[vitus]

Passing std::function by value is almost definitely wrong these days with absl::AnyInvocable (if you need to store the type) and absl::FunctionRef (if you don't). Rough analogues in the standard are std::move_only_function (C++23) and std::function_ref (C++26).

std::string in the case you cited is only really relevant if you std::move() into it and you would otherwise incur a string copy. Yes, it's bigger than 16 bytes (24 bytes), but that pales in comparison to the alternative.

(Taking std::string&& would eliminate the possibility of misuse / accidental copies, but that pattern is generally discouraged by Google's style guide for various reasons.)

Also, just because you see a certain pattern an awful lot even at Google doesn't mean that it's best practice -- there are plenty of instances of protobufs being passed by value...