Document the macros that preview version 4.0 in the roadmap

List the macros that guard breaking changes planned to become the default in version 4.0.0, and explain that 4.0.0 will be the sum of these opt-in flags, which can be tried on the 3.x release train. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-09-27 18:20:32 +00:00 · 2026-09-27 16:39:52 +02:00
parent f682cd2ef1
commit a164f926c5
3 changed files with 49 additions and 5 deletions
@@ -115,3 +115,4 @@ The default value is `0` (disabled — existing behavior is preserved).
 ## Version history

 - Added in version 3.13.0.
+- Planned to become the default (with the macro removed) in version 4.0.0.
@@ -14,7 +14,7 @@ work items are tracked in the [GitHub milestones](https://github.com/nlohmann/js
  opt-in.
 - **Keep the 3.x public API stable.** Releases follow [semantic versioning](https://semver.org). Changes that would
  break existing code are only added behind a feature macro, so users can opt in and test their code before a next
-  major release.
+  major release, see [Version 4.0](#version-40).
 - **Support a broad range of compilers and platforms.** The [CI](quality_assurance.md) keeps testing old and new
  versions of GCC, Clang, MSVC, and other compilers on Linux, macOS, and Windows.
 - **Keep the quality assurance up.** Every change keeps the test coverage at 100%, passes the static and dynamic
@@ -37,7 +37,48 @@ work items are tracked in the [GitHub milestones](https://github.com/nlohmann/js

 ## Version 4.0

-There is no decision yet on whether or when a version 4.0 with breaking changes will be released. Proposals that need
-a major version, for instance stricter type conversions, are collected in issue
-[#3453](https://github.com/nlohmann/json/issues/3453). Until then, such changes are only added as opt-in behavior
-behind feature macros.
+There is no release date for version 4.0 yet. Proposals that need a major version, for instance stricter type
+conversions, are collected in issue [#3453](https://github.com/nlohmann/json/issues/3453).
+
+!!! note "Not final"
+
+    The plan for version 4.0 described below is not final and may still change: macros may be added to or removed from
+    the list, and planned defaults may be revised. Any such change will be documented on this page.
+
+### Trying out 4.0 today
+
+Version 4.0 will not be developed on a separate branch. Instead, every breaking change is first added to a 3.x release
+behind a macro whose default keeps the 3.x behavior. Version 4.0 then switches the defaults and removes the macros.
+Version 4.0 is therefore the sum of these macros: you can try it on the 3.x release train today by defining each macro
+to its 4.0 value and fixing what no longer compiles or behaves differently. Once your code works with all of them, it
+is ready for version 4.0.
+
+The following macros guard changes that are planned to become the default in version 4.0:
+
+| Macro                                                                                                            | 3.x default | 4.0 behavior                                                                                                                  | CMake option                                                                                                       | Added  |
+|------------------------------------------------------------------------------------------------------------------|-------------|-------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------|--------|
+| [`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md)                                | `1`         | `0`: no implicit conversions from `basic_json` to other types; use [`get`](../api/basic_json/get.md) instead                  | [`JSON_ImplicitConversions`](../integration/cmake.md#json_implicitconversions)                                     | 3.9.0  |
+| [`JSON_USE_GLOBAL_UDLS`](../api/macros/json_use_global_udls.md)                                                  | `1`         | `0`: the string literals `_json` and `_json_pointer` are only available in namespace `nlohmann::literals`                    | [`JSON_GlobalUDLs`](../integration/cmake.md#json_globaludls)                                                       | 3.11.0 |
+| [`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../api/macros/json_use_legacy_discarded_value_comparison.md)      | `0`         | removed: the deprecated legacy comparison of discarded values can no longer be enabled                                        | [`JSON_LegacyDiscardedValueComparison`](../integration/cmake.md#json_legacydiscardedvaluecomparison)               | 3.11.0 |
+| [`JSON_BRACE_INIT_COPY_SEMANTICS`](../api/macros/json_brace_init_copy_semantics.md)                              | `0`         | `1`: single-element brace initialization such as `#!cpp json j{obj};` copies the element instead of creating an array         | –                                                                                                                  | 3.13.0 |
+| [`JSON_PRECISE_STREAM_POSITION`](../api/macros/json_precise_stream_position.md)                                  | `0`         | `1`: reading from a stream does not consume the character after a number                                                      | –                                                                                                                  | 3.13.0 |
+| [`JSON_STRICT_NUL_HANDLING`](../api/macros/json_strict_nul_handling.md)                                          | `0`         | `1`: a NUL byte in the input is a parse error instead of the end of input                                                     | [`JSON_StrictNulHandling`](../integration/cmake.md#json_strictnulhandling)                                         | 3.13.0 |
+
+For example, the following makes a 3.x release behave like version 4.0 with respect to these changes:
+
+```cpp
+#define JSON_USE_IMPLICIT_CONVERSIONS 0
+#define JSON_USE_GLOBAL_UDLS 0
+#define JSON_BRACE_INIT_COPY_SEMANTICS 1
+#define JSON_PRECISE_STREAM_POSITION 1
+#define JSON_STRICT_NUL_HANDLING 1
+#include <nlohmann/json.hpp>
+```
+
+The macros must be defined before the library header is included; setting them once in the build system is the easiest
+way to achieve this. In addition,
+version 4.0 will remove all [deprecated functions](../integration/migration_guide.md#replace-deprecated-functions).
+Compiling with deprecation warnings enabled shows which of them your code still uses. The
+[migration guide](../integration/migration_guide.md) describes how to replace them.
+
+New breaking changes will follow the same path: they are added to this table when they land in a 3.x release.
@@ -1,6 +1,8 @@
 # Migration Guide

 This page collects some guidelines on how to future-proof your code for future versions of this library.
+The [roadmap](../community/roadmap.md#trying-out-40-today) lists all macros that let you try the behavior of version
+4.0 with a 3.x release.

 ## Replace deprecated functions