c++11 Return value optimization or move?

Question

I don't understand when I should use std::move and when I should let the compiler optimize... for example:

using SerialBuffer = vector< unsigned char >;

// let compiler optimize it
SerialBuffer read( size_t size ) const
{
    SerialBuffer buffer( size );
    read( begin( buffer ), end( buffer ) );
    // Return Value Optimization
    return buffer;
}

// explicit move
SerialBuffer read( size_t size ) const
{
    SerialBuffer buffer( size );
    read( begin( buffer ), end( buffer ) );
    return move( buffer );
}

Which should I use?

Answer 1

Use exclusively the first method:

Foo f()
{
  Foo result;
  mangle(result);
  return result;
}

This will already allow the use of the move constructor, if one is available. In fact, a local variable can bind to an rvalue reference in a return statement precisely when copy elision is allowed.

Your second version actively prohibits copy elision. The first version is universally better.