Can I write to buffer?

[JaydenFish]

std::string is not a good idea. It's better to write rfl::json::write to a buffer just like char[], std::array...
In this way I can eliminate a copy on socket or something.

[liuzicheng1987]

You can write into any std::ostream or read from any std::istream precisely for that reason.

std::array couldn't work, because you would have to know the size of the string at compile time, which is hardly ever the case.