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Niels LohmannandClaude
494a74c242
* Make basic_json::insert() provide its documented strong exception guarantee (#5986) A failed allocation while the new elements were copied left the value changed, and with JSON_DIAGNOSTICS stale parent pointers behind. The extra work now happens almost only on the failure path: - Array insert(pos, cnt, val) and insert(pos, first, last) append the copies at the end and rotate them into place. If a copy throws, the appended elements are erased again (noexcept) and the parents are set. Moving a basic_json cannot throw, so reallocation is safe. This is no slower than vector::insert (faster for numbers), needs no temporary array, and keeps the elements that libstdc++ lost on reallocation. - Array insert(pos, val) copies a non-scalar value first and inserts it by move, except at the end, where vector::insert is strong. Scalars keep the direct path. - The object range insert(first, last) into an empty object uses the container's own range insertion and clears the object on an exception. Into a non-empty object it inserts member by member and logs one bit per source member (a new member is detected by the size change; no allocation for the first 256 members); on an exception the members added are erased by walking the source range. No assumption that key_type is string_t. - The parents are set when a failed insertion is undone. The regression test fails every allocation of every insert() overload in turn, for json and ordered_json, and checks the value and its parents. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Fix clang-tidy and -Wnrvo findings in insert rollback helpers Declare the rollback helpers non-movable, replace the nested conditional and C array in object_insert_rollback, silence the intentional copy in the allocator test, and avoid the missed NRVO in the test factory lambda. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01N5NasAgbDGG28MqXjQvLqp Signed-off-by: Claude <noreply@anthropic.com> * insert(): strong guarantee for single elements, basic for the rest Rework of the previous two commits, following the discussion in #5996: the standard containers give the strong guarantee only for a single element (std::vector: at the end, or if the exception does not come from copying T; std::map: single-element insert), and copying a basic_json allocates. Rebuilding a rollback for the count, range and object-range overloads on top of array_t/object_t made the code verbose and slower. - insert(pos, const basic_json&): at end() forward as before (the standard covers it); elsewhere copy first and insert by move, as libstdc++ does internally (libc++ shifts the tail before copying). - insert_iterator() and object insert(first, last) set the parents in a scope guard, so a failed insertion leaves a valid value with JSON_DIAGNOSTICS too (the next assert_invariant() aborted before). - Docs: strong guarantee for (1), basic guarantee for (2)-(5); the argument checks throw before anything is changed. - Test: allocation-failure sweep checks the strong guarantee for (1) and validity, type and parents for the other overloads. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * insert(): document the exception safety of overloads 2 to 5 once The four basic-guarantee statements were identical up to array/object. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * insert(): simplify the wording of the basic guarantee Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me> Signed-off-by: Claude <noreply@anthropic.com> Co-authored-by: Claude <noreply@anthropic.com>
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