Copy Elision and the RVO: http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/

rvalue.cpp

std::vector<std::string> 
sorted(std::vector<std::string> names)
{
    std::sort(names);
    return names;
}
 
// names is an lvalue; a copy is required so we don't modify names
std::vector<std::string> sorted_names1 = sorted( names );
 
// get_names() is an rvalue expression; we can omit the copy!
std::vector<std::string> sorted_names2 = sorted( get_names() );