Tour of C++ copy elision in Section 4.2.3

Question

I'm reading Section 4.2.3 Initializing Containers, Tour of C++ Second Edition.

It says:

Vector read(istream& is)
{
     Vector v;
     for (double d; is>>d;)
         v.push_back(d);
     return v;
}

... The way to provide Vector with a move constructor, so that returning a potentially huge amount of data from read() is cheap, is explained in §5.2.2:

Vector v = read(cin);  // no copy of Vector elements here

Is it guaranteed that the above expression will be copy-elided (in C++17)? I think v in the return is an lvalue & a local variable, so it can be copy-elided but is not guaranteed to be elided. Am I missing something here?

Answer 1

Is it guaranteed that the above expression will be copy-elided (in C++17)?

I think v in the return is an lvalue & a local variable, so it can be copy-elided but is not guaranteed to be elided. Am I missing something here?

From copy_elision

Non-mandatory elision of copy/move (since C++11) operations

In a return statement, when the operand is the name of a non-volatile object with automatic storage duration, which isn't a function parameter or a catch clause parameter, and which is of the same class type (ignoring cv-qualification) as the function return type. This variant of copy elision is known as NRVO, "named return value optimization".

So NRVO is not guarantied, even in C++17.

But, if not applied, move-constructor is done (See return statement for details).