json/home/releases/unreleased.md

Unreleased

These changes are part of the develop branch, but not of a release yet.

• Converting an rvalue container to JSON (json j = std::move(vec);, json j = std::map<std::string, std::string>{...};) now moves the elements instead of copying them (e.g., std::vector<std::string> with 100,000 strings: 4.7 ms to 2.3 ms). Two consequences: the source container is left in a valid but unspecified (moved-from) state, and if an exception is thrown during the conversion, the source may be partially moved-from, so the strong exception guarantee no longer holds for the source of an rvalue conversion (the JSON value itself is unaffected). Lvalues and const objects are still copied. See basic_json.
• In C++20 (library support for char8_t, __cpp_lib_char8_t), a #!cpp std::u8string is a JSON string: json j = std::u8string(u8"abc"); stores "abc", and #!cpp j.get<std::u8string>() reads it back. Version 3.12.0 stored a #!cpp std::u8string as an array of code units ([97,98,99]); on the 3.13.0 development branch it was already stored as a string if std::filesystem is available, but could not be read back. Now the result no longer depends on whether std::filesystem::path is supported, and #!cpp get<std::u8string>() accepts both a JSON string and the array of numbers written by 3.12.0. Consequence: JSON documents produced from a #!cpp std::u8string by 3.12.0 (arrays) are still read, but code that checks is_array() on such a value, or that reads it as #!cpp std::vector<char8_t> / #!cpp std::vector<std::uint8_t>, now sees a string; to keep the array form, convert explicitly, e.g., #!cpp json(std::vector<char8_t>(s.begin(), s.end())). #!cpp std::u8string_view and #!cpp char8_t arrays are unchanged (arrays of code units). See basic_json.

API changes