Question

I understand, or at least have an Idea of, why the following code does not work:

class Spambar {
    public:
        Spambar() {};
        Spambar(Spambar& sb) {};

        Spambar operator + (Spambar sb) {
            Spambar new_sb;
            return new_sb;
        }
};

int main() {
    Spambar sb1;
    Spambar sb2;
    Spambar sb3 = sb1 + sb2;  // <<< Error: "No matching function for call to ... "
}

I guess, the problem is that the copy-constructor expects a reference to a Spambar instance. As no reference but a shallow instance is returned, the compilation fails.

So, how do I get that to work?

Answer 1

The problem is that the result of sb1 + sb2 is a temporary; the copy constructor used to initialise sb3 requires a non-const reference; and you can’t take a non-const reference to a temporary.

You almost certainly want to fix this by changing the constructor’s parameter type to Spambar const &. While you’re at it, you should almost certainly do the same to operator+, and also make the operator itself const:

Spambar(Spambar const &);
Spambar operator + (Spambar const &) const;

If you’re doing something very strange, and actually want the copy-constructor to modify its argument, then you’ll have to either avoid passing temporaries to it, or do some nasty hackery with mutable or const_cast. In C++11, you would use a move constructor, with parameter type Spambar &&, for this sort of thing.