声明 object 并在同一行中调用 function 时发生意外优化

Question

I know it is not the best practice writing code like this, but it just happened.

It is basically about a function like

C &foo(C &c); // will assign value to c and return c

and a line initializing an object by a self-assignment through the function, like

C c = foo(c);

Here is my simplified code:

class C {
        public:
        C(int i): _i(i) {}
        C(const C& c): _i(c._i){}
        int _i;
};



C &foo(C &c) {
        c = 10;
        return c;
}



int main() {
        C c = foo(c);
        cout << c._i << endl;
        return 0;
}

The function yields different result if compiled with -O3. Without optimization, the result is 10 as expected. With -O3, the result becomes 0. I know gcc must optimize away something, most likely about the constructor of C, but I cannot figure it out.

If C is replaced with a primitive type, eg int, then the function yields the same result between optimized and non-optimized code.

Answer 1

This has nothing to do with optimisation; your program's behaviour is not well-defined. When you change some compiler settings you observe the consequences of that.

When you pass a reference to c into foo , it has not yet been initialised. You are allowed to use it but only in limited ways. Here you assign a result to it. Then you return the reference again. Then it is copy-initialised from itself. Clearly an initialisation cannot happen after an assignment.

Rework your approach.

Why not make it simple?

C c;
foo(c);

Answer 2

Remember that the "assignment" in the definition is really copy-construction. With

C c = foo(c);

what you do is equivalent to

C c(foo(c));

And the problem here is that when foo(c) is called, the object c haven't been constructed yet, leading to undefined behavior!