Optimal way to return local value in C++11

Question

In the old days, if I wanted a string representation of an object A, I would write something with the signature void to_string(const A& a, string& out) to avoid extra copies. Is this still the best practice in C++11, with move semantics and all?

I have read several comments on other contexts that suggest relying on RVO and instead writing string to_string(const A& a). But RVO is not guaranteed to happen! So, how can I, as the programmer of to_string, guarantee the string is not copied around unnecessarily (independently of the compiler)?

Answer 1

Here is the answer I gathered from feedback and other resources:

The straightforward return by value is the idiom because:

• in practice copy/move elision will take place most of the time;
• the move ctor will be used on fallback;
• preventing the improbable case of a copy actually happening is not worth the less readable code
• passing the reference in requires the object to have already been created
  • not always feasible (for instance, there might be no default ctor) and which
  • one initialization too much also has to be taken into account if the issue is performance

However, if the typical usage is anticipated to be something like

std::string s;
while (i_need_to)
{
    to_string(get_A(), s);
    process(s);
    update(i_need_to);
}

and if the type in question has a default constructor*, then it may still make sense to pass the object that should hold the return by reference.

*considering string here only as an example, but the question and answers could be generalized