Question

Consider the following:

ComplexObject foo()
{
    ComplexObject temp;
    //Do things with temp
    ComplexObject result(temp, SOME_OTHER_SETTING); //1
    //Do things with result. Do not use temp at all
    return result; //2
}

ComplexObject foo()
{
    ComplexObject temp;
    //Do things with temp
    ComplexObject result(std::move(temp), SOME_OTHER_SETTING); //1
    //Do things with result. Do not use temp at all
    return std::move(result); //2
}

with the assumption that ComplexObject has a move constructor which is far more efficient than it’s copy constructor.

Is the compiler allowed to effectively transform the first code into the second code, because it knows that ComplexObject cannot be used for the remainder of that block?

Answer 1

Not for temp, but the compiler can perform other optimizations under the as-if rule, which may have the same effect.

For result, there is a special rule regarding return statements, that will use a move if possible (and elision is preferred over either move or copy).