr,
                        const bool allow_exceptions = true,
                        const bool ignore_comments = false,
@@ -19,10 +19,11 @@ static basic_json parse(IteratorType first, IteratorType last,
 ```

 1. Deserialize from a compatible input.
-2. Deserialize from a pair of character iterators
+2. Deserialize from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)
    
    The `value_type` of the iterator must be an integral type with size of 1, 2, or 4 bytes, which will be interpreted
-    respectively as UTF-8, UTF-16, and UTF-32.
+    respectively as UTF-8, UTF-16, and UTF-32. If `SentinelType` differs from `IteratorType`, it must be comparable to
+    the iterator type with `operator!=`.

 ## Template parameters

@@ -34,7 +35,8 @@ static basic_json parse(IteratorType first, IteratorType last,
    - a C-style array of characters
    - a pointer to a null-terminated string of single byte characters (throws if null)
    - a `std::string`
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)

 `IteratorType`
 :   a compatible iterator type, for instance.
@@ -42,6 +44,12 @@ static basic_json parse(IteratorType first, IteratorType last,
    - a pair of `std::string::iterator` or `std::vector<std::uint8_t>::iterator`
    - a pair of pointers such as `ptr` and `ptr + len`

+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::counted_iterator` with a different sentinel type
+
 ## Parameters

 `i` (in)
@@ -66,7 +74,7 @@ static basic_json parse(IteratorType first, IteratorType last,
 :   iterator to the start of a character range

 `last` (in)
-:   iterator to the end of a character range
+:   iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with `operator!=`

 ## Return value

@@ -235,7 +243,9 @@ Invalid Unicode escapes and unpaired surrogates in the input are reported as
 - Overload for contiguous containers (1) added in version 2.0.3.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
 - Changed [runtime assertion](../../features/assertions.md) in case of `FILE*` null pointers to exception in version 3.12.0.
- Added `ignore_trailing_commas` in version 3.12.x.
+- Added `ignore_trailing_commas` in version 3.13.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

 !!! warning "Deprecation"

@@ -74,4 +74,4 @@ is thrown. In any case, the original value is not changed: the patch is applied

 - Added in version 2.0.0.
 - Added [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) and stopped relying on an internal assertion when an "add" operation's
-  target location has a non-object/non-array parent in version 3.12.x.
+  target location has a non-object/non-array parent in version 3.13.0.
@@ -71,4 +71,4 @@ function throws an exception.

 - Added in version 3.11.0.
 - Added [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) and stopped relying on an internal assertion when an "add" operation's
-  target location has a non-object/non-array parent in version 3.12.x.
+  target location has a non-object/non-array parent in version 3.13.0.
@@ -115,6 +115,7 @@ invalidates all iterators and all references.

 - [emplace_back](emplace_back.md) add a value to an array
 - [operator+=](operator+=.md) add a value to an array/object
+- [Modifying values](../../features/modifying_values.md) - the article on modifying values

 ## Version history

@@ -11,8 +11,8 @@ static bool sax_parse(InputType&& i,
                      const bool ignore_trailing_commas = false);

 // (2)
-template<class IteratorType, class SAX>
-static bool sax_parse(IteratorType first, IteratorType last,
+template<class IteratorType, class SAX, class SentinelType = IteratorType>
+static bool sax_parse(IteratorType first, SentinelType last,
                      SAX* sax,
                      input_format_t format = input_format_t::json,
                      const bool strict = true,
@@ -23,10 +23,11 @@ static bool sax_parse(IteratorType first, IteratorType last,
 Read from input and generate SAX events

 1. Read from a compatible input.
-2. Read from a pair of character iterators
+2. Read from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)
    
    The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted
-    respectively as UTF-8, UTF-16, and UTF-32.
+    respectively as UTF-8, UTF-16, and UTF-32. If `SentinelType` differs from `IteratorType`, it must be comparable to
+    the iterator type with `operator!=`.

 The SAX event lister must follow the interface of [`json_sax`](../json_sax/index.md).

@@ -39,13 +40,16 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
    - a `FILE` pointer
    - a C-style array of characters
    - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of
-      iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)

 `IteratorType`
 :   a compatible iterator type for overload (2); a pair of character iterators whose `value_type` is an integral type
    with a size of 1, 2, or 4 bytes (interpreted respectively as UTF-8, UTF-16, and UTF-32)

+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for overload (2)
+
 `SAX`
 :   a class fulfilling the SAX event listener interface; see [`json_sax`](../json_sax/index.md)

@@ -76,7 +80,7 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
 :   iterator to the start of a character range

 `last` (in)
-:   iterator to the end of a character range
+:   iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with `operator!=`

 ## Return value

@@ -126,7 +130,9 @@ A UTF-8 byte order mark is silently ignored.

 - Added in version 3.2.0.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
- Added `ignore_trailing_commas` in version 3.12.x.
+- Added `ignore_trailing_commas` in version 3.13.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

 !!! warning "Deprecation"

@@ -50,7 +50,8 @@ provides `<format>`, controlled by the [`JSON_HAS_STD_FORMAT`](../macros/json_ha
 - [dump](dump.md) - serialization
 - [operator<<(std::ostream&)](../operator_ltlt.md) - serialize to stream
 - [format_as](format_as.md) - customization point used by `fmt::format` (fmtlib)
+- [Serialization](../../features/serialization.md) - the serialization article

 ## Version history

- Added in version 3.12.x.
+- Added in version 3.13.0.
@@ -18,6 +18,11 @@ JSON class into byte-sized characters during deserialization.
 :   the container to store strings (e.g., `std::string`). Note this container is used for keys/names in objects, see
    [object_t](object_t.md).

+    `StringType` must have a `char`-compatible `value_type`: the library relies on UTF-8/`char`-based storage and
+    processing internally, so `std::wstring`, `std::u16string`, and `std::u32string` are **not** valid choices for
+    `StringType`. To work with wide-character data, convert it to/from UTF-8 at the boundary instead -- see the
+    FAQ's [wide string handling](../../home/faq.md#wide-string-handling) section for a conversion recipe.
+
 ## Notes

 #### Default type
@@ -45,6 +50,15 @@ This implementation is interoperable as it does compare strings code unit by cod
 String values are stored as pointers in a `basic_json` type. That is, for any access to string values, a pointer of type
 `string_t*` must be dereferenced.

+#### Cross-`basic_json` conversion requirements
+
+When converting a string value from one `basic_json` specialization to another via the
+[converting constructor](basic_json.md#overload-4), the target `string_t` must be directly
+constructible from the source `basic_json`'s `string_t` type. If this requirement is not met, the
+conversion does not fail; instead, the string is silently converted as an array of character codes,
+which is incorrect. See [issue #3425](https://github.com/nlohmann/json/issues/3425) for details
+and an example.
+
 ## Examples

 ??? example
@@ -59,6 +59,7 @@ std::string to_string(const BasicJsonType& j)
 ## See also

 - [dump](dump.md)
+- [Serialization](../../features/serialization.md) - the serialization article

 ## Version history

@@ -21,6 +21,12 @@ a string representation of the type ([`value_t`](value_t.md)):
 | array                                              | `"array"`     |
 | binary                                             | `"binary"`    |
 | discarded                                          | `"discarded"` |
+| invalid (corrupted value)                          | `"invalid"`   |
+
+!!! note "The \"invalid\" type"
+
+    The `"invalid"` return value indicates a corrupted JSON value — this can occur if an enum value falls outside the
+    range of valid `value_t` values. This is useful for diagnosing data corruption or internal errors.

 ## Exception safety

@@ -52,3 +58,4 @@ Constant.
 - Part of the public API version since 2.1.0.
 - Changed return value to `const char*` and added `noexcept` in version 3.0.0.
 - Added support for binary type in version 3.8.0.
+- Added `"invalid"` return value for corrupted JSON values in version 3.13.0.
@@ -149,6 +149,7 @@ Basic guarantee: if an exception is thrown during the operation, the JSON value

 - [insert](insert.md) add values to an array/object
 - [merge_patch](merge_patch.md) applies a JSON Merge Patch
+- [Modifying values](../../features/modifying_values.md) - the article on modifying values

 ## Version history

@@ -17,6 +17,8 @@ ValueType value(const json_pointer& ptr,
                const ValueType& default_value) const;
 ```

+This is equivalent to Python's `dict.get(key, default)`.
+
 1. Returns either a copy of an object's element at the specified key `key` or a given default value if no element with
   key `key` exists.
   
@@ -184,4 +186,6 @@ changes to any JSON value.

 1. Added in version 1.0.0. Changed parameter `default_value` type from `const ValueType&` to `ValueType&&` in version 3.11.0.
 2. Added in version 3.11.0. Made `ValueType` the first template parameter in version 3.11.2.
-3. Added in version 2.0.2. Extended to work with arrays in version 3.12.x.
+3. Added in version 2.0.2. Extended to work with arrays in version 3.13.0, including fixing an issue where resolving
+   `ptr` through an array unexpectedly threw `out_of_range` instead of returning the resolved element (or
+   `default_value`, as documented).
@@ -36,4 +36,4 @@ Constant.

 ## Version history

- Added in version 3.12.x.
+- Added in version 3.13.0.
@@ -32,4 +32,4 @@ Linear in the number of reference tokens in the `json_pointer`.

 ## Version history

- Added in version 3.12.x.
+- Added in version 3.13.0.
@@ -35,4 +35,4 @@ Linear in the number of reference tokens in the `json_pointer`.

 ## Version history

- Added in version 3.12.x.
+- Added in version 3.13.0.
@@ -92,4 +92,4 @@ The default value is `0` (disabled — existing behavior is preserved).

 ## Version history

- Added in version 3.12.x.
+- Added in version 3.13.0.
@@ -44,4 +44,4 @@ The default value is detected based on preprocessor macros such as `#!cpp __cplu

 - Added in version 3.10.5.
 - Added `JSON_HAS_CPP_23` in version 3.12.0.
- Added `JSON_HAS_CPP_26` in version 3.12.x.
+- Added `JSON_HAS_CPP_26` in version 3.13.0.
@@ -19,6 +19,20 @@ The default value is detected based on the preprocessor macros `#!cpp __cpp_lib_
 `#!cpp __cpp_lib_experimental_filesystem`, `#!cpp __has_include(<filesystem>)`, or
 `#!cpp __has_include(<experimental/filesystem>)`.

+!!! info "Known compiler/stdlib exclusions"
+
+    Even when the feature-test macro indicates filesystem support is available, the library disables it on the following broken toolchains:
+    
+    - **MinGW + GCC 8** — disabled entirely (broken `std::filesystem` implementation; [MinGW-w64 bug 737](https://sourceforge.net/p/mingw-w64/bugs/737/))
+    - **GCC (non-Clang) < 8** — disabled (no filesystem support)
+    - **Clang < 7** — disabled (no filesystem support)
+    - **MSVC < 19.14** — disabled (no filesystem support)
+    - **iOS < 13** — disabled (no filesystem support)
+    - **macOS < Catalina (10.15)** — disabled (no filesystem support)
+    
+    If `JSON_HAS_FILESYSTEM` or `JSON_HAS_EXPERIMENTAL_FILESYSTEM` is `0` despite `__cpp_lib_filesystem` being defined, one
+    of the exclusions above likely applies to your toolchain.
+
 ## Notes

 - Note that older compilers or older versions of libstdc++ also require the library `stdc++fs` to be linked to for
@@ -13,6 +13,20 @@ The default value is detected based on the preprocessor macro `#!cpp __cpp_lib_r

 When the macro is not defined, the library will define it to its default value.

+!!! info "Known compiler/stdlib exclusions"
+
+    Even when the feature-test macro `__cpp_lib_ranges` indicates ranges support is available, the library disables it on
+    the following incomplete or broken toolchains:
+    
+    - **GCC 11.1.0** — disabled (the shipped `<ranges>` header has a syntax error; [issue #4440](https://github.com/nlohmann/json/issues/4440))
+    - **libstdc++ < 11** — disabled (incomplete C++20 ranges support; [issue #4440](https://github.com/nlohmann/json/issues/4440))
+    - **Clang < 16 with libstdc++** — disabled (incomplete ranges support; [issue #4440](https://github.com/nlohmann/json/issues/4440))
+    - **libc++ < 160000** — disabled (incomplete C++20 ranges support; [issue #4440](https://github.com/nlohmann/json/issues/4440))
+    - **nvcc (CUDA) 12.0.x and 12.1.x** — disabled (the `enable_borrowed_range` variable-template syntax triggers a parse error
+      under these two toolkit versions; fixed in CUDA 12.2; [issue #3907](https://github.com/nlohmann/json/issues/3907))
+    
+    If `JSON_HAS_RANGES` is `0` despite `__cpp_lib_ranges` being defined, one of the exclusions above likely applies to your toolchain.
+
 ## Examples

 ??? example
@@ -38,4 +38,4 @@ When the macro is not defined, the library will define it to its default value.

 ## Version history

- Added in version 3.12.x.
+- Added in version 3.13.0.
@@ -57,7 +57,8 @@ Summary:
 :   name of the base type (class, struct) `type` is derived from

 `member` (in)
-:   name of the member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list
+:   name of the member variable to serialize/deserialize; up to 63 members can be given as a comma-separated
+    list, which may also be empty

 ## Default definition

@@ -127,6 +128,20 @@ void to_json(BasicJsonType& j, const B& b) {
    - Macros 4, 5, and 6 have the same prerequisites of [NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE](nlohmann_define_type_non_intrusive.md).
    - Serialization/deserialization of base types must be defined.

+!!! info "Derived types without own members"
+
+    The member list may be empty. The macro then generates a `to_json`/`from_json` pair that only delegates to
+    the base type, so `type` serializes exactly like `base_type`:
+
+    ```cpp
+    struct derived : base
+    {
+        NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(derived, base)
+    };
+    ```
+
+    The `WITH_NAMES` variants do not support this.
+
 !!! warning "Implementation limits"

    See Implementation limits for [NLOHMANN_DEFINE_TYPE_INTRUSIVE](nlohmann_define_type_intrusive.md) and
@@ -33,7 +33,8 @@ Summary:
 :   name of the type (class, struct) to serialize/deserialize

 `member` (in)
-:   name of the member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list
+:   name of the member variable to serialize/deserialize; up to 63 members can be given as a comma-separated
+    list, which may also be empty

 ## Default definition

@@ -58,10 +59,27 @@ See the examples below for the concrete generated code.

 [GetNonDefNonCopy]: ../../features/arbitrary_types.md#how-can-i-use-get-for-non-default-constructiblenon-copyable-types

+!!! info "Types without members"
+
+    The member list may be empty. The macro then generates a `to_json` that produces an empty JSON object
+    `#!json {}`, and a `from_json` that reads no members:
+
+    ```cpp
+    struct marker
+    {
+        NLOHMANN_DEFINE_TYPE_INTRUSIVE(marker)
+    };
+    ```
+
+    The `WITH_NAMES` variants do not support this.
+
 !!! warning "Implementation limits"

    - The current implementation is limited to at most 63 member variables. If you want to serialize/deserialize types
      with more than 63 member variables, you need to define the `to_json`/`from_json` functions manually.
+    - These macros always produce object-style (named-key) JSON, one key per member. There is no macro variant
+      that serializes a struct's members positionally into a JSON array; for that, write `to_json`/`from_json` by
+      hand, building/reading a `json::array()` of the members in order.

 ## Examples

@@ -33,7 +33,8 @@ Summary:
 :   name of the type (class, struct) to serialize/deserialize

 `member` (in)
-:   name of the (public) member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list
+:   name of the (public) member variable to serialize/deserialize; up to 63 members can be given as a
+    comma-separated list, which may also be empty

 ## Default definition

@@ -59,10 +60,25 @@ See the examples below for the concrete generated code.

 [GetNonDefNonCopy]: ../../features/arbitrary_types.md#how-can-i-use-get-for-non-default-constructiblenon-copyable-types

+!!! info "Types without members"
+
+    The member list may be empty. The macro then generates a `to_json` that produces an empty JSON object
+    `#!json {}`, and a `from_json` that reads no members:
+
+    ```cpp
+    struct marker {};
+    NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(marker)
+    ```
+
+    The `WITH_NAMES` variants do not support this.
+
 !!! warning "Implementation limits"

    - The current implementation is limited to at most 63 member variables. If you want to serialize/deserialize types
      with more than 63 member variables, you need to define the `to_json`/`from_json` functions manually.
+    - These macros always produce object-style (named-key) JSON, one key per member. There is no macro variant
+      that serializes a struct's members positionally into a JSON array; for that, write `to_json`/`from_json` by
+      hand, building/reading a `json::array()` of the members in order.

 ## Examples

@@ -75,4 +75,4 @@ For further information please refer to the corresponding macros without `WITH_N

 ## Version history

-1. Added in version 3.12.x.
+1. Added in version 3.13.0.
@@ -102,4 +102,4 @@ inline void from_json(const BasicJsonType& j, type& e);

 ## Version history

-Added in version 3.12.x.
+Added in version 3.13.0.
@@ -33,6 +33,18 @@ A UTF-8 byte order mark is silently ignored.
 Invalid Unicode escapes and unpaired surrogates in the input are reported as
 [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) with a detailed message.

+`operator>>` parses exactly one JSON value and leaves the stream positioned right after it, so it can be called
+repeatedly to read a sequence of concatenated JSON values from the same stream:
+
+```cpp
+json j1, j2;
+input >> j1;  // parses the first value, stream now positioned right after it
+input >> j2;  // parses the next value
+```
+
+Note this does **not** work for [JSON Lines](../features/parsing/json_lines.md) (newline-delimited JSON) input --
+see that page for why and for the recommended alternative.
+
 !!! warning "Deprecation"

    This function replaces function `#!cpp std::istream& operator<<(basic_json& j, std::istream& i)` which has
@@ -56,8 +56,12 @@ Linear.
    --8<-- "examples/operator_literal_json.output"
    ```

+## See also
+
+- [Creating JSON values](../features/creating_values.md) - the article on creating JSON values
+
 ## Version history

 - Added in version 1.0.0.
 - Moved to namespace `nlohmann::literals::json_literals` in 3.11.0.
- Added `char8_t*` overload in 3.12.x.
+- Added `char8_t*` overload in 3.13.0.
@@ -63,4 +63,4 @@ Linear.

 - Added in version 2.0.0.
 - Moved to namespace `nlohmann::literals::json_literals` in 3.11.0.
- Added `char8_t*` overload in 3.12.x.
+- Added `char8_t*` overload in 3.13.0.
@@ -80,6 +80,12 @@ Linear.
    ```json
    --8<-- "examples/operator_ltlt__json_pointer.output"
    ```
+
+## See also
+
+- [dump](basic_json/dump.md) - serialize to a JSON-formatted string
+- [Serialization](../features/serialization.md) - the serialization article
+
 ## Version history

 1. Added in version 1.0.0. Added support for indentation character and deprecated
@@ -10,6 +10,10 @@ violations will result in a failed build.

    Any compiler with complete C++11 support can compile the library without warnings.

+Note: C++20 modules support may hit compiler-specific issues not covered by the general compiler matrix below. See [Modules](../features/modules.md#known-issues) for known issues and workarounds.
+
+Note: Some modern features (like C++20 ranges or filesystem support) may be disabled on specific broken or incomplete toolchains even when standard feature-test macros indicate support. See [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md) and [`JSON_HAS_FILESYSTEM`](../api/macros/json_has_filesystem.md) for details on known exclusions.
+
 - [x] The library is compiled with 50+ different C++ compilers with different operating systems and platforms,
  including the oldest versions known to compile the library.

@@ -62,12 +66,14 @@ violations will result in a failed build.
        | Clang 20.1.1                                 | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
        | Clang 20.1.8 with GNU-like command-line      | x86_64       | Windows Server 2022 (Build 20348) | GitHub    |
        | Clang 21.1.8                                 | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
-        | CUDA 11.0.221 (nvcc)                         | x86_64       | Ubuntu 20.04 LTS                  | GitHub    |
+        | CUDA 11.8.0 (nvcc)                           | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
+        | CUDA 12.1.1 (nvcc)                           | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
+        | CUDA 12.6.3 (nvcc)                           | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
        | Emscripten 4.0.6                             | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
-        | GNU 4.8.5                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
-        | GNU 4.9.3                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
-        | GNU 5.5.0                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
-        | GNU 6.4.0                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
+        | GNU 4.8.5                                    | x86_64       | Ubuntu 20.04 LTS                  | GitHub    |
+        | GNU 4.9.3                                    | x86_64       | Ubuntu 20.04 LTS                  | GitHub    |
+        | GNU 5.5.0                                    | x86_64       | Ubuntu 20.04 LTS                  | GitHub    |
+        | GNU 6.4.0                                    | x86_64       | Ubuntu 20.04 LTS                  | GitHub    |
        | GNU 7.5.0                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
        | GNU 8.5.0                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
        | GNU 9.3.0                                    | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
@@ -84,7 +90,7 @@ violations will result in a failed build.
        | GNU 15.1.0                                   | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
        | GNU 16.1.0                                   | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
        | GNU 16.1.0                                   | arm64        | Linux 6.1.100                     | Cirrus CI |
-        | icpc (ICC) 2021.5.0 20211109                 | x86_64       | Ubuntu 20.04.3 LTS                | GitHub    |
+        | icpc (ICC) 2021.10.0 20230609                | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
        | icpx (Intel oneAPI DPC++/C++) 2025.3.2       | x86_64       | Ubuntu 24.04 LTS                  | GitHub    |
        | nvc++ (NVIDIA HPC SDK) 25.5-0                | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
        | MSVC 19.0.24241.7                            | x86          | Windows 8.1                       | AppVeyor  |
@@ -0,0 +1,61 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+#include <stdexcept>
+#include <string>
+#include <unordered_set>
+#include <vector>
+
+using json = nlohmann::json;
+
+json parse_strict(const std::string& input)
+{
+    // one key set per nesting depth, reused across sibling objects
+    std::vector<std::unordered_set<std::string>> keys;
+
+    auto reject_duplicate_keys = [&](int depth, json::parse_event_t event, json & parsed)
+    {
+        if (event == json::parse_event_t::object_start)
+        {
+            // keys of this object are reported at depth+1 (see the event table above)
+            const auto child_depth = static_cast<std::size_t>(depth) + 1;
+            if (keys.size() <= child_depth)
+            {
+                keys.resize(child_depth + 1);
+            }
+            keys[child_depth].clear();
+            return true;
+        }
+
+        if (event == json::parse_event_t::key)
+        {
+            auto& seen = keys[static_cast<std::size_t>(depth)];
+            const auto& key = parsed.get_ref<const std::string&>();
+            if (!seen.insert(key).second)
+            {
+                throw std::runtime_error("duplicate JSON object key: " + key);
+            }
+            return true;
+        }
+
+        return true;
+    };
+
+    return json::parse(input, reject_duplicate_keys);
+}
+
+int main()
+{
+    // parsing succeeds when all keys are unique
+    json j = parse_strict(R"({"one": 1, "two": 2})");
+    std::cout << j << '\n';
+
+    // parsing throws when a key is repeated
+    try
+    {
+        parse_strict(R"({"one": 1, "one": 2})");
+    }
+    catch (const std::exception& e)
+    {
+        std::cout << e.what() << '\n';
+    }
+}
@@ -0,0 +1,2 @@
+{"one":1,"two":2}
+duplicate JSON object key: one
@@ -85,7 +85,7 @@ Some important things:

 If you just want to serialize/deserialize some structs, the `to_json`/`from_json` functions can be a lot of boilerplate.

-There are several macros to make your life easier as long as you want to use a JSON object as serialization. The macros are following the naming pattern, and you can chose the macro based on the needed features:
+There are several macros to make your life easier as long as you want to use a JSON object as serialization. The macros are following the naming pattern, and you can choose the macro based on the needed features:

 - All the macros start with `NLOHMANN_DEFINE`.
 - If you want a macro for the derived object, use the [`DERIVED_TYPE`](../api/macros/nlohmann_define_derived_type.md) variant, otherwise use `TYPE`.
@@ -139,7 +139,7 @@ For _derived_ classes and structs, use the following macros
    ```cpp
    namespace ns {
        struct person_derived : person {
-            std:string email;
+            std::string email;
        };
        
        NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(person_derived, person, email)
@@ -180,6 +180,67 @@ For _derived_ classes and structs, use the following macros
    }
    ```

+!!! warning "Overriding conversions for natively-supported types"
+
+    The library already provides built-in `to_json`/`from_json` conversions for STL containers such as
+    `std::vector`, `std::array`, and `std::map`. Defining your own free-function `to_json`/`from_json` overload
+    for one of these container types directly (instead of for your own type) can conflict with the built-in
+    overload during overload resolution, producing compiler errors ("no matching overloaded function",
+    "call is ambiguous") that vary by compiler and library version. If you need different conversion behavior
+    for a container type the library already handles, wrap it in your own type (or use `adl_serializer`
+    specialization, as shown [above](#how-do-i-convert-third-party-types) for `boost::optional`) instead of
+    trying to re-specialize `to_json`/`from_json` for the container type itself.
+
+!!! warning "Raw C-style arrays"
+
+    Members declared as raw C-style arrays (e.g., `char buf[1024]`) do not round-trip safely through
+    `NLOHMANN_DEFINE_TYPE_*` macros or the default (de)serializers: `to_json` serializes any `char` array as a
+    JSON *string* (matching the `std::string`-constructible overload), but the `from_json` overload for
+    fixed-size arrays expects a JSON *array* and iterates it element-wise, which fails with a `type_error` when
+    given a string. Use `std::string`, `std::array<char, N>`, or a manually written `to_json`/`from_json` pair
+    for such members instead.
+
+!!! note "Macros and `nlohmann::ordered_json`"
+
+    The `NLOHMANN_DEFINE_TYPE_*`/`NLOHMANN_DEFINE_DERIVED_TYPE_*` macros are generic over any `basic_json`
+    specialization, including `nlohmann::ordered_json`. Simply use `ordered_json` as the target type and members
+    are serialized in declaration order -- no separate macro or extra code is needed.
+
+    ```cpp
+    namespace ns {
+        NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)
+    }
+
+    ns::person p{"Ned Flanders", "744 Evergreen Terrace", 60};
+    nlohmann::ordered_json j = p; // keys appear in declaration order: name, address, age
+    ```
+
+!!! note "Zero-member types"
+
+    All 12 `NLOHMANN_DEFINE_TYPE_*`/`NLOHMANN_DEFINE_DERIVED_TYPE_*` macros (excluding the `WITH_NAMES` variants)
+    also accept types with no member variables to serialize, producing/accepting an empty JSON object `{}`
+    (or, for the derived-type macros, just the base class's own JSON representation):
+
+    ```cpp
+    namespace ns {
+        struct marker {
+            bool operator==(const marker&) const { return true; }
+            NLOHMANN_DEFINE_TYPE_INTRUSIVE(marker)
+        };
+    }
+
+    ns::marker m{};
+    nlohmann::json j = m; // {}
+    ```
+
+!!! note "No macro for non-default-constructible types"
+
+    There is currently no `NLOHMANN_DEFINE_TYPE_*`-style macro for types that are not
+    [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). This is not an
+    intentional omission of documentation -- no such macro exists yet; see
+    [How can I use `get()` for non-default constructible/non-copyable types?](#how-can-i-use-get-for-non-default-constructiblenon-copyable-types)
+    for the manual pattern to use instead.
+
 ## How do I convert third-party types?

 This requires a bit more advanced technique. But first, let us see how this conversion mechanism works:
@@ -270,6 +331,49 @@ namespace nlohmann {
 }
 ```

+## Why can't I convert to/from `std::any`?
+
+`std::any` is intentionally excluded from `get<T>()`/generic conversion support, so `get<std::any>()` and
+containers like `std::map<std::string, std::any>` fail to compile by design -- there is no way to know, from a
+`json` value alone, which concrete type to store inside the `std::any`. To work with heterogeneous JSON values,
+dispatch on the value's type manually and construct the `std::any` (or extract from it) yourself:
+
+```cpp
+std::any value_to_any(const json& j) {
+    if (j.is_boolean())      { return j.get<bool>(); }
+    if (j.is_number_integer()) { return j.get<int>(); }
+    if (j.is_number_float()) { return j.get<double>(); }
+    if (j.is_string())       { return j.get<std::string>(); }
+    // ... handle other types (arrays, objects) as needed for your use case
+    return {};
+}
+
+json any_to_json(const std::any& a) {
+    if (a.type() == typeid(bool))        { return std::any_cast<bool>(a); }
+    if (a.type() == typeid(int))         { return std::any_cast<int>(a); }
+    if (a.type() == typeid(double))      { return std::any_cast<double>(a); }
+    if (a.type() == typeid(std::string)) { return std::any_cast<std::string>(a); }
+    return nullptr;
+}
+```
+
+## Why does serializing a `std::map`/`std::unordered_map` with non-string keys produce an array?
+
+A `std::map`/`std::unordered_map` whose key type is not string-like (e.g., `std::map<int, std::string>`) is
+serialized as a JSON *array* of 2-element `[key, value]` arrays, not as a JSON object -- JSON object keys must be
+strings, so the library cannot represent an integer-keyed map as an object.
+
+```cpp
+std::map<int, std::string> m{{1, "one"}, {2, "two"}};
+json j = m;
+// j is [[1,"one"],[2,"two"]], not {"1":"one","2":"two"}
+```
+
+## Why does `std::wstring` convert or dump incorrectly?
+
+The library assumes UTF-8 encoding internally, so `std::wstring` is not supported out of the box -- see the FAQ
+entry on [wide string handling](../home/faq.md#wide-string-handling) for why, and for a UTF-8 conversion recipe.
+
 ## Can I write my own serializer? (Advanced use)

 Yes. You might want to take a look at [`unit-udt.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-udt.cpp) in the test suite, to see a few examples.
@@ -11,7 +11,7 @@ Runtime assertions can be switched off by defining the preprocessor macro `NDEBU

 ## Change assertion behavior

-The behavior of runtime assertions can be changes by defining macro [`JSON_ASSERT(x)`](../api/macros/json_assert.md)
+The behavior of runtime assertions can be changed by defining macro [`JSON_ASSERT(x)`](../api/macros/json_assert.md)
 before including the `json.hpp` header.

 ## Function with runtime assertions
@@ -123,7 +123,7 @@ The library uses the following mapping from JSON values types to BJData types ac
    The current version of this library does not yet support automatic detection of and conversion from a nested JSON
    array input to a BJData ND-array.

-    [JDataAAFmt]: https://github.com/NeuroJSON/jdata/blob/master/JData_specification.md#annotated-storage-of-n-d-arrays)
+    [JDataAAFmt]: https://github.com/NeuroJSON/jdata/blob/master/JData_specification.md#annotated-storage-of-n-d-arrays

 !!! info "Restrictions in optimized data types for arrays and objects"

@@ -146,7 +146,7 @@ The library uses the following mapping from JSON values types to BJData types ac
    suggested by the BJData documentation. In particular, this means that the serialization and the deserialization of
    JSON containing binary values into BJData and back will result in a different JSON object.

-    [BJDataBinArr]: https://github.com/NeuroJSON/bjdata/blob/master/Binary_JData_Specification.md#optimized-binary-array)
+    [BJDataBinArr]: https://github.com/NeuroJSON/bjdata/blob/master/Binary_JData_Specification.md#optimized-binary-array

 ??? example

@@ -52,28 +52,29 @@ The library uses the following mapping from JSON values types to BSON types:

 The library maps BSON record types to JSON value types as follows:

-| BSON type             | BSON marker byte | JSON value type |
-|-----------------------|------------------|-----------------|
-| double                | 0x01             | number_float    |
-| string                | 0x02             | string          |
-| document              | 0x03             | object          |
-| array                 | 0x04             | array           |
-| binary                | 0x05             | binary          |
-| undefined             | 0x06             | *unsupported*   |
-| ObjectId              | 0x07             | *unsupported*   |
-| boolean               | 0x08             | boolean         |
-| UTC Date-Time         | 0x09             | *unsupported*   |
-| null                  | 0x0A             | null            |
-| Regular Expr.         | 0x0B             | *unsupported*   |
-| DB Pointer            | 0x0C             | *unsupported*   |
-| JavaScript Code       | 0x0D             | *unsupported*   |
-| Symbol                | 0x0E             | *unsupported*   |
-| JavaScript Code       | 0x0F             | *unsupported*   |
-| int32                 | 0x10             | number_integer  |
-| uint64(Timestamp)     | 0x11             | number_unsigned |
-| 128-bit decimal float | 0x13             | *unsupported*   |
-| Max Key               | 0x7F             | *unsupported*   |
-| Min Key               | 0xFF             | *unsupported*   |
+| BSON type                | BSON marker byte | JSON value type |
+|--------------------------|------------------|-----------------|
+| double                   | 0x01             | number_float    |
+| string                   | 0x02             | string          |
+| document                 | 0x03             | object          |
+| array                    | 0x04             | array           |
+| binary                   | 0x05             | binary          |
+| undefined                | 0x06             | *unsupported*   |
+| ObjectId                 | 0x07             | *unsupported*   |
+| boolean                  | 0x08             | boolean         |
+| UTC Date-Time            | 0x09             | *unsupported*   |
+| null                     | 0x0A             | null            |
+| Regular Expr.            | 0x0B             | *unsupported*   |
+| DB Pointer               | 0x0C             | *unsupported*   |
+| JavaScript Code          | 0x0D             | *unsupported*   |
+| Symbol                   | 0x0E             | *unsupported*   |
+| JavaScript Code w/ scope | 0x0F             | *unsupported*   |
+| int32                    | 0x10             | number_integer  |
+| uint64(Timestamp)        | 0x11             | number_unsigned |
+| int64                    | 0x12             | number_integer  |
+| 128-bit decimal float    | 0x13             | *unsupported*   |
+| Max Key                  | 0x7F             | *unsupported*   |
+| Min Key                  | 0xFF             | *unsupported*   |

 !!! warning "Incomplete mapping"

@@ -5,7 +5,7 @@ extremely small code sizes, fairly small message size, and extensibility without

 !!! abstract "References"

-      - [CBOR Website](http://cbor.io) - the main source on CBOR
+    - [CBOR Website](http://cbor.io) - the main source on CBOR
    - [CBOR Playground](http://cbor.me) - an interactive webpage to translate between JSON and CBOR
    - [RFC 7049](https://tools.ietf.org/html/rfc7049) - the CBOR specification

@@ -37,22 +37,22 @@ The library uses the following mapping from JSON values types to CBOR types acco
 | number_float    | *any value representable by a float*       | Single-Precision Float            | 0xFA       |
 | number_float    | *any value NOT representable by a float*   | Double-Precision Float            | 0xFB       |
 | string          | *length*: 0..23                            | UTF-8 string                      | 0x60..0x77 |
-| string          | *length*: 23..255                          | UTF-8 string (1 byte follow)      | 0x78       |
+| string          | *length*: 24..255                          | UTF-8 string (1 byte follow)      | 0x78       |
 | string          | *length*: 256..65535                       | UTF-8 string (2 bytes follow)     | 0x79       |
 | string          | *length*: 65536..4294967295                | UTF-8 string (4 bytes follow)     | 0x7A       |
 | string          | *length*: 4294967296..18446744073709551615 | UTF-8 string (8 bytes follow)     | 0x7B       |
 | array           | *size*: 0..23                              | array                             | 0x80..0x97 |
-| array           | *size*: 23..255                            | array (1 byte follow)             | 0x98       |
+| array           | *size*: 24..255                            | array (1 byte follow)             | 0x98       |
 | array           | *size*: 256..65535                         | array (2 bytes follow)            | 0x99       |
 | array           | *size*: 65536..4294967295                  | array (4 bytes follow)            | 0x9A       |
 | array           | *size*: 4294967296..18446744073709551615   | array (8 bytes follow)            | 0x9B       |
 | object          | *size*: 0..23                              | map                               | 0xA0..0xB7 |
-| object          | *size*: 23..255                            | map (1 byte follow)               | 0xB8       |
+| object          | *size*: 24..255                            | map (1 byte follow)               | 0xB8       |
 | object          | *size*: 256..65535                         | map (2 bytes follow)              | 0xB9       |
 | object          | *size*: 65536..4294967295                  | map (4 bytes follow)              | 0xBA       |
 | object          | *size*: 4294967296..18446744073709551615   | map (8 bytes follow)              | 0xBB       |
 | binary          | *size*: 0..23                              | byte string                       | 0x40..0x57 |
-| binary          | *size*: 23..255                            | byte string (1 byte follow)       | 0x58       |
+| binary          | *size*: 24..255                            | byte string (1 byte follow)       | 0x58       |
 | binary          | *size*: 256..65535                         | byte string (2 bytes follow)      | 0x59       |
 | binary          | *size*: 65536..4294967295                  | byte string (4 bytes follow)      | 0x5A       |
 | binary          | *size*: 4294967296..18446744073709551615   | byte string (8 bytes follow)      | 0x5B       |
@@ -66,7 +66,15 @@ see "binary" cells in the table above.

 !!! info "NaN/infinity handling"

-    If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the normal JSON serialization which serializes NaN or Infinity to `null`.
+    `NaN`, `Infinity`, and `-Infinity` are serialized as a CBOR half-precision float (type 0xF9, 3 bytes total):
+    `NaN` as `0xF9 0x7E 0x00`, `Infinity` as `0xF9 0x7C 0x00`, and `-Infinity` as `0xF9 0xFC 0x00`. This behavior
+    differs from the normal JSON serialization which serializes NaN or Infinity to `null`.
+
+!!! note
+
+    Prior to version 3.13.0, NaN and Infinity were instead serialized as a CBOR double-precision float (type 0xFB,
+    9 bytes total), because the check used to select a smaller encoding compared magnitudes with NaN, which is
+    always `false` and caused the intended half-precision path to be skipped.

 !!! info "Unused CBOR types"

@@ -160,6 +168,13 @@ The library maps CBOR types to JSON value types as follows:
     - simple values (0xE0..0xF3, 0xF8)
     - undefined (0xF7)

+!!! warning "Negative integer overflow"
+
+    CBOR negative integers (major type 1) are decoded as `-1 - n`. If the encoded magnitude `n` is too large for the
+    result to fit into `number_integer_t` (`std::int64_t` by default), parsing fails with a
+    [`parse_error.112`](../../home/exceptions.md#jsonexceptionparse_error112) exception rather than overflowing
+    silently.
+
 !!! warning "Object keys"

    CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than UTF-8 strings are rejected.
@@ -41,7 +41,7 @@ See [binary values](../binary_values.md) for more information.
 |--------------------|-------------|--------------|-------------------|---------------|
 | BJData             | 53.2 %      | 91.1 %       | 78.1 %            | 96.6 %        |
 | BJData (size)      | 58.6 %      | 92.1 %       | 86.7 %            | 97.4 %        |
-| BJData (size+tyoe) | 58.6 %      | 92.1 %       | 86.5 %            | 97.4 %        |
+| BJData (size+type) | 58.6 %      | 92.1 %       | 86.5 %            | 97.4 %        |
 | BSON               | 85.8 %      | 95.2 %       | 95.8 %            | 106.7 %       |
 | CBOR               | 50.5 %      | 86.3 %       | 68.4 %            | 88.0 %        |
 | MessagePack        | 50.5 %      | 86.0 %       | 68.5 %            | 87.9 %        |
@@ -67,8 +67,15 @@ specification:

 !!! info "NaN/infinity handling"

-    If NaN or Infinity are stored inside a JSON number, they are serialized properly in contrast to the
-    [dump](../../api/basic_json/dump.md) function which serializes NaN or Infinity to `null`.
+    `NaN`, `Infinity`, and `-Infinity` are serialized as a MessagePack float 32 (type 0xCA, 5 bytes total),
+    regardless of magnitude, in contrast to the [dump](../../api/basic_json/dump.md) function which serializes NaN
+    or Infinity to `null`.
+
+!!! note
+
+    Prior to version 3.13.0, NaN and Infinity were instead serialized as a MessagePack float 64 (type 0xCB, 9 bytes
+    total), because the check used to select the smaller float 32 encoding compared magnitudes with NaN, which is
+    always `false` and caused the float 32 path to be skipped.

 ??? example

@@ -41,7 +41,7 @@ binary.has_subtype();                   // returns false
 binary_with_subtype.has_subtype();      // returns true

 binary_with_subtype.clear_subtype();
-binary_with_subtype.has_subtype();      // returns true
+binary_with_subtype.has_subtype();      // returns false

 binary_with_subtype.set_subtype(42);
 binary.set_subtype(23);
@@ -146,7 +146,7 @@ as an array of uint8 values. The library implements this translation.
    auto v = json::to_bjdata(j);      
    ```
            
-    `v` is a `std::vector<std::uint8t>` with the following 20 elements:
+    `v` is a `std::vector<std::uint8_t>` with the following 20 elements:

    ```c
    0x7B                                             // '{'
@@ -158,10 +158,10 @@ as an array of uint8 values. The library implements this translation.
    0x7D                                             // '}'
    ```

-    The following code uses the type and size optimization for UBJSON:
+    The following code uses the type and size optimization for BJData:

    ```cpp
-    // convert to UBJSON using the size and type optimization
+    // convert to BJData using the size and type optimization
    auto v = json::to_bjdata(j, true, true);
    ```

@@ -178,7 +178,7 @@ as an array of uint8 values. The library implements this translation.
            0xCA 0xFE 0xBA 0xBE         // content
    ```

-    Note that subtype (42) is **not** serialized and that UBJSON has **no binary type**, and deserializing `v` would
+    Note that subtype (42) is **not** serialized and that BJData has **no binary type**, and deserializing `v` would
    yield the following value:

    ```json
@@ -205,7 +205,7 @@ unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00
    auto v = json::to_bson(j);      
    ```
            
-    `v` is a `std::vector<std::uint8t>` with the following 22 elements:
+    `v` is a `std::vector<std::uint8_t>` with the following 22 elements:
    
    ```c
    0x16 0x00 0x00 0x00                         // number of bytes in the document
@@ -247,7 +247,7 @@ byte array.
    auto v = json::to_cbor(j);      
    ```
            
-    `v` is a `std::vector<std::uint8t>` with the following 15 elements:
+    `v` is a `std::vector<std::uint8_t>` with the following 15 elements:
    
    ```c
    0xA1                                   // map(1)
@@ -291,7 +291,7 @@ If no subtype is given, the bin family (bin8, bin16, bin32) is used.
    auto v = json::to_msgpack(j);      
    ```
            
-    `v` is a `std::vector<std::uint8t>` with the following 14 elements:
+    `v` is a `std::vector<std::uint8_t>` with the following 14 elements:
    
    ```c
    0x81                                   // fixmap1
@@ -331,7 +331,7 @@ as an array of uint8 values. The library implements this translation.
    auto v = json::to_ubjson(j);      
    ```
            
-    `v` is a `std::vector<std::uint8t>` with the following 20 elements:
+    `v` is a `std::vector<std::uint8_t>` with the following 20 elements:
    
    ```c
    0x7B                                             // '{'
@@ -11,7 +11,7 @@ This library does not support comments *by default*. It does so for three reason

 3. It is dangerous for interoperability if some libraries add comment support while others do not. Please check [The Harmful Consequences of the Robustness Principle](https://tools.ietf.org/html/draft-iab-protocol-maintenance-01) on this.

-However, you can set parameter `ignore_comments` to `#!cpp true` in the [`parse`](../api/basic_json/parse.md) function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace.
+However, you can set parameter `ignore_comments` to `#!cpp true` in the [`parse`](../api/basic_json/parse.md) function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace. Combined with `ignore_trailing_commas` (also a `parse` parameter), this covers what is commonly referred to as **JSONC** (JSON with Comments, as used e.g. by Visual Studio Code's `.jsonc` files) -- comments and trailing commas, nothing more. This is a different, smaller extension than [JSON5](https://json5.org), which additionally allows unquoted keys, single-quoted strings, and other syntax changes that this library does not support.

 For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).

@@ -0,0 +1,152 @@
+# Converting values
+
+A `basic_json` value stores JSON data, but most of the time you want to move that data into ordinary C++ types (an
+`#!cpp int`, a `#!cpp std::string`, a `#!cpp std::vector`, or one of your own structs) and back. This page describes how
+these conversions work.
+
+## Getting values out
+
+The [`get`](../api/basic_json/get.md) function template returns a copy of the stored value converted to the requested
+type:
+
+```cpp
+json j = R"({"name": "Mary", "age": 42, "hobbies": ["hiking", "reading"]})"_json;
+
+auto name = j["name"].get<std::string>();               // "Mary"
+auto age  = j["age"].get<int>();                        // 42
+auto hobbies = j["hobbies"].get<std::vector<std::string>>();  // {"hiking", "reading"}
+```
+
+!!! note "Getting a string without quotes"
+
+    A frequent point of confusion: use [`get`](../api/basic_json/get.md), **not** [`dump`](serialization.md), to read a
+    string value. `#!cpp j["name"].get<std::string>()` yields `#!cpp Mary`, whereas `#!cpp j["name"].dump()` yields the
+    JSON text `#!cpp "Mary"` (**with** quotes), because `dump` always produces a JSON text.
+
+Alternatively, [`get_to`](../api/basic_json/get_to.md) writes into an existing variable and deduces the target type,
+which avoids repeating it:
+
+??? example
+
+    ```cpp
+    --8<-- "examples/get_to.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/get_to.output"
+    ```
+
+The library already knows how to convert to and from the scalar types and the STL containers (such as
+`#!cpp std::vector`, `#!cpp std::map`, `#!cpp std::array`, `#!cpp std::optional`, and many more). Converting a JSON
+object back to a `#!cpp std::map` or a JSON array back to a `#!cpp std::vector` therefore works without any extra code:
+
+```cpp
+json j = {{"one", 1}, {"two", 2}};
+auto m = j.get<std::map<std::string, int>>();  // {{"one", 1}, {"two", 2}}
+```
+
+## Implicit conversions
+
+By default, a JSON value implicitly converts to a compatible C++ type, so the explicit `get` call can often be omitted:
+
+```cpp
+json j = "Hello";
+std::string s = j;   // implicit conversion, same as j.get<std::string>()
+```
+
+Implicit conversions are convenient but can be surprising (for example, in overload resolution or with `auto`). They can
+be disabled by defining [`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md) to `#!cpp 0`,
+which forces the explicit `get` form and can catch unintended conversions at compile time.
+
+!!! warning "Conversions do not range-check numbers"
+
+    Just like C++ itself, the `get` family performs numeric conversions without range checks — retrieving a
+    floating-point value as an integer truncates it, and narrowing conversions may overflow. See
+    [number conversion](types/number_handling.md#number-conversion) for details and how to guard against it.
+
+!!! warning "std::optional direct construction from JSON null throws"
+
+    Constructing or assigning `std::optional<T>` directly from a JSON value does not correctly produce
+    `std::nullopt` for a JSON `null`:
+
+    ```cpp
+    json j_null;
+    std::optional<std::string> opt = j_null;  // ❌ throws type_error 302
+    ```
+
+    This is due to C++ language rules: `std::optional<T>` has its own converting constructor that is chosen over
+    `basic_json::operator T()` when both are viable. Use `get<std::optional<T>>()` or `get_to()` instead:
+
+    ```cpp
+    auto opt = j_null.get<std::optional<std::string>>();  // ✅ std::nullopt
+    j_null.get_to(opt);                                     // ✅ std::nullopt
+    ```
+
+!!! warning "`static_cast` and `get<std::optional<T>>()` are not guaranteed equivalent"
+
+    `operator ValueType()` (used by `static_cast` and implicit conversions) intentionally excludes
+    `std::optional<T>` from delegating to `get<T>()`, to avoid a constructor ambiguity with
+    `std::optional<T>`'s own converting constructor from `basic_json`. As a result,
+    `static_cast<std::optional<T>>(json_value)` goes through `std::optional<T>`'s own converting
+    constructor rather than through `get<std::optional<T>>()`, which can behave differently -- for example,
+    with a custom `adl_serializer<std::optional<T>>` specialization. Prefer `get<std::optional<T>>()`/`get_to()`
+    over `static_cast` for optional types.
+
+!!! warning "Converting to a fixed-size `std::array` does not check length"
+
+    Converting a JSON array to `#!cpp std::array<T, N>` does not check that the JSON array's size matches `N`:
+    if the JSON array is longer, the extra elements are silently dropped; if it is shorter, the remaining
+    `std::array` elements are left default-constructed. No exception is thrown in either case.
+
+    ```cpp
+    json j = {1, 2, 3, 4, 5};
+    auto a = j.get<std::array<int, 3>>();  // {1, 2, 3} -- elements 4 and 5 silently dropped
+    ```
+
+## Omitting a field when serializing `std::optional`
+
+By default, `to_json` for `std::optional<T>` writes either the value or `#!json null` -- there is no built-in way
+to make a field disappear from the serialized object entirely when the `std::optional` is `std::nullopt`. Because
+a specialization of `adl_serializer<std::optional<T>>` only controls how the *value* is converted (it cannot
+prevent the containing object's `to_json` from inserting the key in the first place), omission has to be
+implemented in the *containing* type's `to_json`:
+
+```cpp
+struct person {
+    std::string name;
+    std::optional<int> age;
+};
+
+void to_json(json& j, const person& p) {
+    j = json{{"name", p.name}};
+    if (p.age) {
+        j["age"] = *p.age;  // key is only inserted when the optional has a value
+    }
+}
+```
+
+## Putting values in
+
+The reverse direction works the same way: assigning or constructing a `json` from a C++ value converts it to JSON.
+
+```cpp
+std::vector<int> numbers = {1, 2, 3};
+json j = numbers;   // [1,2,3]
+```
+
+## Your own types
+
+The conversions above are built in for standard types. To make the same syntax work for **your own** types, provide
+`to_json`/`from_json` functions (or use one of the convenience macros). This is described in detail on the
+[arbitrary types conversions](arbitrary_types.md) page. Enums can be mapped to strings as described in
+[specializing enum conversion](enum_conversion.md).
+
+## See also
+
+- [`get`](../api/basic_json/get.md) - get a copy converted to a given type
+- [`get_to`](../api/basic_json/get_to.md) - convert into an existing variable
+- [`get_ref`](../api/basic_json/get_ref.md) / [`get_ptr`](../api/basic_json/get_ptr.md) - access the stored value without copying
+- [Arbitrary types conversions](arbitrary_types.md) - support your own types
+- [`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md) - toggle implicit conversions
@@ -0,0 +1,103 @@
+# Creating JSON values
+
+There are several ways to create a JSON value in memory. This page gives an overview; to read a value from JSON text
+instead, see [parsing](parsing/index.md).
+
+## From C++ values
+
+Any value of a supported C++ type can be assigned to or used to construct a `json`:
+
+```cpp
+json j_number  = 42;
+json j_float   = 3.141;
+json j_string  = "Hello";
+json j_boolean = true;
+json j_null    = nullptr;
+json j_vector  = std::vector<int>{1, 2, 3};   // array
+```
+
+See [converting values](conversions.md) for the full set of supported types.
+
+## With initializer lists
+
+Objects and arrays can be written concisely with brace-enclosed initializer lists:
+
+```cpp
+// an array
+json array = {1, 2, 3, 4};
+
+// an object (a list of key/value pairs)
+json object = {
+    {"pi", 3.141},
+    {"happy", true},
+    {"name", "Niels"},
+    {"nothing", nullptr},
+    {"list", {1, 0, 2}},
+    {"object", {{"currency", "USD"}, {"value", 42.99}}}
+};
+```
+
+The library decides between an array and an object based on the content: a list whose elements are all two-element lists
+with a string as the first element is treated as an object, everything else as an array.
+
+!!! warning "Ambiguous cases: `#!cpp {}` vs. `#!cpp []`"
+
+    Because the same `#!cpp {}` syntax is used for both arrays and objects, some cases are ambiguous. To force a
+    particular type, use the explicit factory functions [`json::array`](../api/basic_json/array.md) and
+    [`json::object`](../api/basic_json/object.md):
+
+    ```cpp
+    json empty_array_explicit = json::array();               // []
+    json empty_object_explicit = json::object();             // {}
+
+    // a JSON array with one object, not an object with one member
+    json array_of_objects = json::array({{"key", "value"}}); // [{"key":"value"}]
+    ```
+
+    Related to this, single-element brace initialization such as `#!cpp json j{value};` wraps the element in a
+    single-element **array** by default, and its behavior even differs between compilers. See the
+    [FAQ](../home/faq.md#brace-initialization-yields-arrays) for details and the opt-in
+    [`JSON_BRACE_INIT_COPY_SEMANTICS`](../api/macros/json_brace_init_copy_semantics.md) macro.
+
+## Building incrementally
+
+A value can also be built up piece by piece. Accessing a non-existing object key or array index with
+[`operator[]`](element_access/unchecked_access.md) creates the element on the fly:
+
+```cpp
+json j;                    // null
+j["answer"]["everything"] = 42;   // becomes an object
+j["list"] = {1, 0, 2};
+j["list"].push_back(3);           // [1,0,2,3]
+```
+
+See [modifying values](modifying_values.md) for [`push_back`](../api/basic_json/push_back.md),
+[`emplace`](../api/basic_json/emplace.md), and related functions.
+
+## With the `_json` literal
+
+The `_json` [user-defined literal](../api/operator_literal_json.md) parses a string at the call site and is a
+convenient way to write a JSON value inline:
+
+??? example
+
+    ```cpp
+    --8<-- "examples/operator_literal_json.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/operator_literal_json.output"
+    ```
+
+Note this **parses** the string, so `#!cpp "42"_json` is the number `#!cpp 42`, whereas `#!cpp json("42")` is the JSON
+string `#!json "42"`.
+
+## See also
+
+- [`basic_json` constructors](../api/basic_json/basic_json.md) - all ways to construct a value
+- [`array`](../api/basic_json/array.md) / [`object`](../api/basic_json/object.md) - force array or object type
+- [`operator""_json`](../api/operator_literal_json.md) - the `_json` literal
+- [Converting values](conversions.md) - which C++ types can be used
+- [Parsing](parsing/index.md) - create a value from JSON text
@@ -29,7 +29,7 @@ otherwise.
    | `#!cpp j.at("hobbies").at(0)` | `#!json "hiking"`                                                            |
    | `#!cpp j.at("hobbies").at(1)` | `#!json "reading"`                                                           |

-The return value is a reference, so it can be modified by the original value.
+The return value is a reference, so it can be used to modify the original value.

 ??? example "Write access"

@@ -62,7 +62,7 @@ non-existing, an exception is thrown.
    [json.exception.out_of_range.401] array index 3 is out of range
    ```

-    When you [extended diagnostic messages](../../home/exceptions.md#extended-diagnostic-messages) are enabled by
+    When [extended diagnostic messages](../../home/exceptions.md#extended-diagnostic-messages) are enabled by
    defining [`JSON_DIAGNOSTICS`](../../api/macros/json_diagnostics.md), the exception further gives information where
    the key or index is missing or out of range.
    
@@ -4,7 +4,8 @@

 In many situations, such as configuration files, missing values are not exceptional, but may be treated as if a default
 value was present. For this case, use [`value(key, default_value)`](../../api/basic_json/value.md) which takes the key
-you want to access and a default value in case there is no value stored with that key.
+you want to access and a default value in case there is no value stored with that key. This is equivalent to Python's
+`dict.get(key, default)`.

 ## Example

@@ -41,7 +42,7 @@ you want to access and a default value in case there is no value stored with tha
    The value function is a template, and the return type of the function is determined by the type of the provided
    default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit
    unsigned integer. We get exactly that value when using [`operator[]`](../../api/basic_json/operator[].md). However,
-    when we call `value` and provide `#!c 0` as default value, then `#!c -1` is returned. The occurs, because `#!c 0`
+    when we call `value` and provide `#!c 0` as default value, then `#!c -1` is returned. This occurs, because `#!c 0`
    has type `#!c int` which overflows when handling the value `#!c 18446744073709551615`.

    To address this issue, either provide a correctly typed default value or use the template parameter to specify the
@@ -102,6 +102,20 @@ that the passed index is the new maximal index. Intermediate values are filled w
    `operator[]` can only be used with objects (with a string argument) or with arrays (with a numeric argument). For
    other types, a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error305) is thrown.

+## Performance: reserving array capacity
+
+There is no public `reserve(count)` member on `basic_json` for pre-allocating array capacity. If you are building
+a large array incrementally (e.g., via repeated `push_back()`) and know its final size ahead of time, you can
+reserve capacity via `get_ref()` to access the underlying `array_t` directly:
+
+```cpp
+json j = json::array();
+j.get_ref<json::array_t&>().reserve(1000);
+for (int i = 0; i < 1000; ++i) {
+    j.push_back(i);
+}
+```
+
 ## Summary

 | scenario                          | non-const value                                                                                                                                      | const value                                                                 |
@@ -0,0 +1,53 @@
+# Features
+
+This section describes the features of the library in detail. If you are new to the library, the pages below are
+roughly ordered along a typical workflow: create or parse a value, access and modify it, convert it to and from your own
+C++ types, and finally serialize it again.
+
+## Creating and reading values
+
+- [Creating JSON values](creating_values.md) — build values from literals, initializer lists, and STL containers, and
+  understand the `#!cpp {}` vs. `#!cpp []` ambiguity.
+- [Parsing](parsing/index.md) — read a JSON value from a string, file, or stream, including
+  [JSON Lines](parsing/json_lines.md), [callbacks](parsing/parser_callbacks.md), the
+  [SAX interface](parsing/sax_interface.md), and [error handling](parsing/parse_exceptions.md).
+- [Comments](comments.md) and [trailing commas](trailing_commas.md) — opt-in relaxations of the JSON grammar.
+
+## Accessing and modifying values
+
+- [Element access](element_access/index.md) — unchecked ([`operator[]`](element_access/unchecked_access.md)),
+  checked ([`at`](element_access/checked_access.md)), and access with a
+  [default value](element_access/default_value.md).
+- [JSON Pointer](json_pointer.md) — address values deep inside a document with [RFC 6901](https://tools.ietf.org/html/rfc6901) pointers.
+- [Iterators](iterators.md) — traverse arrays and objects.
+- [Modifying values](modifying_values.md) — add, update, merge, and remove elements.
+- [JSON Patch and Diff](json_patch.md) and [JSON Merge Patch](merge_patch.md) — apply and compute structured changes.
+
+## Converting to and from C++ types
+
+- [Converting values](conversions.md) — get values out with [`get`](../api/basic_json/get.md)/[`get_to`](../api/basic_json/get_to.md),
+  and understand implicit conversions.
+- [Arbitrary types conversions](arbitrary_types.md) — teach the library about your own structs and classes.
+- [Specializing enum conversion](enum_conversion.md) — map enums to strings instead of integers.
+
+## Serializing values
+
+- [Serialization](serialization.md) — turn a value back into JSON text with [`dump`](../api/basic_json/dump.md),
+  including pretty-printing and handling of non-ASCII and invalid UTF-8.
+- [Binary formats](binary_formats/index.md) — encode values more compactly as
+  [BJData](binary_formats/bjdata.md), [BSON](binary_formats/bson.md), [CBOR](binary_formats/cbor.md),
+  [MessagePack](binary_formats/messagepack.md), or [UBJSON](binary_formats/ubjson.md).
+- [Binary values](binary_values.md) — store and exchange raw byte sequences.
+
+## How values are stored and configured
+
+- [Types](types/index.md) and [number handling](types/number_handling.md) — how JSON types map to C++ types and how
+  numbers are treated.
+- [Object order](object_order.md) — keep insertion order with [`ordered_json`](../api/ordered_json.md).
+- [Runtime assertions](assertions.md), [supported macros](macros.md), the [`nlohmann` namespace](namespace.md), and
+  [C++ modules](modules.md) — build-time and runtime configuration.
+
+!!! tip "Looking for a specific function?"
+
+    This section gives conceptual overviews. For the precise signature, parameters, and return value of a function, see
+    the [API Documentation](../api/basic_json/index.md).
@@ -130,7 +130,7 @@ for (auto& [key, val] : j_object.items())

 ### Iterating strings and binary values

-Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, `*begin()` returns the complete string or binary array and is also safe the underlying string or binary array is empty.
+Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, `*begin()` returns the complete string or binary array and is also safe if the underlying string or binary array is empty.

 ??? example

@@ -28,7 +28,7 @@ The library can also calculate a JSON patch (i.e., a **diff**) given two JSON va

    For two JSON values *source* and *target*, the following code yields always true:

-    ```cüü
+    ```cpp
    source.patch(diff(source, target)) == target;
    ```

@@ -77,6 +77,11 @@ auto val2 = j.at(json::json_pointer("/nested/three/1"));    // false
 auto val3 = j.value(json::json_pointer("/nested/four"), 0); // 0
 ```

+!!! note "Creating intermediate levels that don't exist"
+
+    See the [`operator[]` notes](../api/basic_json/operator%5B%5D.md#return-value) for how array vs. object is
+    decided when a pointer creates intermediate levels that don't exist yet.
+
 ## Flatten / unflatten

 The library implements a function [`flatten`](../api/basic_json/flatten.md) to convert any JSON document into a JSON
@@ -0,0 +1,77 @@
+# Modifying values
+
+Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page
+gives an overview of the available operations. For read access, see [element access](element_access/index.md).
+
+## Adding to arrays
+
+New elements are appended to an array with [`push_back`](../api/basic_json/push_back.md) or constructed in place with
+[`emplace_back`](../api/basic_json/emplace_back.md). If the value is `#!json null`, it is converted to an array first, so
+these functions can also be used to build an array from scratch.
+
+```cpp
+json j;                 // null
+j.push_back(1);         // [1]
+j.push_back(2);         // [1,2]
+j.emplace_back(3);      // [1,2,3]
+
+// operator+= is a shorthand for push_back
+j += 4;                 // [1,2,3,4]
+```
+
+## Adding to objects
+
+The most common way to add or replace a member is [`operator[]`](element_access/unchecked_access.md), which inserts the
+key if it does not exist yet:
+
+```cpp
+json j;
+j["name"] = "Mary";     // {"name":"Mary"}
+j["name"] = "John";     // {"name":"John"}  (replaced)
+```
+
+[`emplace`](../api/basic_json/emplace.md) inserts a member only if the key is not already present, and reports whether
+the insertion happened — useful for "add if absent" semantics.
+
+## Merging objects
+
+To merge one object into another, [`update`](../api/basic_json/update.md) copies all members from another object,
+overwriting existing keys (similar to Python's `dict.update`). This is the idiomatic way to combine two objects.
+
+??? example
+
+    ```cpp
+    --8<-- "examples/update.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/update.output"
+    ```
+
+For a recursive merge that follows [RFC 7386](https://tools.ietf.org/html/rfc7386), see
+[JSON Merge Patch](merge_patch.md). To apply a sequence of well-defined edit operations, see
+[JSON Patch](json_patch.md).
+
+## Removing elements
+
+Elements are removed with [`erase`](../api/basic_json/erase.md), which accepts an object key, an array index, or an
+iterator. [`clear`](../api/basic_json/clear.md) empties a value while keeping its type, and
+[`operator[]`](element_access/unchecked_access.md) combined with assignment can overwrite a value entirely.
+
+```cpp
+json j = {{"a", 1}, {"b", 2}, {"c", 3}};
+j.erase("b");           // {"a":1,"c":3}
+
+json a = {1, 2, 3, 4};
+a.erase(1);             // [1,3,4]  (erase by index)
+```
+
+## See also
+
+- [`push_back`](../api/basic_json/push_back.md) / [`emplace_back`](../api/basic_json/emplace_back.md) - append to an array
+- [`emplace`](../api/basic_json/emplace.md) - insert into an object if the key is absent
+- [`update`](../api/basic_json/update.md) - merge objects
+- [`erase`](../api/basic_json/erase.md) / [`clear`](../api/basic_json/clear.md) - remove elements
+- [JSON Patch and Diff](json_patch.md) and [JSON Merge Patch](merge_patch.md) - structured modifications
@@ -27,6 +27,7 @@ json data = json::parse(f);
 It should be noted that as modules do not export macros, the `nlohmann.json` module will not export any macros.

 ## Exported symbols
+
 Only the following symbols are exported from `nlohmann.json`:

 - `nlohmann::adl_serializer`
@@ -38,3 +39,21 @@ Only the following symbols are exported from `nlohmann.json`:
 - `nlohmann::to_string`
 - `nlohmann::literals::json_literals::operator""_json`
 - `nlohmann::literals::json_literals::operator""_json_pointer`
+
+Additionally, the following `nlohmann::detail` symbols are exported, solely to work around an MSVC compilation issue
+([#3970](https://github.com/nlohmann/json/issues/3970)). They are implementation details, not part of the public API,
+and should not be used directly:
+
+- `nlohmann::detail::json_sax_dom_callback_parser`
+- `nlohmann::detail::unknown_size`
+
+## Known issues
+
+C++20 modules support is exercised in CI against current GCC and Clang on Ubuntu, and the default MSVC toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md).
+
+!!! info "Known compiler issues"
+
+    - **GCC** may emit "redefinition" errors when `#include <nlohmann/json.hpp>` appears in a module preamble together with other imports. This is an upstream GCC bug, not yet resolved as of GCC 16. Workarounds: include `nlohmann/json.hpp` before other `#include`s, use `import nlohmann.json;` instead, or upgrade GCC. ([issue #5103](https://github.com/nlohmann/json/issues/5103))
+    - **MSVC** could fail with `C2039: 'json_sax_dom_callback_parser' is not a member of ... detail`; fixed by exporting the required internal symbols from `json.cppm` (see [Exported symbols](#exported-symbols) above). ([issue #3970](https://github.com/nlohmann/json/issues/3970))
+
+    If you hit a different module-related build failure, search [existing issues](https://github.com/nlohmann/json/issues?q=is%3Aissue+modules) before filing a new one.
@@ -59,7 +59,7 @@ may complain about undefined references.

 Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and
 recommends that all parts of a codebase exchanging library types be built with the same version. Users can, **at their
-own risk**, disable the version component of the linline namespace, allowing different versions – but not
+own risk**, disable the version component of the inline namespace, allowing different versions – but not
 configurations – to be used in cases where the linker would otherwise output undefined reference errors.

 To do so, define [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](../api/macros/nlohmann_json_namespace_no_version.md) to `1`.
@@ -71,6 +71,7 @@ section to emulate the effect of the `NLOHMANN_JSON_NAMESPACE_NO_VERSION` macro.

    Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect
    behavior. You have been warned!
+
 ## Disabling the inline namespace completely

 When interoperability with code using a pre-3.11.0 version of the library is required, users can, **at their own risk**
@@ -37,7 +37,7 @@ The default type `nlohmann::json` uses a `std::map` to store JSON objects, and t

 ## Alternative behavior: preserve insertion order

-If you do want to preserve the **insertion order**, you can try the type [`nlohmann::ordered_json`](https://github.com/nlohmann/json/issues/2179).
+If you do want to preserve the **insertion order**, you can use the type [`nlohmann::ordered_json`](../api/ordered_json.md).

 ??? example

@@ -103,7 +103,7 @@ Assume file `input.json` contains the JSON object above:
    ```json
    {
      "one": 1,
-      "three": 3
-      "two": 2,
+      "three": 3,
+      "two": 2
    }
    ```
@@ -1,13 +1,61 @@
 # Parsing

-!!! note
-
-    This page is under construction.
+This library can create a JSON value from a wide range of inputs. This page gives an overview of the available parsing
+functions and how they behave; the linked pages go into more detail.

 ## Input

+The [`parse`](../../api/basic_json/parse.md) function reads a JSON value from an input. The input can be
+
+- a string (`#!cpp std::string`, C string, or string literal),
+- a `#!cpp std::istream` (e.g., an `#!cpp std::ifstream` reading from a file),
+- a `#!cpp FILE*` pointer,
+- a pair of iterators over a contiguous range (e.g., a `#!cpp std::vector<std::uint8_t>`), or
+- a contiguous container.
+
+```cpp
+// parse from a string
+json j = json::parse(R"({"happy": true, "pi": 3.141})");
+
+// parse from a file
+std::ifstream f("example.json");
+json data = json::parse(f);
+```
+
+The input must be encoded in UTF-8; other encodings are not supported. A single input may contain only one JSON value.
+Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](json_lines.md) format.
+
+By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the `parse`
+function — see [comments](../comments.md) and [trailing commas](../trailing_commas.md).
+
 ## SAX vs. DOM parsing

+The library offers two parsing models:
+
+- **DOM parsing** (the default): the complete input is read and stored as an in-memory `basic_json` value that can be
+  traversed and modified freely. This is what [`parse`](../../api/basic_json/parse.md) does, and it is the right choice
+  for most use cases.
+- **SAX parsing**: instead of building a value, the parser reports events (such as "a string was read" or "an object
+  started") to a handler that you implement. This avoids building the full value in memory and is useful for very large
+  inputs or when you only need to extract parts of the input. See the [SAX interface](sax_interface.md) for details and
+  [`sax_parse`](../../api/basic_json/sax_parse.md) for the API.
+
+You can influence a DOM parse without switching to the SAX interface by passing a
+[parser callback](parser_callbacks.md), which is called during parsing and can, for example, discard parts of the input.
+
 ## Exceptions

-See [parsing and exceptions](parse_exceptions.md).
+When the input is not valid JSON, the `parse` function throws an exception by default. If exceptions are undesired or
+unavailable, the parser can instead return a discarded value, or [`accept`](../../api/basic_json/accept.md) can be used
+to only check whether an input is valid JSON. See [parsing and exceptions](parse_exceptions.md) for the available
+options.
+
+## See also
+
+- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
+- [`accept`](../../api/basic_json/accept.md) - check if the input is valid JSON
+- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
+- [JSON Lines](json_lines.md) - parse newline-delimited JSON
+- [parser callbacks](parser_callbacks.md) - influence the parsing by a callback function
+- [SAX interface](sax_interface.md) - implement a custom SAX handler
+- [parsing and exceptions](parse_exceptions.md) - control error handling
@@ -47,3 +47,6 @@ JSON Lines input with more than one value is treated as invalid JSON by the [`pa
    ```
    
    with a JSON Lines input does not work, because the parser will try to parse one value after the last one.
+
+    This is different from parsing a stream of *concatenated* (non-newline-delimited) JSON values, for which
+    `operator>>` does work -- see its [notes](../../api/operator_gtgt.md#notes) for details.
@@ -58,6 +58,14 @@ table describes the values of the parameters `depth`, `event`, and `parsed`.
    | `array_end`    | 1     | `#!json [52.519444,13.406667]` |
    | `object_end`   | 0     | `#!json {"location":[52.519444,13.406667],"name":"Berlin"}` |

+!!! note "No built-in nesting depth limit"
+
+    The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only
+    *discard* content it has already parsed (by returning `#!c false`); it cannot make parsing fail once a
+    nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep
+    untrusted input outright, track `depth` in a callback and `throw` from it once your limit is exceeded (a
+    thrown exception propagates out of `parse()` as usual).
+
 ## Return value

 Discarding a value (i.e., returning `#!c false`) has different effects depending on the context in which the function
@@ -81,3 +89,82 @@ was called:
    ```json
    --8<-- "examples/parse__string__parser_callback_t.output"
    ```
+
+## Recipe: rejecting duplicate object keys
+
+The JSON specification leaves the handling of objects with repeated keys up to the implementation. As described in
+[`object_t`](../../api/basic_json/object_t.md#behavior), it is unspecified which value for a repeated key ends up in
+the resulting `#!c json` value -- once parsing has produced that value, the duplicate is already gone, because object
+storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback
+can detect them while the object is still being read, before that ambiguity ever applies.
+
+??? example
+
+    ```cpp
+    --8<-- "examples/reject_duplicate_keys.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/reject_duplicate_keys.output"
+    ```
+
+This approach has two limitations:
+
+- The depth-indexed bookkeeping must account for the fact that `object_start` reports the depth of the *parent* of
+  the object, while the `key` events inside that object are reported one depth deeper (see the event table above);
+  it is easy to get this off by one for nested objects.
+- The thrown exception cannot carry a `parse_error`-style byte offset, because position tracking only exists inside
+  the parser and lexer, not at the callback layer.
+
+For strict validation with precise error positions, implementing a [SAX interface](sax_interface.md) instead gives
+access to the parser's position information directly.
+
+## Recipe: streaming a large homogeneous array
+
+A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in
+memory as a `#!c json` value. A parser callback can hand off each completed element to a user function and then
+discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather
+than the whole document. Since the top-level array's `array_start`/`array_end` are reported at `depth == 0` (its
+parent is the document root), the object elements it contains are reported at `depth == 1`:
+
+??? example
+
+    ```cpp
+    std::ifstream input("large_array.json");
+
+    auto callback = [](int depth, json::parse_event_t event, json& parsed) -> bool {
+        if (depth == 1 && event == json::parse_event_t::object_end) {
+            handle_element(parsed); // process the element, e.g. write it elsewhere
+            return false;           // discard it -- frees its memory before the next one is parsed
+        }
+        return true; // keep everything else, including the (by then empty) top-level array
+    };
+
+    json::parse(input, callback);
+    ```
+
+If the array's elements are scalars or nested arrays instead of objects, check for `parse_event_t::value` or
+`parse_event_t::array_end` at `depth == 1` instead. The same approach works for a top-level *object* of many
+homogeneous values by checking `object_end`/`value` events at `depth == 1` there too.
+
+## Recipe: max nesting depth via a callback
+
+Since there is no built-in nesting-depth limit (see the note above), a callback can enforce one manually by
+tracking the maximum `depth` seen and throwing once it is exceeded:
+
+??? example
+
+    ```cpp
+    constexpr int max_depth = 32;
+
+    auto callback = [](int depth, json::parse_event_t /*event*/, json& /*parsed*/) -> bool {
+        if (depth > max_depth) {
+            throw std::runtime_error("maximum nesting depth exceeded");
+        }
+        return true;
+    };
+
+    json::parse(input, callback);
+    ```
@@ -0,0 +1,129 @@
+# Serialization
+
+Serialization is the process of turning a JSON value back into JSON text. It is the counterpart to
+[parsing](parsing/index.md). The central function is [`dump`](../api/basic_json/dump.md), which returns the JSON text as
+a string.
+
+```cpp
+json j = {{"pi", 3.141}, {"happy", true}};
+
+std::string s = j.dump();   // {"happy":true,"pi":3.141}
+```
+
+To write a value directly to a stream (for example, a file or `#!cpp std::cout`), the
+[`operator<<`](../api/operator_ltlt.md) is provided:
+
+```cpp
+std::cout << j << std::endl;
+```
+
+!!! note "String, not raw value"
+
+    `dump` always returns a **JSON text**. Serializing a JSON string therefore includes the surrounding quotes and
+    escapes special characters. To obtain the *contained* string value without quotes, use
+    [`get<std::string>()`](conversions.md) instead of `dump`. See the [converting values](conversions.md) page.
+
+## Pretty-printing
+
+By default, `dump` produces the most compact representation without any superfluous whitespace. Passing a non-negative
+`indent` argument pretty-prints the output with the given number of spaces per level:
+
+??? example
+
+    ```cpp
+    --8<-- "examples/dump.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/dump.output"
+    ```
+
+The indentation character can be changed with the second argument (e.g., a tab `#!cpp '\t'`). An `indent` of `0` inserts
+newlines but no leading spaces, and the default of `#!cpp -1` selects the compact single-line form.
+
+## Non-ASCII characters
+
+Strings are stored and serialized as UTF-8 (see [types](types/index.md#strings)). By default, `dump` copies valid
+non-ASCII characters as-is. Setting the third argument `ensure_ascii` to `#!cpp true` escapes all non-ASCII characters
+with `\uXXXX` sequences, so that the output contains only ASCII characters:
+
+```cpp
+json j = "苹果";
+j.dump();               // "苹果"
+j.dump(-1, ' ', true);  // "苹果"
+```
+
+## Handling invalid UTF-8
+
+If a string contains invalid UTF-8 sequences (for example, because it holds data in another encoding such as Latin-1),
+serialization fails by default. The fourth argument of `dump` selects an
+[`error_handler`](../api/basic_json/error_handler_t.md):
+
+- `strict` (default) — throw a [`type_error.316`](../home/exceptions.md#jsonexceptiontype_error316) exception.
+- `replace` — replace invalid bytes with the Unicode replacement character U+FFFD (`�`).
+- `ignore` — silently drop invalid bytes.
+
+??? example
+
+    ```cpp
+    --8<-- "examples/error_handler_t.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/error_handler_t.output"
+    ```
+
+!!! tip "Avoiding invalid UTF-8"
+
+    The best fix is to ensure that all strings are UTF-8 encoded before storing them. See the
+    [FAQ on non-ASCII characters](../home/faq.md#parse-errors-reading-non-ascii-characters) for how to convert wide or
+    Latin-1 strings.
+
+## Numbers, NaN, and binary values
+
+- **Numbers** are serialized with enough precision to round-trip; see [number serialization](types/number_handling.md#number-serialization).
+- **NaN and infinity** cannot be represented in JSON and are serialized as `#!json null`; see
+  [NaN handling](types/number_handling.md#nan-handling). The [binary formats](binary_formats/index.md) can preserve
+  them.
+- **Binary values** have no JSON representation and are serialized as a helper object for debugging only; see
+  [binary values](binary_values.md#serialization).
+
+## Using `std::format`, `std::print`, and `fmt`
+
+Since version 3.12.0, JSON values can be formatted directly with C++20's
+[`std::format`](https://en.cppreference.com/w/cpp/utility/format/format) whenever the standard library provides the
+`<format>` header (controlled by [`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md)). This is enabled by the
+[`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) specialization, which also makes JSON values work with
+`std::format_to` and with C++23's `std::print`/`std::println`:
+
+```cpp
+std::print("{}", j);      // compact, like j.dump()
+std::print("{:2}", j);    // pretty-printed with indent 2 (like j.dump(2))
+std::println("{:#}", j);  // pretty-printed with the default indent
+```
+
+The format spec mirrors the `dump` parameters: `#!cpp "{:#}"` pretty-prints, a width such as `#!cpp "{:2}"` sets the
+indent, and a fill-and-align prefix such as `#!cpp "{:.>#}"` sets the indent character.
+
+For the [{fmt}](https://github.com/fmtlib/fmt) library, the library ships a
+[`format_as`](../api/basic_json/format_as.md) helper. Note its behavior depends on the `fmt` version; see the
+[FAQ entry](../home/faq.md#using-json-values-with-stdformat-or-fmt) for the details and a recipe for a full
+`fmt::formatter` specialization.
+
+## Serializing to other formats
+
+Besides JSON text, a value can also be serialized to the more compact [binary formats](binary_formats/index.md)
+(BJData, BSON, CBOR, MessagePack, UBJSON).
+
+## See also
+
+- [`dump`](../api/basic_json/dump.md) - serialize to a JSON-formatted string
+- [`operator<<`](../api/operator_ltlt.md) - serialize to a stream
+- [`to_string`](../api/basic_json/to_string.md) - user-defined-conversion helper
+- [`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) - use JSON values with `std::format` and `std::print`
+- [`format_as`](../api/basic_json/format_as.md) - use JSON values with the {fmt} library
+- [Parsing](parsing/index.md) - the reverse operation
@@ -131,13 +131,13 @@ std::map<
 The choice of `object_t` influences the behavior of the JSON class. With the default type, objects have the following behavior:

 - When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.
- When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, `#!json {"key": 2, "key": 1}` could be equal to either `#!json {"key": 1}` or `#!json {"key": 2}`.
+- When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, `#!json {"key": 2, "key": 1}` could be equal to either `#!json {"key": 1}` or `#!json {"key": 2}`. To reject duplicate keys instead of silently resolving them one way or another, see [this parsing recipe](../parsing/parser_callbacks.md#recipe-rejecting-duplicate-object-keys).
 - Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see `dump`) in this order. For instance, both `#!json {"b": 1, "a": 2}` and `#!json {"a": 2, "b": 1}` will be stored and serialized as `#!json {"a": 2, "b": 1}`.
 - When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, `#!json {"b": 1, "a": 2}` and `#!json {"a": 2, "b": 1}` will be treated as equal.

 ### Key order

-The order name/value pairs are added to the object are *not* preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as `std::map` with `std::less` is used by default. Please note this behavior conforms to [RFC 8259](https://tools.ietf.org/html/rfc8259), because any order implements the specified "unordered" nature of JSON objects.
+The order in which name/value pairs are added to the object is *not* preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as `std::map` with `std::less` is used by default. Please note this behavior conforms to [RFC 8259](https://tools.ietf.org/html/rfc8259), because any order implements the specified "unordered" nature of JSON objects.

 ### Limits

@@ -151,6 +151,18 @@ In this class, the object's limit of nesting is not explicitly constrained. Howe

 Objects are stored as pointers in a `basic_json` type. That is, for any access to object values, a pointer of type `object_t*` must be dereferenced.

+### Converting maps with non-string keys
+
+A `std::map`/`std::unordered_map` whose key type is not string-like (e.g., `std::map<int, std::string>`) is
+converted to a JSON *array* of 2-element `[key, value]` arrays rather than a JSON object, because JSON object
+keys must be strings:
+
+```cpp
+std::map<int, std::string> m{{1, "one"}, {2, "two"}};
+json j = m;
+// j is [[1,"one"],[2,"two"]], not {"1":"one","2":"two"}
+```
+

 ## Arrays

@@ -63,6 +63,10 @@ In the default [`json`](../../api/json.md) type, numbers are stored as `#!c std:
 number without loss of precision. If this is impossible (e.g., if the number is too large), the number is stored as
 `#!c double`.

+Positive integers are stored as `#!c std::uint64_t`, while negative integers are stored as `#!c std::int64_t`. This
+distinction is determined at parse time: if the JSON number has a leading minus sign, it uses signed integer storage;
+otherwise, it uses unsigned integer storage.
+
 !!! info "Notes"

    - Numbers with a decimal digit or scientific notation are always stored as `#!c double`.
@@ -91,7 +95,7 @@ This is the same behavior as the code `#!c double x = 3.141592653589793238462643

 !!! success "Interoperability"

-    - The library interoperable with respect to the specification, because its supported range $[-2^{63}, 2^{64}-1]$ is
+    - The library is interoperable with respect to the specification, because its supported range $[-2^{63}, 2^{64}-1]$ is
      larger than the described range $[-2^{53}+1, 2^{53}-1]$.
    - All integers outside the range $[-2^{63}, 2^{64}-1]$, as well as floating-point numbers are stored as `double`.
      This also concurs with the specification above.
@@ -153,7 +157,7 @@ NaN (not-a-number) cannot be expressed with the number syntax described above an

 That is, there is no way to *parse* a NaN value. However, assignments can store NaN values in a JSON value.

-This library serializes NaN values  as `#!js null`. This corresponds to the behavior of JavaScript's
+This library serializes NaN values as `#!js null`. This corresponds to the behavior of JavaScript's
 [`JSON.stringify`](https://www.w3schools.com/js/js_json_stringify.asp) function.

 !!! example
@@ -182,7 +186,7 @@ This library serializes NaN values  as `#!js null`. This corresponds to the beha

 ### Number comparison

-Floating-point inside JSON values numbers are compared with `#!c json::number_float_t::operator==` which is
+Floating-point numbers inside JSON values are compared with `#!c json::number_float_t::operator==` which is
 `#!c double::operator==` by default.

 !!! example "Alternative comparison functions"
@@ -203,8 +207,8 @@ Floating-point inside JSON values numbers are compared with `#!c json::number_fl
    ```cpp
    bool my_equal(const_reference lhs, const_reference rhs)
    {
-        const auto lhs_type lhs.type();
-        const auto rhs_type rhs.type();
+        const auto lhs_type = lhs.type();
+        const auto rhs_type = rhs.type();
        if (lhs_type == rhs_type)
        {
            switch(lhs_type)
@@ -230,7 +234,7 @@ Floating-point inside JSON values numbers are compared with `#!c json::number_fl
 ### Number conversion

 Just like the C++ language itself, the `get` family of functions allows conversions between unsigned and signed
-integers, and between integers and floating-point values to integers. This behavior may be surprising.
+integers, and between integers and floating-point values. This behavior may be surprising.

 !!! warning "Unconditional number conversions"

@@ -0,0 +1,31 @@
+# Debugging
+
+This page collects the library's built-in debugger integrations and other debugging-related features. They are
+not linked from a single place elsewhere in the docs, so are collected here.
+
+## Visual Studio (natvis)
+
+The repository ships [`nlohmann_json.natvis`](https://github.com/nlohmann/json/blob/develop/nlohmann_json.natvis)
+at its root, a [Natvis](https://learn.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects)
+file that gives `json`/`ordered_json` values a friendly, key/value debugger view instead of showing raw internal
+fields, when debugging with the MSVC debug engine (`cppvsdbg`) in Visual Studio or VS Code.
+
+Debug engines that wrap LLDB instead of the MSVC debug engine (for example, `codelldb` in VS Code) only have
+partial/experimental Natvis support, and commonly fall back to showing raw internal fields even with the
+`.natvis` file present. Switching to `cppvsdbg` where available, or checking your debug extension's own Natvis
+support/version, are the next things to try if this happens. There is currently no bundled LLDB-native
+pretty-printer script in this repository.
+
+## GDB
+
+The repository ships a [GDB Python pretty printer](https://github.com/nlohmann/json/tree/develop/tools/gdb_pretty_printer)
+under `tools/gdb_pretty_printer`, with its own usage instructions in that directory's `README.md`.
+
+## Extended exception diagnostics
+
+Defining [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) before including the library augments
+`type_error`/`out_of_range`-style exceptions with a JSON Pointer to the offending value, which can help pinpoint
+where in a large document a runtime error occurred. This only applies to exceptions thrown *after* a value
+exists (e.g. during element access); parse errors, which happen before any value exists to point at, are not
+covered by this mechanism -- see [Parsing and exceptions](../features/parsing/parse_exceptions.md) for how parse
+errors report their own location instead.
@@ -326,6 +326,9 @@ An unexpected byte was read in a [binary format](../features/binary_formats/inde
    ```
    [json.exception.parse_error.112] parse error at byte 15: syntax error while parsing BSON binary: byte array length cannot be negative, is -1
    ```
+    ```
+    [json.exception.parse_error.112] parse error at byte 9: syntax error while parsing CBOR value: negative integer overflow
+    ```

 ### json.exception.parse_error.113

@@ -893,7 +896,7 @@ A JSON Patch `add` operation cannot be applied because the target location's par

 !!! note

-    This exception was added in version 3.12.x. Before that, this situation hit an internal assertion (aborting the program in debug builds) or was silently ignored when assertions were disabled.
+    This exception was added in version 3.13.0. Before that, this situation hit an internal assertion (aborting the program in debug builds) or was silently ignored when assertions were disabled.

 ## Further exceptions

--- a/Show More
+++ b/Show More