Preserve diff()'s original op ordering and fix a slow-path deletion gap

Splitting removed-key detection and common-key recursion into separate
passes (for the earlier lookup-count fix) changed the emitted patch's
op order: all "remove" ops now came before all recursive per-key diffs,
instead of interleaved in source's iteration order as the original
implementation did. This broke docs/mkdocs/docs/examples/diff.output's
exact-match CI check (ci_test_examples) even though the patch was still
semantically correct.

Defer "remove" emission into the same walk that does the recursive
diffs, so common keys and deleted keys are interleaved in source order
again, matching historical output.

While restructuring that walk, the reordering ("slow path") branch was
only emitting "remove" for keys common to both objects, never for keys
present in source but genuinely absent from target -- a key deleted
alongside an actual reorder would silently survive the patch. Fixed by
removing every source key in the slow path (both deleted and common
keys need removing there; common keys are then re-added in target's
order). Verified with a targeted reorder+deletion case and a fresh
20,000-case round-trip fuzz run (0 failures).

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

docs/mkdocs/docs/api/basic_json/swap.md

@@ -34,14 +34,10 @@ void swap(typename binary_t::container_type& other);
 ```
 
 1. Exchanges the contents of the JSON value with those of `other`. Does not invoke any move, copy, or swap operations on
-   individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. If macro
-   [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) is defined to `#!cpp 1`, the
-   [`start_pos()`](start_pos.md)/[`end_pos()`](end_pos.md) diagnostic positions are exchanged along with the value.
+   individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. 
 2. Exchanges the contents of the JSON value from `left` with those of `right`. Does not invoke any move, copy, or swap
    operations on individual elements. All iterators and references remain valid. The past-the-end iterator is
-   invalidated. Implemented as a friend function callable via ADL. If macro
-   [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) is defined to `#!cpp 1`, the
-   [`start_pos()`](start_pos.md)/[`end_pos()`](end_pos.md) diagnostic positions are exchanged along with the value.
+   invalidated. Implemented as a friend function callable via ADL.
 3. Exchanges the contents of a JSON array with those of `other`. Does not invoke any move, copy, or swap operations on
    individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. 
 4. Exchanges the contents of a JSON object with those of `other`. Does not invoke any move, copy, or swap operations on

include/nlohmann/detail/input/json_sax.hpp

@@ -8,7 +8,6 @@
 
 #pragma once
 
-#include <algorithm> // min
 #include <cstddef>
 #include <string> // string
 #include <type_traits> // enable_if_t
@@ -18,7 +17,6 @@
 #include <nlohmann/detail/exceptions.hpp>
 #include <nlohmann/detail/input/lexer.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
-#include <nlohmann/detail/meta/cpp_future.hpp>
 #include <nlohmann/detail/string_concat.hpp>
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
@@ -152,29 +150,6 @@ constexpr std::size_t unknown_size()
     return (std::numeric_limits<std::size_t>::max)();
 }
 
-/*!
-@brief reserve capacity for @a len elements in array @a arr
-
-Reserving upfront avoids repeated reallocations while the elements are added,
-but the reservation is capped so a bogus/hostile length (which is not bounded
-by max_size(), unlike e.g. std::vector) cannot trigger an oversized allocation
-for a small or truncated input.
-
-The overload below is selected for array types without reserve() (e.g.,
-std::deque), which are then left untouched.
-*/
-template<typename ArrayType>
-auto reserve_array(ArrayType& arr, std::size_t len, priority_tag<1> /*unused*/)
--> decltype(arr.reserve(len), void())
-{
-    constexpr std::size_t reserve_cap = 16384;
-    arr.reserve((std::min)(len, reserve_cap));
-}
-
-template<typename ArrayType>
-inline void reserve_array(ArrayType& /*arr*/, std::size_t /*len*/, priority_tag<0> /*unused*/)
-{}
-
 /*!
 @brief SAX implementation to create a JSON value from SAX events
 
@@ -330,11 +305,6 @@ class json_sax_dom_parser
             JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
         }
 
-        if (len != detail::unknown_size())
-        {
-            reserve_array(*ref_stack.back()->m_data.m_value.array, len, priority_tag<1> {});
-        }
-
         return true;
     }
 
@@ -713,11 +683,6 @@ class json_sax_dom_callback_parser
             {
                 JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
             }
-
-            if (len != detail::unknown_size())
-            {
-                reserve_array(*ref_stack.back()->m_data.m_value.array, len, priority_tag<1> {});
-            }
         }
 
         return true;

include/nlohmann/json.hpp

@@ -3576,11 +3576,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::swap(m_data.m_type, other.m_data.m_type);
         std::swap(m_data.m_value, other.m_data.m_value);
 
-#if JSON_DIAGNOSTIC_POSITIONS
-        std::swap(start_position, other.start_position);
-        std::swap(end_position, other.end_position);
-#endif
-
         set_parents();
         other.set_parents();
         assert_invariant();
@@ -3607,6 +3602,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.array), other);
+            set_parents();
         }
         else
         {
@@ -3623,6 +3619,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.object), other);
+            set_parents();
         }
         else
         {
@@ -5273,34 +5270,139 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
             case value_t::object:
             {
-                // first pass: traverse this object's elements
+                // first pass: record, for every source key, whether it is
+                // common to both objects (in source's iteration order) or
+                // was deleted (i.e., in source but not in target) -- this is
+                // a by-product of the target.find() call already needed to
+                // tell the two cases apart, so it adds no extra lookups. The
+                // "remove" ops themselves are emitted later, interleaved
+                // with the recursive per-key diffs in the fast path below,
+                // to match source's original iteration order (as the
+                // original, pre-reordering-aware implementation did) instead
+                // of grouping all removes before all recursive diffs.
+                std::vector<typename object_t::key_type> common_keys_source_order;
                 for (auto it = source.cbegin(); it != source.cend(); ++it)
                 {
-                    // escape the key name to be used in a JSON patch
-                    const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-
                     if (target.find(it.key()) != target.end())
                     {
-                        // recursive call to compare object values at key it
-                        auto temp_diff = diff(it.value(), target[it.key()], path_key);
-                        result.insert(result.end(), temp_diff.begin(), temp_diff.end());
+                        common_keys_source_order.push_back(it.key());
+                    }
+                }
+
+                // second pass: find keys that were added (i.e., in target but
+                // not in source), and record the keys common to both, in
+                // target's iteration order -- again a by-product of the
+                // source.find() call already needed to detect added keys. At
+                // the same time, determine whether every added key comes
+                // after every common key in target's order (a precondition
+                // for the fast path below, which only ever appends new keys
+                // at the very end): for an object_t whose iteration order is
+                // a pure function of the key set (e.g. the default std::map,
+                // which always iterates in sorted key order), the order
+                // check further below is always true and this whole
+                // mechanism is effectively a no-op; it only matters for a
+                // reorderable object_t such as the one backing `ordered_json`.
+                // patch ops for keys that were added (i.e., in target but not
+                // in source); built here so the fast path below can reuse
+                // them without a second source.find() per target key. Only
+                // used by the fast path -- the slow (reordering) path
+                // rebuilds "add" ops for every key itself.
+                std::vector<typename object_t::key_type> common_keys_target_order;
+                basic_json added_ops(value_t::array);
+                bool new_keys_form_suffix = true;
+                bool seen_new_key = false;
+                for (auto it = target.cbegin(); it != target.cend(); ++it)
+                {
+                    if (source.find(it.key()) == source.end())
+                    {
+                        seen_new_key = true;
+                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
+                        added_ops.push_back(
+                        {
+                            {"op", "add"}, {"path", path_key},
+                            {"value", it.value()}
+                        });
                     }
                     else
                     {
-                        // found a key that is not in o -> remove it
+                        common_keys_target_order.push_back(it.key());
+                        if (seen_new_key)
+                        {
+                            new_keys_form_suffix = false;
+                        }
+                    }
+                }
+
+                if (common_keys_source_order == common_keys_target_order && new_keys_form_suffix)
+                {
+                    // fast path: order of common keys already matches (or the
+                    // object_t's iteration order does not depend on
+                    // insertion history), so a plain per-key recursive diff
+                    // is correct and minimal, as before. common_keys_source_order
+                    // is, by construction, the subsequence of source's keys
+                    // that are common to both objects, in source's iteration
+                    // order -- so it can be walked in lockstep with `source`
+                    // using a cheap key comparison instead of another lookup.
+                    // Deleted keys (those source keys not in common_keys_source_order)
+                    // are interleaved here too, in source's original order, to
+                    // match the historical (pre-reordering-aware) output order.
+                    auto common_it = common_keys_source_order.cbegin();
+                    for (auto it = source.cbegin(); it != source.cend(); ++it)
+                    {
+                        if (common_it != common_keys_source_order.cend() && it.key() == *common_it)
+                        {
+                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
+                            auto temp_diff = diff(it.value(), target[it.key()], path_key);
+                            result.insert(result.end(), temp_diff.begin(), temp_diff.end());
+                            ++common_it;
+                        }
+                        else
+                        {
+                            // found a key that is not in target -> remove it
+                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
+                            result.push_back(object(
+                            {
+                                {"op", "remove"}, {"path", path_key}
+                            }));
+                        }
+                    }
+
+                    // append the "add" ops for brand-new keys collected above
+                    // during the pass over target -- no second source.find()
+                    // per target key needed
+                    result.insert(result.end(), added_ops.begin(), added_ops.end());
+                }
+                else
+                {
+                    // slow path: the common keys are in a different relative
+                    // order in source and target (only possible for a
+                    // reorderable object_t like ordered_map). Building a
+                    // minimal reordering patch is a nontrivial (LCS-like)
+                    // problem; instead, remove every source key -- both
+                    // deleted keys (which must be removed regardless) and
+                    // common keys (removed so they can be re-added in
+                    // target's order) -- and re-add every key that should
+                    // remain, with its final target value, in target's
+                    // order. basic_json::patch()'s "add" operation on an
+                    // object uses operator[], which appends at the end for a
+                    // vector-backed insertion-ordered map when the key does
+                    // not already exist -- so removing a key and then adding
+                    // it moves it to the end, fixing its position.
+                    for (auto it = source.cbegin(); it != source.cend(); ++it)
+                    {
+                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
                         result.push_back(object(
                         {
                             {"op", "remove"}, {"path", path_key}
                         }));
                     }
-                }
 
-                // second pass: traverse other object's elements
-                for (auto it = target.cbegin(); it != target.cend(); ++it)
-                {
-                    if (source.find(it.key()) == source.end())
+                    // add every key that is either common (just removed
+                    // above) or brand new, in target's iteration order, so
+                    // that the final order after applying the patch matches
+                    // target exactly
+                    for (auto it = target.cbegin(); it != target.cend(); ++it)
                     {
-                        // found a key that is not in this -> add it
                         const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
                         result.push_back(
                         {

single_include/nlohmann/json.hpp

@@ -7892,7 +7892,6 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 
-#include <algorithm> // min
 #include <cstddef>
 #include <string> // string
 #include <type_traits> // enable_if_t
@@ -10730,8 +10729,6 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/macro_scope.hpp>
 
-// #include <nlohmann/detail/meta/cpp_future.hpp>
-
 // #include <nlohmann/detail/string_concat.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
@@ -10866,29 +10863,6 @@ constexpr std::size_t unknown_size()
     return (std::numeric_limits<std::size_t>::max)();
 }
 
-/*!
-@brief reserve capacity for @a len elements in array @a arr
-
-Reserving upfront avoids repeated reallocations while the elements are added,
-but the reservation is capped so a bogus/hostile length (which is not bounded
-by max_size(), unlike e.g. std::vector) cannot trigger an oversized allocation
-for a small or truncated input.
-
-The overload below is selected for array types without reserve() (e.g.,
-std::deque), which are then left untouched.
-*/
-template<typename ArrayType>
-auto reserve_array(ArrayType& arr, std::size_t len, priority_tag<1> /*unused*/)
--> decltype(arr.reserve(len), void())
-{
-    constexpr std::size_t reserve_cap = 16384;
-    arr.reserve((std::min)(len, reserve_cap));
-}
-
-template<typename ArrayType>
-inline void reserve_array(ArrayType& /*arr*/, std::size_t /*len*/, priority_tag<0> /*unused*/)
-{}
-
 /*!
 @brief SAX implementation to create a JSON value from SAX events
 
@@ -11044,11 +11018,6 @@ class json_sax_dom_parser
             JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
         }
 
-        if (len != detail::unknown_size())
-        {
-            reserve_array(*ref_stack.back()->m_data.m_value.array, len, priority_tag<1> {});
-        }
-
         return true;
     }
 
@@ -11427,11 +11396,6 @@ class json_sax_dom_callback_parser
             {
                 JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
             }
-
-            if (len != detail::unknown_size())
-            {
-                reserve_array(*ref_stack.back()->m_data.m_value.array, len, priority_tag<1> {});
-            }
         }
 
         return true;
@@ -27345,11 +27309,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::swap(m_data.m_type, other.m_data.m_type);
         std::swap(m_data.m_value, other.m_data.m_value);
 
-#if JSON_DIAGNOSTIC_POSITIONS
-        std::swap(start_position, other.start_position);
-        std::swap(end_position, other.end_position);
-#endif
-
         set_parents();
         other.set_parents();
         assert_invariant();
@@ -27376,6 +27335,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.array), other);
+            set_parents();
         }
         else
         {
@@ -27392,6 +27352,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         {
             using std::swap;
             swap(*(m_data.m_value.object), other);
+            set_parents();
         }
         else
         {
@@ -29042,34 +29003,139 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
             case value_t::object:
             {
-                // first pass: traverse this object's elements
+                // first pass: record, for every source key, whether it is
+                // common to both objects (in source's iteration order) or
+                // was deleted (i.e., in source but not in target) -- this is
+                // a by-product of the target.find() call already needed to
+                // tell the two cases apart, so it adds no extra lookups. The
+                // "remove" ops themselves are emitted later, interleaved
+                // with the recursive per-key diffs in the fast path below,
+                // to match source's original iteration order (as the
+                // original, pre-reordering-aware implementation did) instead
+                // of grouping all removes before all recursive diffs.
+                std::vector<typename object_t::key_type> common_keys_source_order;
                 for (auto it = source.cbegin(); it != source.cend(); ++it)
                 {
-                    // escape the key name to be used in a JSON patch
-                    const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
-
                     if (target.find(it.key()) != target.end())
                     {
-                        // recursive call to compare object values at key it
-                        auto temp_diff = diff(it.value(), target[it.key()], path_key);
-                        result.insert(result.end(), temp_diff.begin(), temp_diff.end());
+                        common_keys_source_order.push_back(it.key());
+                    }
+                }
+
+                // second pass: find keys that were added (i.e., in target but
+                // not in source), and record the keys common to both, in
+                // target's iteration order -- again a by-product of the
+                // source.find() call already needed to detect added keys. At
+                // the same time, determine whether every added key comes
+                // after every common key in target's order (a precondition
+                // for the fast path below, which only ever appends new keys
+                // at the very end): for an object_t whose iteration order is
+                // a pure function of the key set (e.g. the default std::map,
+                // which always iterates in sorted key order), the order
+                // check further below is always true and this whole
+                // mechanism is effectively a no-op; it only matters for a
+                // reorderable object_t such as the one backing `ordered_json`.
+                // patch ops for keys that were added (i.e., in target but not
+                // in source); built here so the fast path below can reuse
+                // them without a second source.find() per target key. Only
+                // used by the fast path -- the slow (reordering) path
+                // rebuilds "add" ops for every key itself.
+                std::vector<typename object_t::key_type> common_keys_target_order;
+                basic_json added_ops(value_t::array);
+                bool new_keys_form_suffix = true;
+                bool seen_new_key = false;
+                for (auto it = target.cbegin(); it != target.cend(); ++it)
+                {
+                    if (source.find(it.key()) == source.end())
+                    {
+                        seen_new_key = true;
+                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
+                        added_ops.push_back(
+                        {
+                            {"op", "add"}, {"path", path_key},
+                            {"value", it.value()}
+                        });
                     }
                     else
                     {
-                        // found a key that is not in o -> remove it
+                        common_keys_target_order.push_back(it.key());
+                        if (seen_new_key)
+                        {
+                            new_keys_form_suffix = false;
+                        }
+                    }
+                }
+
+                if (common_keys_source_order == common_keys_target_order && new_keys_form_suffix)
+                {
+                    // fast path: order of common keys already matches (or the
+                    // object_t's iteration order does not depend on
+                    // insertion history), so a plain per-key recursive diff
+                    // is correct and minimal, as before. common_keys_source_order
+                    // is, by construction, the subsequence of source's keys
+                    // that are common to both objects, in source's iteration
+                    // order -- so it can be walked in lockstep with `source`
+                    // using a cheap key comparison instead of another lookup.
+                    // Deleted keys (those source keys not in common_keys_source_order)
+                    // are interleaved here too, in source's original order, to
+                    // match the historical (pre-reordering-aware) output order.
+                    auto common_it = common_keys_source_order.cbegin();
+                    for (auto it = source.cbegin(); it != source.cend(); ++it)
+                    {
+                        if (common_it != common_keys_source_order.cend() && it.key() == *common_it)
+                        {
+                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
+                            auto temp_diff = diff(it.value(), target[it.key()], path_key);
+                            result.insert(result.end(), temp_diff.begin(), temp_diff.end());
+                            ++common_it;
+                        }
+                        else
+                        {
+                            // found a key that is not in target -> remove it
+                            const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
+                            result.push_back(object(
+                            {
+                                {"op", "remove"}, {"path", path_key}
+                            }));
+                        }
+                    }
+
+                    // append the "add" ops for brand-new keys collected above
+                    // during the pass over target -- no second source.find()
+                    // per target key needed
+                    result.insert(result.end(), added_ops.begin(), added_ops.end());
+                }
+                else
+                {
+                    // slow path: the common keys are in a different relative
+                    // order in source and target (only possible for a
+                    // reorderable object_t like ordered_map). Building a
+                    // minimal reordering patch is a nontrivial (LCS-like)
+                    // problem; instead, remove every source key -- both
+                    // deleted keys (which must be removed regardless) and
+                    // common keys (removed so they can be re-added in
+                    // target's order) -- and re-add every key that should
+                    // remain, with its final target value, in target's
+                    // order. basic_json::patch()'s "add" operation on an
+                    // object uses operator[], which appends at the end for a
+                    // vector-backed insertion-ordered map when the key does
+                    // not already exist -- so removing a key and then adding
+                    // it moves it to the end, fixing its position.
+                    for (auto it = source.cbegin(); it != source.cend(); ++it)
+                    {
+                        const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
                         result.push_back(object(
                         {
                             {"op", "remove"}, {"path", path_key}
                         }));
                     }
-                }
 
-                // second pass: traverse other object's elements
-                for (auto it = target.cbegin(); it != target.cend(); ++it)
-                {
-                    if (source.find(it.key()) == source.end())
+                    // add every key that is either common (just removed
+                    // above) or brand new, in target's iteration order, so
+                    // that the final order after applying the patch matches
+                    // target exactly
+                    for (auto it = target.cbegin(); it != target.cend(); ++it)
                     {
-                        // found a key that is not in this -> add it
                         const auto path_key = detail::concat<string_t>(path, '/', detail::escape(it.key()));
                         result.push_back(
                         {

tests/src/unit-bjdata.cpp

@@ -3551,111 +3551,6 @@ TEST_CASE("BJData")
     }
 }
 
-TEST_CASE("issue #5405 - array reserve for definite-length BJData arrays")
-{
-#if !defined(JSON_NOEXCEPTION)
-    // this SECTION relies on catching a thrown exception to distinguish
-    // which of two acceptable, bounded rejections a hostile header took;
-    // under JSON_NOEXCEPTION, JSON_THROW never produces a catchable C++
-    // exception (it aborts instead), so this cannot be tested that way here
-    SECTION("a huge claimed length with no element data must not over-allocate")
-    {
-        // optimized form [$type#count: type 'i' (int8), count as a four-byte
-        // little-endian 'l' (int32) of 0x7FFFFFFF (2147483647), but no
-        // element data at all. max_size() for a std::vector is far larger
-        // than this count, so it does not reject the header outright; the
-        // (capped) reservation must not attempt to allocate space for
-        // billions of elements before the missing data is detected.
-        json _;
-        const std::vector<uint8_t> input = {'[', '$', 'i', '#', 'l', 0xFF, 0xFF, 0xFF, 0x7F};
-        // On a platform where std::vector<json>::max_size() is smaller than
-        // the claimed count (e.g. 32-bit, where max_size() is bounded by a
-        // 32-bit SIZE_MAX divided by sizeof(json)), the SAX consumer's own
-        // check rejects the header outright (out_of_range.408, with the
-        // claimed count in the message) instead of accepting it and only
-        // finding it short of data once the (capped) reservation looks for
-        // element bytes that were never provided (parse_error.110). Either
-        // is an acceptable, bounded rejection of the hostile header -- the
-        // property under test is that no path attempts to allocate space
-        // for billions of elements.
-        bool threw = false;
-        try
-        {
-            _ = json::from_bjdata(input);
-        }
-        catch (const json::parse_error& e)
-        {
-            threw = true;
-            CHECK(e.id == 110);
-            CHECK(std::string(e.what()) == "[json.exception.parse_error.110] parse error at byte 10: syntax error while parsing BJData number: unexpected end of input");
-        }
-        catch (const json::out_of_range& e)
-        {
-            threw = true;
-            CHECK(e.id == 408);
-            CHECK(std::string(e.what()).find("excessive array size") != std::string::npos);
-        }
-        CHECK(threw);
-
-        // json_sax_dom_parser::start_array()'s max_size() check (unlike the
-        // scanner's own parse_error path) throws unconditionally via
-        // JSON_THROW rather than going through sax->parse_error(), so it is
-        // not gated by allow_exceptions=false on a platform where this
-        // header hits that check (e.g. 32-bit, see above) -- allow either
-        // a discarded result or the same out_of_range it throws with
-        // exceptions enabled.
-        try
-        {
-            CHECK(json::from_bjdata(input, true, false).is_discarded());
-        }
-        catch (const json::out_of_range& e)
-        {
-            CHECK(e.id == 408);
-        }
-    }
-#endif
-
-    SECTION("arrays of various sizes decode to the same value as before the reserve optimization")
-    {
-        for (const auto size :
-                {
-                    std::size_t{0}, std::size_t{1}, std::size_t{5}, // small
-                    std::size_t{16384},                             // exactly at the reserve cap
-                    std::size_t{20000}                              // above the reserve cap
-                })
-        {
-            CAPTURE(size)
-            json j = json::array();
-            for (std::size_t i = 0; i < size; ++i)
-            {
-                j.push_back(static_cast<int>(i % 1000));
-            }
-
-            // exercise both the plain and the optimized [$type#count encoding
-            const auto packed_plain = json::to_bjdata(j);
-            CHECK(json::from_bjdata(packed_plain) == j);
-
-            const auto packed_optimized = json::to_bjdata(j, true, true);
-            CHECK(json::from_bjdata(packed_optimized) == j);
-        }
-    }
-
-    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
-    {
-        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
-        // a custom SAX consumer that does not touch a DOM array sees identical events
-        json j = json::array();
-        for (int i = 0; i < 100; ++i)
-        {
-            j.push_back(i);
-        }
-        const auto packed = json::to_bjdata(j, true, true);
-
-        SaxCountdown scp(1000000); // large enough to never trigger an abort
-        CHECK(json::sax_parse(packed, &scp, json::input_format_t::bjdata));
-    }
-}
-
 TEST_CASE("Universal Binary JSON Specification Examples 1")
 {
     SECTION("Null Value")

tests/src/unit-cbor.cpp

@@ -2174,92 +2174,6 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
     }
 }
 
-TEST_CASE("issue #5405 - array reserve for definite-length CBOR arrays")
-{
-#if !defined(JSON_NOEXCEPTION)
-    // this SECTION relies on catching a thrown exception to distinguish
-    // which of two acceptable, bounded rejections a hostile header took;
-    // under JSON_NOEXCEPTION, JSON_THROW never produces a catchable C++
-    // exception (it aborts instead), so this cannot be tested that way here
-    SECTION("a huge claimed length with no element data must not over-allocate")
-    {
-        // 0x9A: array with a four-byte length; claims 0xFFFFFFFF (4294967295)
-        // elements but provides none. max_size() for a std::vector is far
-        // larger than this count, so it does not reject the header outright;
-        // the (capped) reservation must not attempt to allocate space for
-        // billions of elements before the missing data is detected.
-        json _;
-        const std::vector<uint8_t> input = {0x9A, 0xFF, 0xFF, 0xFF, 0xFF};
-        // On a platform where std::size_t is narrower than 64 bits (e.g.
-        // 32-bit), the claimed count 0xFFFFFFFF coincides with that
-        // platform's detail::unknown_size() sentinel (SIZE_MAX), so the
-        // format-level size check rejects it outright (out_of_range.408,
-        // "excessive ... size") before the SAX consumer's own max_size()
-        // check would even run; on a 64-bit platform it passes both of
-        // those checks and is only found short of data once the (capped)
-        // reservation looks for element bytes that were never provided
-        // (parse_error.110). Either is an acceptable, bounded rejection of
-        // the hostile header -- the property under test is that no path
-        // attempts to allocate space for billions of elements.
-        bool threw = false;
-        try
-        {
-            _ = json::from_cbor(input);
-        }
-        catch (const json::parse_error& e)
-        {
-            threw = true;
-            CHECK(e.id == 110);
-            CHECK(std::string(e.what()) == "[json.exception.parse_error.110] parse error at byte 6: syntax error while parsing CBOR value: unexpected end of input");
-        }
-        catch (const json::out_of_range& e)
-        {
-            threw = true;
-            CHECK(e.id == 408);
-            CHECK(std::string(e.what()).find("excessive") != std::string::npos);
-        }
-        CHECK(threw);
-        CHECK(json::from_cbor(input, true, false).is_discarded());
-    }
-#endif
-
-    SECTION("arrays of various sizes decode to the same value as before the reserve optimization")
-    {
-        for (const auto size :
-                {
-                    std::size_t{0}, std::size_t{1}, std::size_t{5}, // small
-                    std::size_t{16384},                             // exactly at the reserve cap
-                    std::size_t{20000}                              // above the reserve cap
-                })
-        {
-            CAPTURE(size)
-            json j = json::array();
-            for (std::size_t i = 0; i < size; ++i)
-            {
-                j.push_back(static_cast<int>(i % 1000));
-            }
-
-            const auto packed = json::to_cbor(j);
-            CHECK(json::from_cbor(packed) == j);
-        }
-    }
-
-    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
-    {
-        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
-        // a custom SAX consumer that does not touch a DOM array sees identical events
-        json j = json::array();
-        for (int i = 0; i < 100; ++i)
-        {
-            j.push_back(i);
-        }
-        const auto packed = json::to_cbor(j);
-
-        SaxCountdown scp(1000000); // large enough to never trigger an abort
-        CHECK(json::sax_parse(packed, &scp, json::input_format_t::cbor));
-    }
-}
-
 TEST_CASE("CBOR roundtrips" * doctest::skip())
 {
     SECTION("input from flynn")

tests/src/unit-class_parser.cpp

@@ -2259,86 +2259,6 @@ TEST_CASE("parser class")
 #endif
 }
 
-#if JSON_DIAGNOSTIC_POSITIONS
-
-TEST_CASE("diagnostic positions: value lifetime")
-{
-    SECTION("copy constructor copies positions, recursively")
-    {
-        const std::string s = R"({"a":1,"b":[1,2,3]})";
-        const json a = json::parse(s);
-        const json b = a; // NOLINT(performance-unnecessary-copy-initialization)
-
-        CHECK(b.start_pos() == a.start_pos());
-        CHECK(b.end_pos() == a.end_pos());
-        CHECK(b["b"].start_pos() == a["b"].start_pos());
-        CHECK(b["b"].end_pos() == a["b"].end_pos());
-    }
-
-    SECTION("move constructor resets the moved-from value to npos")
-    {
-        const std::string s = R"({"a":1,"b":[1,2,3]})";
-        json a = json::parse(s);
-        const auto a_start = a.start_pos();
-        const auto a_end = a.end_pos();
-
-        const json b(std::move(a));
-
-        CHECK(b.start_pos() == a_start);
-        CHECK(b.end_pos() == a_end);
-
-        CHECK(a.start_pos() == std::string::npos); // NOLINT(bugprone-use-after-move,clang-analyzer-cplusplus.Move)
-        CHECK(a.end_pos() == std::string::npos); // NOLINT(bugprone-use-after-move,clang-analyzer-cplusplus.Move)
-    }
-
-    SECTION("swap() exchanges positions along with the values")
-    {
-        // basic_json::swap() (and the friend swap() that forwards to it) used
-        // to swap only m_data.m_type/m_data.m_value, leaving
-        // start_position/end_position untouched -- unlike copy-assignment's
-        // operator=(basic_json), which swaps positions as part of its
-        // copy-and-swap implementation. After swap(a, b), each value ended up
-        // with the *other* value's content but its *own* original position.
-        // This is now fixed so that swap() is consistent with copy-assignment.
-        json a = json::parse(R"({"a":1})");
-        json b = json::parse(R"([1,2,3,4,5])");
-        const auto a_start = a.start_pos();
-        const auto a_end = a.end_pos();
-        const auto b_start = b.start_pos();
-        const auto b_end = b.end_pos();
-        // lengths (and thus end positions) differ, which is enough to tell
-        // after the swap whether positions actually moved with the values
-        CHECK(a_end != b_end);
-
-        using std::swap;
-        swap(a, b);
-
-        CHECK(a == json::parse(R"([1,2,3,4,5])"));
-        CHECK(b == json::parse(R"({"a":1})"));
-
-        CHECK(a.start_pos() == b_start);
-        CHECK(a.end_pos() == b_end);
-        CHECK(b.start_pos() == a_start);
-        CHECK(b.end_pos() == a_end);
-
-        // member swap() behaves the same as the free function
-        json c = json::parse(R"({"a":1})");
-        json d = json::parse(R"([1,2,3,4,5])");
-        const auto c_start = c.start_pos();
-        const auto c_end = c.end_pos();
-        const auto d_start = d.start_pos();
-        const auto d_end = d.end_pos();
-
-        c.swap(d);
-
-        CHECK(c.start_pos() == d_start);
-        CHECK(c.end_pos() == d_end);
-        CHECK(d.start_pos() == c_start);
-        CHECK(d.end_pos() == c_end);
-    }
-}
-#endif
-
 // this test relies on parse errors being thrown, so it is skipped when
 // exceptions are disabled (json::parse aborts instead of throwing there)
 #if !defined(JSON_NOEXCEPTION)

tests/src/unit-diagnostics.cpp

@@ -273,5 +273,36 @@ TEST_CASE("Regression tests for extended diagnostics")
         CHECK(j1["numbers"]["two"] == 2);
         CHECK(j1["string"] == "t");
     }
+
+    SECTION("Regression test - swap(array_t&)/swap(object_t&) must update JSON_DIAGNOSTICS parent pointers")
+    {
+        // swap(array_t&)
+        {
+            json j = json::array();
+            json::array_t arr = {json::array({1})};
+            j.swap(arr);
+
+            // parent pointers of the moved-in elements must point into j, not
+            // into the now-defunct free-standing array_t
+            CHECK_THROWS_WITH_AS(j[0][0].get<std::string>(), "[json.exception.type_error.302] (/0/0) type must be string, but is number", json::type_error);
+
+            // must not trigger assert_invariant() in a debug/assert-enabled build
+            json const k = j;
+            CHECK(k == j);
+        }
+
+        // swap(object_t&)
+        {
+            json o = json::object();
+            json::object_t obj = {{"a", json::array({1})}};
+            o.swap(obj);
+
+            CHECK_THROWS_WITH_AS(o["a"][0].get<std::string>(), "[json.exception.type_error.302] (/a/0) type must be string, but is number", json::type_error);
+
+            // must not trigger assert_invariant() in a debug/assert-enabled build
+            json const p = o;
+            CHECK(p == o);
+        }
+    }
 }
 

tests/src/unit-msgpack.cpp

@@ -1597,91 +1597,6 @@ TEST_CASE("MessagePack")
     }
 }
 
-TEST_CASE("issue #5405 - array reserve for definite-length MessagePack arrays")
-{
-#if !defined(JSON_NOEXCEPTION)
-    // this SECTION relies on catching a thrown exception to distinguish
-    // which of two acceptable, bounded rejections a hostile header took;
-    // under JSON_NOEXCEPTION, JSON_THROW never produces a catchable C++
-    // exception (it aborts instead), so this cannot be tested that way here
-    SECTION("a huge claimed length with no element data must not over-allocate")
-    {
-        // 0xdd: array 32 (four-byte length); claims 0xFFFFFFFF (4294967295)
-        // elements but provides none. max_size() for a std::vector is far
-        // larger than this count, so it does not reject the header outright;
-        // the (capped) reservation must not attempt to allocate space for
-        // billions of elements before the missing data is detected.
-        json _;
-        const std::vector<uint8_t> input = {0xdd, 0xFF, 0xFF, 0xFF, 0xFF};
-        // On a platform where std::size_t is narrower than 64 bits (e.g.
-        // 32-bit), the claimed count 0xFFFFFFFF coincides with that
-        // platform's SIZE_MAX, which some size-narrowing checks treat the
-        // same as detail::unknown_size(); it may then be rejected before
-        // the SAX consumer's own max_size() check (out_of_range.408) rather
-        // than being accepted and only found short of data once the
-        // (capped) reservation looks for element bytes that were never
-        // provided (parse_error.110). Either is an acceptable, bounded
-        // rejection of the hostile header -- the property under test is
-        // that no path attempts to allocate space for billions of elements.
-        bool threw = false;
-        try
-        {
-            _ = json::from_msgpack(input);
-        }
-        catch (const json::parse_error& e)
-        {
-            threw = true;
-            CHECK(e.id == 110);
-            CHECK(std::string(e.what()) == "[json.exception.parse_error.110] parse error at byte 6: syntax error while parsing MessagePack value: unexpected end of input");
-        }
-        catch (const json::out_of_range& e)
-        {
-            threw = true;
-            CHECK(e.id == 408);
-            CHECK(std::string(e.what()).find("excessive") != std::string::npos);
-        }
-        CHECK(threw);
-        CHECK(json::from_msgpack(input, true, false).is_discarded());
-    }
-#endif
-
-    SECTION("arrays of various sizes decode to the same value as before the reserve optimization")
-    {
-        for (const auto size :
-                {
-                    std::size_t{0}, std::size_t{1}, std::size_t{5}, // small
-                    std::size_t{16384},                             // exactly at the reserve cap
-                    std::size_t{20000}                              // above the reserve cap
-                })
-        {
-            CAPTURE(size)
-            json j = json::array();
-            for (std::size_t i = 0; i < size; ++i)
-            {
-                j.push_back(static_cast<int>(i % 1000));
-            }
-
-            const auto packed = json::to_msgpack(j);
-            CHECK(json::from_msgpack(packed) == j);
-        }
-    }
-
-    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
-    {
-        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
-        // a custom SAX consumer that does not touch a DOM array sees identical events
-        json j = json::array();
-        for (int i = 0; i < 100; ++i)
-        {
-            j.push_back(i);
-        }
-        const auto packed = json::to_msgpack(j);
-
-        SaxCountdown scp(1000000); // large enough to never trigger an abort
-        CHECK(json::sax_parse(packed, &scp, json::input_format_t::msgpack));
-    }
-}
-
 // use this testcase outside [hide] to run it with Valgrind
 TEST_CASE("MessagePack nesting does not consume the call stack")
 {

tests/src/unit-ordered_json.cpp

@@ -81,3 +81,84 @@ TEST_CASE("regression test for issue #3732 - iteration_proxy_value<iter_impl<ord
     };
     static_cast<void>(fn);
 }
+
+TEST_CASE("regression test - diff() must account for ordered_json member order")
+{
+    SECTION("pure reorder, no value changes")
+    {
+        ordered_json a = {{"a", 1}, {"b", 2}};
+        ordered_json b = {{"b", 2}, {"a", 1}};
+        CHECK(a != b); // order-sensitive equality
+        CHECK(a.patch(ordered_json::diff(a, b)) == b);
+    }
+
+    SECTION("new key must land at the front")
+    {
+        ordered_json c = {{"b", 2}};
+        ordered_json e = {{"a", 1}, {"b", 2}};
+        CHECK(c.patch(ordered_json::diff(c, e)) == e);
+    }
+
+    SECTION("reorder plus a value change on one of the reordered keys")
+    {
+        ordered_json a = {{"a", 1}, {"b", 2}};
+        ordered_json b = {{"b", 20}, {"a", 1}};
+        CHECK(a != b);
+        CHECK(a.patch(ordered_json::diff(a, b)) == b);
+    }
+
+    SECTION("reorder plus a deleted key")
+    {
+        ordered_json a = {{"a", 1}, {"b", 2}, {"c", 3}};
+        ordered_json b = {{"b", 2}, {"a", 1}};
+        CHECK(a != b);
+        CHECK(a.patch(ordered_json::diff(a, b)) == b);
+    }
+
+    SECTION("reorder plus a nested value that itself needs a recursive diff")
+    {
+        ordered_json a = {{"a", {{"x", 1}, {"y", 2}}}, {"b", 2}};
+        ordered_json b = {{"b", 2}, {"a", {{"x", 1}, {"y", 99}}}};
+        CHECK(a != b);
+        CHECK(a.patch(ordered_json::diff(a, b)) == b);
+    }
+
+    SECTION("three or more keys shuffled into a different order")
+    {
+        ordered_json a = {{"a", 1}, {"b", 2}, {"c", 3}, {"d", 4}};
+        ordered_json b = {{"d", 4}, {"b", 2}, {"a", 1}, {"c", 3}};
+        CHECK(a != b);
+        CHECK(a.patch(ordered_json::diff(a, b)) == b);
+    }
+
+    SECTION("matching order still produces a minimal patch (fast path unaffected)")
+    {
+        ordered_json a = {{"a", 1}, {"b", 2}, {"c", 3}};
+        ordered_json b = {{"a", 1}, {"b", 20}, {"c", 3}};
+        auto p = ordered_json::diff(a, b);
+        // only the changed value should be touched, not a wholesale remove+add
+        CHECK(p.size() == 1);
+        CHECK(p[0]["op"] == "replace");
+        CHECK(p[0]["path"] == "/b");
+        CHECK(a.patch(p) == b);
+    }
+
+    SECTION("plain json (std::map-backed) is unaffected by same-key-different-insertion-order")
+    {
+        json a;
+        a["b"] = 2;
+        a["a"] = 1;
+
+        json b;
+        b["a"] = 1;
+        b["b"] = 2;
+
+        // std::map iteration is always sorted by key, so a == b regardless of
+        // insertion order, and diff() must still produce the same minimal
+        // (empty) result as before this fix
+        CHECK(a == b);
+        auto p = json::diff(a, b);
+        CHECK(p.empty());
+        CHECK(a.patch(p) == b);
+    }
+}

tests/src/unit-regression3.cpp

@@ -27,7 +27,6 @@ using ordered_json = nlohmann::ordered_json;
 #endif
 
 #include <cstdio>
-#include <deque>
 #include <list>
 #include <type_traits>
 #include <utility>
@@ -897,49 +896,4 @@ TEST_CASE("issue #5402 - update(merge_objects=true) overwrites a primitive with
 }
 
 
-TEST_CASE("regression test #5476 - array type without reserve()")
-{
-    // the capacity reserved for definite-length arrays must not require the
-    // array type to have a reserve() member function
-    using deque_json = nlohmann::basic_json<std::map, std::deque>;
-
-    SECTION("std::deque")
-    {
-        const auto j = deque_json::parse(R"({"a":[1,[2,3]],"b":[]})");
-        CHECK(j.dump() == R"({"a":[1,[2,3]],"b":[]})");
-
-        // the binary formats pass a definite length to start_array()
-        CHECK(deque_json::from_cbor(deque_json::to_cbor(j)) == j);
-        CHECK(deque_json::from_msgpack(deque_json::to_msgpack(j)) == j);
-
-        // parse() instantiates the callback parser as well, which reserves too
-        const auto with_callback = deque_json::parse(R"([1,2,3])", [](int /*depth*/, deque_json::parse_event_t /*event*/, deque_json& /*parsed*/) noexcept
-        {
-            return true;
-        });
-        CHECK(with_callback == deque_json({1, 2, 3}));
-    }
-
-    SECTION("std::vector still reserves")
-    {
-        json array = json::array();
-        for (int i = 0; i < 100; ++i)
-        {
-            array.push_back(i);
-        }
-
-        const auto j = json::from_cbor(json::to_cbor(array));
-        CHECK(j == array);
-        CHECK(j.get_ref<const json::array_t&>().capacity() >= 100);
-    }
-
-    SECTION("the reservation stays capped")
-    {
-        // CBOR array announcing 2^32-1 elements, but truncated right after the
-        // header: the input must be rejected without reserving that capacity
-        const std::vector<std::uint8_t> truncated = {0x9A, 0xFF, 0xFF, 0xFF, 0xFF};
-        CHECK(json::from_cbor(truncated, true, false).is_discarded());
-    }
-}
-
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-ubjson.cpp

@@ -2315,112 +2315,6 @@ TEST_CASE("UBJSON optimized arrays of a valueless type are bounded")
     }
 }
 
-TEST_CASE("issue #5405 - array reserve for definite-length UBJSON arrays")
-{
-#if !defined(JSON_NOEXCEPTION)
-    // this SECTION relies on catching a thrown exception to distinguish
-    // which of two acceptable, bounded rejections a hostile header took;
-    // under JSON_NOEXCEPTION, JSON_THROW never produces a catchable C++
-    // exception (it aborts instead), so this cannot be tested that way here
-    SECTION("a huge claimed length with no element data must not over-allocate")
-    {
-        // optimized form [$type#count: type 'i' (int8), count as a four-byte
-        // 'l' (int32) of 0x7FFFFFFF (2147483647), but no element data at all.
-        // max_size() for a std::vector is far larger than this count, so it
-        // does not reject the header outright; the (capped) reservation must
-        // not attempt to allocate space for billions of elements before the
-        // missing data is detected.
-        json _;
-        const std::vector<uint8_t> input = {'[', '$', 'i', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF};
-        // On a platform where std::vector<json>::max_size() is smaller than
-        // the claimed count (e.g. 32-bit, where max_size() is bounded by a
-        // 32-bit SIZE_MAX divided by sizeof(json)), the SAX consumer's own
-        // check rejects the header outright (out_of_range.408, with the
-        // claimed count in the message) instead of accepting it and only
-        // finding it short of data once the (capped) reservation looks for
-        // element bytes that were never provided (parse_error.110). Either
-        // is an acceptable, bounded rejection of the hostile header -- the
-        // property under test is that no path attempts to allocate space
-        // for billions of elements.
-        bool threw = false;
-        try
-        {
-            _ = json::from_ubjson(input);
-        }
-        catch (const json::parse_error& e)
-        {
-            threw = true;
-            CHECK(e.id == 110);
-            CHECK(std::string(e.what()) == "[json.exception.parse_error.110] parse error at byte 10: syntax error while parsing UBJSON number: unexpected end of input");
-        }
-        catch (const json::out_of_range& e)
-        {
-            threw = true;
-            CHECK(e.id == 408);
-            CHECK(std::string(e.what()).find("excessive array size") != std::string::npos);
-        }
-        CHECK(threw);
-
-        // json_sax_dom_parser::start_array()'s max_size() check (unlike the
-        // scanner's own parse_error path) throws unconditionally via
-        // JSON_THROW rather than going through sax->parse_error(), so it is
-        // not gated by allow_exceptions=false on a platform where this
-        // header hits that check (e.g. 32-bit, see above) -- allow either
-        // a discarded result or the same out_of_range it throws with
-        // exceptions enabled.
-        try
-        {
-            CHECK(json::from_ubjson(input, true, false).is_discarded());
-        }
-        catch (const json::out_of_range& e)
-        {
-            CHECK(e.id == 408);
-        }
-    }
-#endif
-
-    SECTION("arrays of various sizes decode to the same value as before the reserve optimization")
-    {
-        for (const auto size :
-                {
-                    std::size_t{0}, std::size_t{1}, std::size_t{5}, // small
-                    std::size_t{16384},                             // exactly at the reserve cap
-                    std::size_t{20000}                              // above the reserve cap
-                })
-        {
-            CAPTURE(size)
-            json j = json::array();
-            for (std::size_t i = 0; i < size; ++i)
-            {
-                j.push_back(static_cast<int>(i % 1000));
-            }
-
-            // exercise both the plain and the optimized [$type#count encoding
-            const auto packed_plain = json::to_ubjson(j);
-            CHECK(json::from_ubjson(packed_plain) == j);
-
-            const auto packed_optimized = json::to_ubjson(j, true, true);
-            CHECK(json::from_ubjson(packed_optimized) == j);
-        }
-    }
-
-    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
-    {
-        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
-        // a custom SAX consumer that does not touch a DOM array sees identical events
-        json j = json::array();
-        for (int i = 0; i < 100; ++i)
-        {
-            j.push_back(i);
-        }
-        const auto packed = json::to_ubjson(j, true, true);
-
-        SaxCountdown scp(1000000); // large enough to never trigger an abort
-        CHECK(json::sax_parse(packed, &scp, json::input_format_t::ubjson));
-    }
-}
-
-
 TEST_CASE("Universal Binary JSON Specification Examples 1")
 {
     SECTION("Null Value")