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Niels Lohmann
2241161307
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>