overloading operator+ (heap? stack?)

[thinhare]

Hello,

I am reading some c++ tutorial about operator overloading, and the following code given as an example for operator+ overloading has had me very much confused.

const MyClass MyClass::operator+(const MyClass &other) const {
    MyClass result = *this;     // Make a copy of myself.  Same as MyClass result(*this);
    result += other;            // Use += to add other to the copy.
    return result;              // All done!
}

As I understand it, "MyClass result" only creates the object on stack, so it is only "alive" within the scope of the function, and will be destoried at the end of the function?

[cyberfish]

"return" will make a copy and return that.

The original "result" will be destroyed.

[iMalc]

Returning by value is just like passing by value - a copy is made. At least that's how you should think about it. In practice RVO might optimise the copy away whilst keeping the intent of the code the same.

There are at least a dozen other ways it could have been implemented too. Here's a few of them:

const MyClass MyClass::operator+(MyClass other) const {
    other += *this;
    return other;
}

const MyClass MyClass::operator+(MyClass other) const {
    return other += *this;
}

const MyClass MyClass::operator+(const MyClass &other) const {
    return MyClass(*this) += other;
}

The two-parameter non-member version, which is better as it allows implicit conversion of both arguments:

const MyClass operator+(const MyClass &lhs, const MyClass &rhs) {
    return MyClass(lhs) += *rhs;
}

Even shorter variant on the above (though less symmetrical looking):

const MyClass operator+(MyClass lhs, const MyClass &rhs) {
    return lhs += *rhs;
}

In every single one of the above, one copy is made of one of the parameters, the other parameter is added to this (without copying it), and the resulting value is copied upon the return.

[thinhare]

Thanks for the answer.

I know pass by reference and pass by value, but haven't paied any attention to return by value and reture by reference before, and apparently they are two apples on the same tree. :-)