Feature: Support unicode character types

[gracicot]

This is a feature request. I wondered if this is possible. The JSON format is UTF-8. C++11 added char16_t and char32_t for unicode characters, and lately C++20 added char8_t.

Since JSON is specified as UTF-8, this library should at least support char8_t. This add more safety since we can validate and check the character encoding at compile time, prohibiting sending strings to functions that expects another encoding.

What is the issue you have?

Right now, trying to use char8_t with std::u8string in a basic_json result in error.

Please describe the steps to reproduce the issue.

using u8json = basic_json<std::map, std::vector, std::u8string>;
u8json value = {};

Compiler explorer link

What is the expected behavior?

The u8json alias should work, and conversions to std::u8string and std::u8string_view should work instead of std::string using this alias.

auto json = u8json{
    {u8"key", u8"value"}
};

auto key = std::u8string{json[u8"key"]};

And what is the actual behavior instead?

A compilation error.

Which compiler and operating system are you using?

• Compiler: GCC (trunk)
• Operating system: Any

Which version of the library did you use?

• latest release version 3.9.0
• other release - please state the version: ___
• the develop branch

If you experience a compilation error: can you compile and run the unit tests?

• yes
• no - please copy/paste the error message below

[nlohmann]

This has been substantially addressed since: include/nlohmann/json.hpp now provides a char8_t-based operator""_json (guarded by __cpp_char8_t), and from_json/to_json have explicit char8_t/std::u8string_view conversion paths guarded by __cpp_lib_char8_t. Using std::u8string directly as a custom StringType alias may still have rough edges, but native char8_t string literal and conversion support — the core of this request — is now in place.

---

This reply was drafted by Claude Code on behalf of @nlohmann, as part of a review of open discussions.

[gracicot]

Thank you so much!