[eco]

This is done so you can use std::move to take ownership of the allocated memory in these objects rather than do a new allocation. Passing by value rather than rvalue reference let's your function be more flexible at the call site. You can pass an rvalue/move or just make a copy at the call site which means the caller (who actually knows if a copy or a move is more appropriate) gets to control how the memory gets allocated.

An unnecessary memory allocation is much more of a performance hit than suboptimal calling convention.

[jstimpfle]

In that case, the optimal interface should take std::string&& no? But it's awkward.

[wbkang]

The vanilla interface often allows what's right depending on the context (e.g., if an unnamed return value is passed in, it can be move constructed, otherwise it can be copied). I saw some code bases provide an override for a const ref and a normal type.

[andersa]

Wouldn't this be very annoying to work with, because now you have to explicitly move or copy the string whenever you want to construct one of these objects?

[nitnelave]

It's kinda what Rust forces you to do, except that std::move is implied. Anything taken by value is equivalent to taking by && unless the type is explicitly marked as Copy (i.e. it can be trivially copied and the copies are implicit).

But yeah, in a c++ codebase, good modern practices are often verbose and clunky.

[ismailmaj]

The function could accept an universal reference instead of an rvalue reference, this avoids the dance the caller has to do to pass a copy.

IMO it's hard to beat pass by value considering both performance and cognitive load.

[mort96]

Yeah, making it accept a universal reference would fix it...

...but that requires the argument to be a type from a template D: so you'd have to write:

    template<typename String = std::string>
    void dosomething(String &&str)

...and that's not quite right either, since you'd want it to be either an rvalue reference or a const lvalue reference

[secondcoming]

Explicity having to copy or move is a desired coding style IMO.