A base64 implementation is very small, but not completely trivial to the point where you should implement it yourself, or paste in a one liner from Stack Overflow. It's widely used with JSON. C++ has no package manager to make it easy to import tiny libraries for purposes like this, so C++ libraries should be a little more self contained than npm packages or rust crates. It would absolutely be reasonable to include a base64 implementation in a C++ JSON library for those reasons.