Merge branch 'json-view/13-view-dump' into json-view/14-view-compare

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

BUILD.bazel

@@ -67,6 +67,7 @@ cc_library(
         "include/nlohmann/detail/string_utils.hpp",
         "include/nlohmann/detail/value_t.hpp",
         "include/nlohmann/detail/view/builder.hpp",
+        "include/nlohmann/detail/view/compare.hpp",
         "include/nlohmann/detail/view/document_data.hpp",
         "include/nlohmann/detail/view/errors.hpp",
         "include/nlohmann/detail/view/input.hpp",

docs/docset/docSet.sql

@@ -178,6 +178,8 @@ INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::number_token
 INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::operator bool', 'Method', 'api/basic_json_view/operator_bool/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::operator<<', 'Operator', 'api/basic_json_view/operator_ltlt/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::operator[]', 'Operator', 'api/basic_json_view/operator[]/index.html');
+INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::operator==', 'Operator', 'api/basic_json_view/operator_eq/index.html');
+INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::operator!=', 'Operator', 'api/basic_json_view/operator_ne/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::size', 'Method', 'api/basic_json_view/size/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::source_offset', 'Method', 'api/basic_json_view/source_offset/index.html');
 INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::type', 'Method', 'api/basic_json_view/type/index.html');

docs/mkdocs/docs/api/basic_json/operator_eq.md

@@ -166,6 +166,8 @@ Linear.
 
 - [operator!=](operator_ne.md) compare for inequality
 - [operator<=>](operator_spaceship.md) comparison: 3-way (C++20)
+- [basic_json_view::operator==](../basic_json_view/operator_eq.md) - the same comparison on a zero-copy view, without
+  building a `basic_json` value for it
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/operator_ne.md

@@ -89,6 +89,12 @@ Linear.
     --8<-- "examples/operator__notequal__nullptr_t.output"
     ```
 
+## See also
+
+- [operator==](operator_eq.md) compare for equality
+- [basic_json_view::operator!=](../basic_json_view/operator_ne.md) - the same comparison on a zero-copy view, without
+  building a `basic_json` value for it
+
 ## Version history
 
 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 

docs/mkdocs/docs/api/basic_json_view/index.md

@@ -20,11 +20,12 @@ Moving the document itself does not invalidate its views: the index is heap-allo
 `basic_json_document` object.
 
 `basic_json_view` provides the read-only part of the `BasicJsonType` interface: the type-inspection functions, element
-access, lookup, iteration, and conversion -- [`get<T>()`](get.md), [`get_string()`](get_string.md),
+access, lookup, iteration, conversion, and comparison -- [`get<T>()`](get.md), [`get_string()`](get_string.md),
 [`number_token()`](number_token.md), and [`materialize()`](materialize.md) to build the `BasicJsonType` value of a
 subtree on demand. [`operator[]`](operator%5B%5D.md), [`at`](at.md), [`contains`](contains.md), and
-[`value`](value.md) also accept a [`json_pointer`](../json_pointer/index.md). It does not (yet) provide
-comparison.
+[`value`](value.md) also accept a [`json_pointer`](../json_pointer/index.md). [`operator==`](operator_eq.md) and
+[`operator!=`](operator_ne.md) compare two views, or a view and a `BasicJsonType` value, without ever building a
+`BasicJsonType` value for a view; no ordering comparison (`#!cpp operator<`) is provided.
 
 ## Template parameters
 
@@ -107,6 +108,11 @@ comparison.
 - [**number_token**](number_token.md) - get a number's token text without a copy
 - [**materialize**](materialize.md) - build the `BasicJsonType` value of this subtree
 
+### Comparison
+
+- [**operator==**](operator_eq.md) - comparison: equal
+- [**operator!=**](operator_ne.md) - comparison: not equal
+
 ### Serialization
 
 - [**dump**](dump.md) - serialize to a JSON-formatted string

docs/mkdocs/docs/api/basic_json_view/operator_eq.md

@@ -0,0 +1,107 @@
+# <small>nlohmann::basic_json_view::</small>operator==
+
+```cpp
+// (1)
+bool operator==(const basic_json_view& lhs, const basic_json_view& rhs);
+
+// (2)
+bool operator==(const basic_json_view& lhs, const BasicJsonType& rhs);
+bool operator==(const BasicJsonType& lhs, const basic_json_view& rhs);
+```
+
+1. Compares two views for equality: whether the values [`BasicJsonType::parse()`](../basic_json/parse.md) would
+   produce for `lhs` and `rhs` are equal, according to `BasicJsonType`'s [`operator==`](../basic_json/operator_eq.md).
+2. Compares a view and a `BasicJsonType` value for equality, in either order: whether the value `parse()` would
+   produce for the view and the other operand are equal, according to `BasicJsonType`'s
+   [`operator==`](../basic_json/operator_eq.md).
+
+Neither overload builds a `BasicJsonType` value for a view to do the comparison (see [Notes](#notes) below). Numbers
+compare by value across their types (`#!cpp 1 == 1.0`), and an object compares by its members, with duplicate keys
+resolved exactly as `parse()` resolves them -- the last value, at the position of the first occurrence of the key.
+
+## Parameters
+
+`lhs` (in)
+:   first value to consider
+
+`rhs` (in)
+:   second value to consider
+
+## Return value
+
+whether the values `lhs` and `rhs` are equal
+
+## Exception safety
+
+Strong exception safety: if an exception is thrown, there are no changes to either operand, or to the document(s) a
+view refers to.
+
+## Exceptions
+
+May throw `#!cpp std::bad_alloc`. Unlike the other comparison and most other `basic_json_view` functions,
+`operator==` is not `#!cpp noexcept`: resolving an object's members needs a temporary array to sort them by key (see
+[Complexity](#complexity) below), and that allocation can fail.
+
+## Complexity
+
+Linear in the size of the compared values: every number, string, array element, and object member is visited at most
+once, and the walk is iterative, so the nesting depth it can compare is limited by available memory only, not by the
+call stack (as for [`materialize()`](materialize.md)). Resolving an object's members takes an additional O(n log n)
+in the number of members at that level, since they are sorted by key to detect and resolve duplicates before being
+compared. Two arrays of different [`size()`](size.md) are rejected without visiting either one's elements.
+
+## Notes
+
+Only a single number, boolean, or `#!cpp null` value is ever materialized into a `BasicJsonType`, to reuse its
+`operator==` -- for numbers, so that values written differently in the source text but equal in value (e.g. an
+integer and a floating-point literal) still compare equal, following the same rules `BasicJsonType` does for special
+values such as `#!cpp NaN`. Constructing one of these scalars never allocates. Strings are compared directly, without
+allocating, either from the source text on both sides or, for overload 2, against `BasicJsonType`'s own string.
+Arrays and objects are never materialized at all; only their elements or members are visited, one pair at a time.
+
+!!! info "How objects are compared"
+
+    For a [`json_view`](../json_view.md) (`BasicJsonType::object_t` is `#!cpp std::map`), members are compared by
+    key, regardless of the order they appear in the source text. For an
+    [`ordered_json_view`](../ordered_json_view.md) (`object_t` is `ordered_map`), they are compared in the order
+    they occur, so the very same two objects with their members reordered can compare equal as `json_view`s but not
+    as `ordered_json_view`s. This is exactly how [`json`](../json.md) and [`ordered_json`](../ordered_json.md)
+    compare, see ["Comparing different `basic_json` specializations"](../basic_json/operator_eq.md#notes).
+
+!!! info "Discarded views"
+
+    A [discarded](is_discarded.md) view compares the same way a discarded `BasicJsonType` value does, which is
+    governed by
+    [`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../macros/json_use_legacy_discarded_value_comparison.md): by
+    default, a discarded view is never equal to anything, not even another discarded view.
+
+No ordering comparison (`#!cpp operator<`) is provided for `basic_json_view`; [`materialize()`](materialize.md) is
+the way to get a `BasicJsonType` value that supports it.
+
+## Examples
+
+??? example
+
+    The example below checks whether a newly received configuration differs from the previous one, and whether a
+    received document matches what a test expects -- directly on views, without ever materializing a `BasicJsonType`
+    value for either side.
+
+    ```cpp
+    --8<-- "examples/basic_json_view__operator_eq.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_view__operator_eq.output"
+    ```
+
+## See also
+
+- [operator!=](operator_ne.md) - compare for inequality
+- [materialize](materialize.md) - build a `BasicJsonType` value, e.g. to keep comparing after the document is gone
+- [`BasicJsonType::operator==`](../basic_json/operator_eq.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.

docs/mkdocs/docs/api/basic_json_view/operator_ne.md

@@ -0,0 +1,82 @@
+# <small>nlohmann::basic_json_view::</small>operator!=
+
+```cpp
+// (1)
+bool operator!=(const basic_json_view& lhs, const basic_json_view& rhs);
+
+// (2)
+bool operator!=(const basic_json_view& lhs, const BasicJsonType& rhs);
+bool operator!=(const BasicJsonType& lhs, const basic_json_view& rhs);
+```
+
+1. Compares two views for inequality. Returns `#!cpp !(lhs == rhs)`, see [operator==](operator_eq.md).
+2. Compares a view and a `BasicJsonType` value for inequality, in either order. Returns `#!cpp !(lhs == rhs)` (or,
+   for the reversed order, `#!cpp !(rhs == lhs)`), see [operator==](operator_eq.md).
+
+Since `operator!=` is defined as the negation of [`operator==`](operator_eq.md), it follows the same rules for
+special cases: for instance, since a [discarded](is_discarded.md) view is never equal to anything by default (see
+[operator=='s Notes](operator_eq.md#notes)), it is never *unequal* to anything either -- `#!cpp discarded != discarded`
+is also `#!cpp false`, exactly as for a discarded `BasicJsonType` value.
+
+## Parameters
+
+`lhs` (in)
+:   first value to consider
+
+`rhs` (in)
+:   second value to consider
+
+## Return value
+
+whether the values `lhs` and `rhs` are not equal
+
+## Exception safety
+
+Strong exception safety: if an exception is thrown, there are no changes to either operand, or to the document(s) a
+view refers to.
+
+## Exceptions
+
+May throw `#!cpp std::bad_alloc`, propagated from [`operator==`](operator_eq.md#exceptions). Unlike most other
+`basic_json_view` functions, `operator!=` is not `#!cpp noexcept`.
+
+## Complexity
+
+Linear, as [`operator==`](operator_eq.md#complexity).
+
+## Notes
+
+See the [Notes](operator_eq.md#notes) of `operator==` -- in particular for how an object's members are compared
+(order matters for [`ordered_json_view`](../ordered_json_view.md) but not for [`json_view`](../json_view.md)) and
+for how discarded views compare.
+
+No ordering comparison (`#!cpp operator<`) is provided for `basic_json_view`; [`materialize()`](materialize.md) is
+the way to get a `BasicJsonType` value that supports it.
+
+## Examples
+
+??? example
+
+    The example below asserts, as a test would, that a received document differs from an unwanted value, and shows
+    that -- as for [`json`](../json.md)/[`ordered_json`](../ordered_json.md) -- reordering an object's members is
+    detected as a difference for an `ordered_json_view` but not for a `json_view`.
+
+    ```cpp
+    --8<-- "examples/basic_json_view__operator_ne.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_view__operator_ne.output"
+    ```
+
+## See also
+
+- [operator==](operator_eq.md) - compare for equality
+- [materialize](materialize.md) - build a `BasicJsonType` value, e.g. to keep comparing after the document is gone
+- [`BasicJsonType::operator!=`](../basic_json/operator_ne.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.

docs/mkdocs/docs/examples/basic_json_view__operator_eq.cpp

@@ -0,0 +1,30 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+using json = nlohmann::json;
+
+int main()
+{
+    // two snapshots of a polled configuration endpoint -- compare them
+    // directly as views, without ever building a nlohmann::json value for
+    // either one
+    const json_document previous = json_document::parse(
+                                       R"({"name": "cache", "port": 6379, "timeout": 30})");
+    const json_document current = json_document::parse(
+                                      R"({"port": 6379.0, "timeout": 30, "name": "cache"})");
+
+    // same members, reordered, and 6379 written as a float -- operator==
+    // treats them the same way BasicJsonType::operator== would
+    std::cout << std::boolalpha << (previous.root() == current.root()) << '\n';
+
+    // an actually changed value is detected the same way
+    const json_document changed = json_document::parse(
+                                      R"({"name": "cache", "port": 6380, "timeout": 30})");
+    std::cout << (previous.root() == changed.root()) << '\n';
+
+    // comparing a view directly against an expected json value -- handy in a
+    // test, without materializing the received document at all
+    const json expected = {{"name", "cache"}, {"port", 6379}, {"timeout", 30}};
+    std::cout << (previous.root() == expected) << '\n';
+}

docs/mkdocs/docs/examples/basic_json_view__operator_eq.output

@@ -0,0 +1,3 @@
+true
+false
+true

docs/mkdocs/docs/examples/basic_json_view__operator_ne.cpp

@@ -0,0 +1,28 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+using ordered_json_document = nlohmann::ordered_json_document;
+using json = nlohmann::json;
+
+int main()
+{
+    // assert, as a test would, that a received document differs from an
+    // unwanted shape -- without ever materializing it into a json value just
+    // to compare
+    const json_document received = json_document::parse(
+                                       R"({"status": "ok", "code": 200})");
+    const json unwanted = {{"status", "error"}, {"code", 500}};
+    std::cout << std::boolalpha << (received.root() != unwanted) << '\n';
+
+    // json (std::map) compares object members regardless of order ...
+    const json_document a = json_document::parse(R"({"a": 1, "b": 2})");
+    const json_document b = json_document::parse(R"({"b": 2, "a": 1})");
+    std::cout << (a.root() != b.root()) << '\n';
+
+    // ... but ordered_json (ordered_map) compares them in the order they
+    // appear, so the very same reordering is detected as a difference
+    const ordered_json_document oa = ordered_json_document::parse(R"({"a": 1, "b": 2})");
+    const ordered_json_document ob = ordered_json_document::parse(R"({"b": 2, "a": 1})");
+    std::cout << (oa.root() != ob.root()) << '\n';
+}

docs/mkdocs/docs/examples/basic_json_view__operator_ne.output

@@ -0,0 +1,3 @@
+true
+false
+true

docs/mkdocs/docs/features/json_view.md

@@ -139,7 +139,11 @@ whenever any of the other conditions above was not met.
   element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the
   view has no `basic_json` value to point at, so the exception is created without one, regardless of how
   `BasicJsonType` was built.
-- **Comparison is not (yet) provided** by `basic_json_view`. For now,
+- **Ordering comparisons are not provided** by `basic_json_view` -- there is no `#!cpp operator<`.
+  [`operator==`](../api/basic_json_view/operator_eq.md) and [`operator!=`](../api/basic_json_view/operator_ne.md) are
+  provided, though: two views, or a view and a `BasicJsonType` value, compare equal exactly when
+  [`materialize()`](../api/basic_json_view/materialize.md) or [`parse()`](../api/basic_json/parse.md) would produce
+  equal values for them, without ever building a tree to do it. For ordering, too,
   [`materialize()`](../api/basic_json_view/materialize.md) is the way to get a value you can compare.
 
 ## Getting values out without copying

docs/mkdocs/mkdocs.yml

@@ -281,6 +281,8 @@ nav:
           - 'operator bool': api/basic_json_view/operator_bool.md
           - 'operator<<': api/basic_json_view/operator_ltlt.md
           - 'operator[]': api/basic_json_view/operator[].md
+          - 'operator==': api/basic_json_view/operator_eq.md
+          - 'operator!=': api/basic_json_view/operator_ne.md
           - 'size': api/basic_json_view/size.md
           - 'source_offset': api/basic_json_view/source_offset.md
           - 'type': api/basic_json_view/type.md

include/nlohmann/detail/view/compare.hpp

@@ -0,0 +1,317 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <algorithm> // sort, stable_sort
+#include <cstddef> // size_t
+#include <string> // string
+#include <utility> // move, pair
+#include <vector> // vector
+
+#include <nlohmann/json.hpp>
+#include <nlohmann/detail/view/macro_scope.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+namespace view
+{
+
+// Equality of views, and of views with basic_json values, with the semantics
+// of basic_json's operator== applied to the values parse() would produce:
+// numbers compare by value across their types, an object is compared by its
+// members with duplicate keys resolved as parse() resolves them (the last
+// value, at the position of the first occurrence), and in document order if
+// the object type keeps an order (ordered_json), by key otherwise.
+
+/// one side of a comparison: a view
+template<typename BasicJsonType, typename View>
+class view_side
+{
+  public:
+    using string_view_t = typename View::string_view_t;
+
+    explicit view_side(const View& v) noexcept
+        : m_view(v)
+    {}
+
+    value_t type() const noexcept
+    {
+        return m_view.type();
+    }
+
+    std::size_t size() const noexcept
+    {
+        return m_view.size();
+    }
+
+    string_view_t string() const
+    {
+        return m_view.get_string();
+    }
+
+    /// a number, boolean, or null as a basic_json value (no allocation)
+    BasicJsonType scalar() const
+    {
+        switch (m_view.type())
+        {
+            case value_t::number_integer:
+                return BasicJsonType(m_view.template get<typename BasicJsonType::number_integer_t>());
+            case value_t::number_unsigned:
+                return BasicJsonType(m_view.template get<typename BasicJsonType::number_unsigned_t>());
+            case value_t::number_float:
+                return BasicJsonType(m_view.template get<typename BasicJsonType::number_float_t>());
+            case value_t::boolean:
+                return BasicJsonType(m_view.template get<bool>());
+            case value_t::null:
+            case value_t::object:
+            case value_t::array:
+            case value_t::string:
+            case value_t::binary:
+            case value_t::discarded:
+            default:
+                return BasicJsonType(nullptr);
+        }
+    }
+
+    void elements(std::vector<view_side>& out) const
+    {
+        out.reserve(m_view.size());
+        for (const View e : m_view)
+        {
+            out.emplace_back(e);
+        }
+    }
+
+    /// the members as parse() keeps them: one per key, the last value at the
+    /// position of the first occurrence; in that order, or sorted by key
+    void members(std::vector<std::pair<string_view_t, view_side>>& out, bool ordered) const
+    {
+        struct member
+        {
+            string_view_t key;
+            View value;
+            std::size_t position;
+        };
+        std::vector<member> all;
+        all.reserve(m_view.size());
+        std::size_t position = 0;
+        for (auto it = m_view.begin(); it != m_view.end(); ++it)
+        {
+            all.push_back(member{it.key(), it.value(), position++});
+        }
+        std::stable_sort(all.begin(), all.end(), [](const member & a, const member & b)
+        {
+            return a.key < b.key;
+        });
+        std::vector<member> unique;
+        unique.reserve(all.size());
+        for (std::size_t i = 0; i < all.size();)
+        {
+            std::size_t last = i;
+            while (last + 1 < all.size() && all[last + 1].key == all[i].key)
+            {
+                ++last;
+            }
+            unique.push_back(member{all[i].key, all[last].value, all[i].position});
+            i = last + 1;
+        }
+        if (ordered)
+        {
+            std::sort(unique.begin(), unique.end(), [](const member & a, const member & b)
+            {
+                return a.position < b.position;
+            });
+        }
+        out.reserve(unique.size());
+        for (const member& m : unique)
+        {
+            out.emplace_back(m.key, view_side(m.value));
+        }
+    }
+
+  private:
+    View m_view;
+};
+
+/// the other side of a comparison: a basic_json value
+template<typename BasicJsonType, typename StringView>
+class json_side
+{
+  public:
+    using string_view_t = StringView;
+
+    explicit json_side(const BasicJsonType& j) noexcept
+        : m_json(&j)
+    {}
+
+    value_t type() const noexcept
+    {
+        return m_json->type();
+    }
+
+    std::size_t size() const noexcept
+    {
+        return m_json->size();
+    }
+
+    string_view_t string() const
+    {
+        const auto& s = m_json->template get_ref<const typename BasicJsonType::string_t&>();
+        return string_view_t(s.data(), s.size());
+    }
+
+    BasicJsonType scalar() const
+    {
+        return *m_json;
+    }
+
+    void elements(std::vector<json_side>& out) const
+    {
+        out.reserve(m_json->size());
+        for (const auto& e : *m_json)
+        {
+            out.emplace_back(e);
+        }
+    }
+
+    void members(std::vector<std::pair<string_view_t, json_side>>& out, bool ordered) const
+    {
+        out.reserve(m_json->size());
+        for (auto it = m_json->cbegin(); it != m_json->cend(); ++it)
+        {
+            out.emplace_back(string_view_t(it.key().data(), it.key().size()), json_side(it.value()));
+        }
+        if (!ordered)
+        {
+            std::sort(out.begin(), out.end(), [](const std::pair<string_view_t, json_side>& a, const std::pair<string_view_t, json_side>& b)
+            {
+                return a.first < b.first;
+            });
+        }
+    }
+
+  private:
+    const BasicJsonType* m_json;
+};
+
+/// whether two sides are equal; iterative, so that the nesting depth is
+/// limited by memory only
+template<typename BasicJsonType, typename A, typename B>
+bool equal(const A& a0, const B& b0)
+{
+    using string_view_t = typename A::string_view_t;
+    const bool ordered = is_ordered_map<typename BasicJsonType::object_t>::value;
+
+    struct frame
+    {
+        std::vector<A> elements_a{};
+        std::vector<B> elements_b{};
+        std::vector<std::pair<string_view_t, A>> members_a{};
+        std::vector<std::pair<string_view_t, B>> members_b{};
+        bool object = false;
+        std::size_t next = 0;
+    };
+    std::vector<frame> stack;
+    A a = a0;
+    B b = b0;
+    for (;;)
+    {
+        const value_t ta = a.type();
+        const value_t tb = b.type();
+        const bool numbers = (ta == value_t::number_integer || ta == value_t::number_unsigned || ta == value_t::number_float)
+                             && (tb == value_t::number_integer || tb == value_t::number_unsigned || tb == value_t::number_float);
+        if (ta == value_t::discarded || tb == value_t::discarded)
+        {
+            // basic_json decides (JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON)
+            if (ta != tb || !(BasicJsonType(value_t::discarded) == BasicJsonType(value_t::discarded)))
+            {
+                return false;
+            }
+        }
+        else
+        {
+            if (!numbers && ta != tb)
+            {
+                return false;
+            }
+            if (ta == value_t::string)
+            {
+                if (!(a.string() == b.string()))
+                {
+                    return false;
+                }
+            }
+            else if (ta == value_t::array || ta == value_t::object)
+            {
+                if (a.size() != b.size() && ta == value_t::array)
+                {
+                    return false;
+                }
+                frame f;
+                f.object = ta == value_t::object;
+                if (f.object)
+                {
+                    a.members(f.members_a, ordered);
+                    b.members(f.members_b, ordered);
+                    if (f.members_a.size() != f.members_b.size())
+                    {
+                        return false;
+                    }
+                }
+                else
+                {
+                    a.elements(f.elements_a);
+                    b.elements(f.elements_b);
+                }
+                stack.push_back(std::move(f));
+            }
+            else if (!(a.scalar() == b.scalar())) // numbers (also of different types), null, boolean
+            {
+                return false;
+            }
+        }
+
+        // the next pair of values
+        for (;;)
+        {
+            if (stack.empty())
+            {
+                return true;
+            }
+            frame& f = stack.back();
+            const std::size_t count = f.object ? f.members_a.size() : f.elements_a.size();
+            if (f.next == count)
+            {
+                stack.pop_back();
+                continue;
+            }
+            if (f.object)
+            {
+                if (!(f.members_a[f.next].first == f.members_b[f.next].first))
+                {
+                    return false;
+                }
+                a = f.members_a[f.next].second;
+                b = f.members_b[f.next].second;
+            }
+            else
+            {
+                a = f.elements_a[f.next];
+                b = f.elements_b[f.next];
+            }
+            ++f.next;
+            break;
+        }
+    }
+}
+
+}  // namespace view
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/json_view.hpp

@@ -47,6 +47,7 @@
 #endif
 
 #include <nlohmann/detail/view/builder.hpp>
+#include <nlohmann/detail/view/compare.hpp>
 #include <nlohmann/detail/view/document_data.hpp>
 #include <nlohmann/detail/view/errors.hpp>
 #include <nlohmann/detail/view/input.hpp>
@@ -603,6 +604,44 @@ class basic_json_view
     }
 #endif
 
+    ////////////////
+    // comparison //
+    ////////////////
+
+    /// whether the values parse() would produce for two views are equal, as
+    /// by BasicJsonType's operator== (numbers by value, objects by their
+    /// members with duplicate keys resolved as parse() resolves them)
+    friend bool operator==(const basic_json_view& a, const basic_json_view& b)
+    {
+        return detail::view::equal<BasicJsonType>(side(a), side(b));
+    }
+
+    friend bool operator!=(const basic_json_view& a, const basic_json_view& b)
+    {
+        return !(a == b);
+    }
+
+    /// whether the value parse() would produce for a view equals a value
+    friend bool operator==(const basic_json_view& a, const BasicJsonType& j)
+    {
+        return detail::view::equal<BasicJsonType>(side(a), json_side_t(j));
+    }
+
+    friend bool operator==(const BasicJsonType& j, const basic_json_view& a)
+    {
+        return a == j;
+    }
+
+    friend bool operator!=(const basic_json_view& a, const BasicJsonType& j)
+    {
+        return !(a == j);
+    }
+
+    friend bool operator!=(const BasicJsonType& j, const basic_json_view& a)
+    {
+        return !(a == j);
+    }
+
     /////////////////
     // materialize //
     /////////////////
@@ -635,6 +674,13 @@ class basic_json_view
         : m_doc(d), m_node(n)
     {}
 
+    using json_side_t = detail::view::json_side<BasicJsonType, string_view_t>;
+
+    static detail::view::view_side<BasicJsonType, basic_json_view> side(const basic_json_view& v) noexcept
+    {
+        return detail::view::view_side<BasicJsonType, basic_json_view>(v);
+    }
+
     /// the number of source bytes of this value (estimated for values with
     /// decoded strings)
     std::size_t source_extent() const noexcept

single_include/nlohmann/json_view.hpp

@@ -1576,6 +1576,326 @@ inline bool build(document_data& d, const char* src, std::size_t size, bool comm
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END
 
+// #include <nlohmann/detail/view/compare.hpp>
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+
+
+#include <algorithm> // sort, stable_sort
+#include <cstddef> // size_t
+#include <string> // string
+#include <utility> // move, pair
+#include <vector> // vector
+
+// #include <nlohmann/json.hpp>
+// #include <nlohmann/detail/view/macro_scope.hpp>
+
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+namespace view
+{
+
+// Equality of views, and of views with basic_json values, with the semantics
+// of basic_json's operator== applied to the values parse() would produce:
+// numbers compare by value across their types, an object is compared by its
+// members with duplicate keys resolved as parse() resolves them (the last
+// value, at the position of the first occurrence), and in document order if
+// the object type keeps an order (ordered_json), by key otherwise.
+
+/// one side of a comparison: a view
+template<typename BasicJsonType, typename View>
+class view_side
+{
+  public:
+    using string_view_t = typename View::string_view_t;
+
+    explicit view_side(const View& v) noexcept
+        : m_view(v)
+    {}
+
+    value_t type() const noexcept
+    {
+        return m_view.type();
+    }
+
+    std::size_t size() const noexcept
+    {
+        return m_view.size();
+    }
+
+    string_view_t string() const
+    {
+        return m_view.get_string();
+    }
+
+    /// a number, boolean, or null as a basic_json value (no allocation)
+    BasicJsonType scalar() const
+    {
+        switch (m_view.type())
+        {
+            case value_t::number_integer:
+                return BasicJsonType(m_view.template get<typename BasicJsonType::number_integer_t>());
+            case value_t::number_unsigned:
+                return BasicJsonType(m_view.template get<typename BasicJsonType::number_unsigned_t>());
+            case value_t::number_float:
+                return BasicJsonType(m_view.template get<typename BasicJsonType::number_float_t>());
+            case value_t::boolean:
+                return BasicJsonType(m_view.template get<bool>());
+            case value_t::null:
+            case value_t::object:
+            case value_t::array:
+            case value_t::string:
+            case value_t::binary:
+            case value_t::discarded:
+            default:
+                return BasicJsonType(nullptr);
+        }
+    }
+
+    void elements(std::vector<view_side>& out) const
+    {
+        out.reserve(m_view.size());
+        for (const View e : m_view)
+        {
+            out.emplace_back(e);
+        }
+    }
+
+    /// the members as parse() keeps them: one per key, the last value at the
+    /// position of the first occurrence; in that order, or sorted by key
+    void members(std::vector<std::pair<string_view_t, view_side>>& out, bool ordered) const
+    {
+        struct member
+        {
+            string_view_t key;
+            View value;
+            std::size_t position;
+        };
+        std::vector<member> all;
+        all.reserve(m_view.size());
+        std::size_t position = 0;
+        for (auto it = m_view.begin(); it != m_view.end(); ++it)
+        {
+            all.push_back(member{it.key(), it.value(), position++});
+        }
+        std::stable_sort(all.begin(), all.end(), [](const member & a, const member & b)
+        {
+            return a.key < b.key;
+        });
+        std::vector<member> unique;
+        unique.reserve(all.size());
+        for (std::size_t i = 0; i < all.size();)
+        {
+            std::size_t last = i;
+            while (last + 1 < all.size() && all[last + 1].key == all[i].key)
+            {
+                ++last;
+            }
+            unique.push_back(member{all[i].key, all[last].value, all[i].position});
+            i = last + 1;
+        }
+        if (ordered)
+        {
+            std::sort(unique.begin(), unique.end(), [](const member & a, const member & b)
+            {
+                return a.position < b.position;
+            });
+        }
+        out.reserve(unique.size());
+        for (const member& m : unique)
+        {
+            out.emplace_back(m.key, view_side(m.value));
+        }
+    }
+
+  private:
+    View m_view;
+};
+
+/// the other side of a comparison: a basic_json value
+template<typename BasicJsonType, typename StringView>
+class json_side
+{
+  public:
+    using string_view_t = StringView;
+
+    explicit json_side(const BasicJsonType& j) noexcept
+        : m_json(&j)
+    {}
+
+    value_t type() const noexcept
+    {
+        return m_json->type();
+    }
+
+    std::size_t size() const noexcept
+    {
+        return m_json->size();
+    }
+
+    string_view_t string() const
+    {
+        const auto& s = m_json->template get_ref<const typename BasicJsonType::string_t&>();
+        return string_view_t(s.data(), s.size());
+    }
+
+    BasicJsonType scalar() const
+    {
+        return *m_json;
+    }
+
+    void elements(std::vector<json_side>& out) const
+    {
+        out.reserve(m_json->size());
+        for (const auto& e : *m_json)
+        {
+            out.emplace_back(e);
+        }
+    }
+
+    void members(std::vector<std::pair<string_view_t, json_side>>& out, bool ordered) const
+    {
+        out.reserve(m_json->size());
+        for (auto it = m_json->cbegin(); it != m_json->cend(); ++it)
+        {
+            out.emplace_back(string_view_t(it.key().data(), it.key().size()), json_side(it.value()));
+        }
+        if (!ordered)
+        {
+            std::sort(out.begin(), out.end(), [](const std::pair<string_view_t, json_side>& a, const std::pair<string_view_t, json_side>& b)
+            {
+                return a.first < b.first;
+            });
+        }
+    }
+
+  private:
+    const BasicJsonType* m_json;
+};
+
+/// whether two sides are equal; iterative, so that the nesting depth is
+/// limited by memory only
+template<typename BasicJsonType, typename A, typename B>
+bool equal(const A& a0, const B& b0)
+{
+    using string_view_t = typename A::string_view_t;
+    const bool ordered = is_ordered_map<typename BasicJsonType::object_t>::value;
+
+    struct frame
+    {
+        std::vector<A> elements_a{};
+        std::vector<B> elements_b{};
+        std::vector<std::pair<string_view_t, A>> members_a{};
+        std::vector<std::pair<string_view_t, B>> members_b{};
+        bool object = false;
+        std::size_t next = 0;
+    };
+    std::vector<frame> stack;
+    A a = a0;
+    B b = b0;
+    for (;;)
+    {
+        const value_t ta = a.type();
+        const value_t tb = b.type();
+        const bool numbers = (ta == value_t::number_integer || ta == value_t::number_unsigned || ta == value_t::number_float)
+                             && (tb == value_t::number_integer || tb == value_t::number_unsigned || tb == value_t::number_float);
+        if (ta == value_t::discarded || tb == value_t::discarded)
+        {
+            // basic_json decides (JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON)
+            if (ta != tb || !(BasicJsonType(value_t::discarded) == BasicJsonType(value_t::discarded)))
+            {
+                return false;
+            }
+        }
+        else
+        {
+            if (!numbers && ta != tb)
+            {
+                return false;
+            }
+            if (ta == value_t::string)
+            {
+                if (!(a.string() == b.string()))
+                {
+                    return false;
+                }
+            }
+            else if (ta == value_t::array || ta == value_t::object)
+            {
+                if (a.size() != b.size() && ta == value_t::array)
+                {
+                    return false;
+                }
+                frame f;
+                f.object = ta == value_t::object;
+                if (f.object)
+                {
+                    a.members(f.members_a, ordered);
+                    b.members(f.members_b, ordered);
+                    if (f.members_a.size() != f.members_b.size())
+                    {
+                        return false;
+                    }
+                }
+                else
+                {
+                    a.elements(f.elements_a);
+                    b.elements(f.elements_b);
+                }
+                stack.push_back(std::move(f));
+            }
+            else if (!(a.scalar() == b.scalar())) // numbers (also of different types), null, boolean
+            {
+                return false;
+            }
+        }
+
+        // the next pair of values
+        for (;;)
+        {
+            if (stack.empty())
+            {
+                return true;
+            }
+            frame& f = stack.back();
+            const std::size_t count = f.object ? f.members_a.size() : f.elements_a.size();
+            if (f.next == count)
+            {
+                stack.pop_back();
+                continue;
+            }
+            if (f.object)
+            {
+                if (!(f.members_a[f.next].first == f.members_b[f.next].first))
+                {
+                    return false;
+                }
+                a = f.members_a[f.next].second;
+                b = f.members_b[f.next].second;
+            }
+            else
+            {
+                a = f.elements_a[f.next];
+                b = f.elements_b[f.next];
+            }
+            ++f.next;
+            break;
+        }
+    }
+}
+
+}  // namespace view
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END
+
 // #include <nlohmann/detail/view/document_data.hpp>
 
 // #include <nlohmann/detail/view/errors.hpp>
@@ -3766,6 +4086,44 @@ class basic_json_view
     }
 #endif
 
+    ////////////////
+    // comparison //
+    ////////////////
+
+    /// whether the values parse() would produce for two views are equal, as
+    /// by BasicJsonType's operator== (numbers by value, objects by their
+    /// members with duplicate keys resolved as parse() resolves them)
+    friend bool operator==(const basic_json_view& a, const basic_json_view& b)
+    {
+        return detail::view::equal<BasicJsonType>(side(a), side(b));
+    }
+
+    friend bool operator!=(const basic_json_view& a, const basic_json_view& b)
+    {
+        return !(a == b);
+    }
+
+    /// whether the value parse() would produce for a view equals a value
+    friend bool operator==(const basic_json_view& a, const BasicJsonType& j)
+    {
+        return detail::view::equal<BasicJsonType>(side(a), json_side_t(j));
+    }
+
+    friend bool operator==(const BasicJsonType& j, const basic_json_view& a)
+    {
+        return a == j;
+    }
+
+    friend bool operator!=(const basic_json_view& a, const BasicJsonType& j)
+    {
+        return !(a == j);
+    }
+
+    friend bool operator!=(const BasicJsonType& j, const basic_json_view& a)
+    {
+        return !(a == j);
+    }
+
     /////////////////
     // materialize //
     /////////////////
@@ -3798,6 +4156,13 @@ class basic_json_view
         : m_doc(d), m_node(n)
     {}
 
+    using json_side_t = detail::view::json_side<BasicJsonType, string_view_t>;
+
+    static detail::view::view_side<BasicJsonType, basic_json_view> side(const basic_json_view& v) noexcept
+    {
+        return detail::view::view_side<BasicJsonType, basic_json_view>(v);
+    }
+
     /// the number of source bytes of this value (estimated for values with
     /// decoded strings)
     std::size_t source_extent() const noexcept

tests/src/unit-json_view.cpp

@@ -1155,3 +1155,87 @@ TEST_CASE("json_view dump")
         CHECK(json_view().dump() == json(json::value_t::discarded).dump());
     }
 }
+
+TEST_CASE("json_view comparison")
+{
+    SECTION("equality of the values parse() produces")
+    {
+        generator g;
+        std::vector<std::string> texts;
+        for (int i = 0; i < 600; ++i)
+        {
+            std::string text;
+            g.value(text, 0);
+            texts.push_back(text);
+            // the same value written differently: sorted keys, canonical numbers
+            texts.push_back(json::parse(text).dump(1));
+        }
+        for (std::size_t i = 0; i + 2 < texts.size(); ++i)
+        {
+            for (std::size_t k = i; k < i + 3; ++k)
+            {
+                CAPTURE(texts[i]);
+                CAPTURE(texts[k]);
+                const json_document a = json_document::parse(texts[i]);
+                const json_document b = json_document::parse(texts[k]);
+                const json ja = json::parse(texts[i]);
+                const json jb = json::parse(texts[k]);
+                CHECK((a.root() == b.root()) == (ja == jb));
+                CHECK((a.root() != b.root()) == (ja != jb));
+                CHECK((a.root() == jb) == (ja == jb));
+                CHECK((jb == a.root()) == (ja == jb));
+                CHECK((a.root() != jb) == (ja != jb));
+                CHECK((jb != a.root()) == (ja != jb));
+
+                // ordered_json compares members in order
+                const ordered_json_document oa = ordered_json_document::parse(texts[i]);
+                const ordered_json_document ob = ordered_json_document::parse(texts[k]);
+                const ordered_json oja = ordered_json::parse(texts[i]);
+                const ordered_json ojb = ordered_json::parse(texts[k]);
+                CHECK((oa.root() == ob.root()) == (oja == ojb));
+                CHECK((oa.root() == ojb) == (oja == ojb));
+            }
+        }
+    }
+
+    SECTION("numbers, duplicate keys, member order")
+    {
+        const auto same = [](const char* x, const char* y)
+        {
+            return json_document::parse(x).root() == json_document::parse(y).root();
+        };
+        CHECK(same("1", "1.0"));
+        CHECK(same("[1, -1, 2.5]", "[1.0, -1.0, 25e-1]"));
+        CHECK(!same("1", "1.5"));
+        CHECK(same("18446744073709551615", "18446744073709551615"));
+        CHECK(same(R"({"a": 1, "a": 2})", R"({"a": 2})"));
+        CHECK(!same(R"({"a": 1, "a": 2})", R"({"a": 1})"));
+        CHECK(same(R"({"a": 1, "b": 2})", R"({"b": 2, "a": 1})"));
+        CHECK(!same(R"({"a": 1})", R"({"a": 1, "b": 2})"));
+        CHECK(!same("[1, 2]", "[2, 1]"));
+        CHECK(!same("\"a\"", "\"b\""));
+        CHECK(same("\"\\u00e9\"", "\"\xc3\xa9\""));
+        CHECK(!same("null", "false"));
+        CHECK(!same("[]", "{}"));
+        CHECK(ordered_json_document::parse(R"({"a": 1, "b": 2, "a": 3})").root() == ordered_json_document::parse(R"({"a": 3, "b": 2})").root());
+        CHECK(ordered_json_document::parse(R"({"a": 1, "b": 2})").root() != ordered_json_document::parse(R"({"b": 2, "a": 1})").root());
+
+        // discarded values compare as basic_json's do
+        const json discarded(json::value_t::discarded);
+        CHECK((json_view() == json_view()) == (discarded == discarded)); // NOLINT(readability-container-size-empty): operator== is tested
+        CHECK((json_view() == discarded) == (discarded == discarded));
+        CHECK(!(json_view() == json_document::parse("null").root())); // NOLINT(readability-container-size-empty)
+        CHECK(!(json_document::parse("null").root() == discarded));
+    }
+
+    SECTION("deep nesting")
+    {
+        const std::string deep = std::string(100000, '[') + std::string(100000, ']');
+        const json_document a = json_document::parse(deep);
+        const json_document b = json_document::parse(deep);
+        CHECK(a.root() == b.root());
+        CHECK(a.root() == json::parse(deep));
+        const std::string other = std::string(100000, '[') + "1" + std::string(100000, ']');
+        CHECK(a.root() != json_document::parse(other).root());
+    }
+}