What is the meaning of assigning to return value of getter taking this pointer by value?

Question

This doubt came to me when I jumped on an existing code and mistakenly used a getter to set a property,

obj.getProp() = otherProp;

instead of calling the setter,

obj.setProp(otherProp);

I did not realize the mistake because there was no error at compilation or runtime; the assignment resulted in a no-op.

So I came up with the following example, which outputs 337:

#include <iostream>

struct A {
    int x = 0;
    A(int x) : x(x) {}
    A(A& a) : x(a.x) {}
    void operator=(A const& other) { x = other.x; }
};

struct B {
    A a{3};
    int x{3};
    A  getAbyVal() { return a; }
    A& getAbyRef() { return a; }
    int getXbyVal() { return x; }
};

int main() {
    B b;
    std::cout << b.a.x;   // this and the other two cout print what I expect, but...
    b.getAbyVal() = A{7}; // ... I expected this to fail at compilation time in the first place...
    //b.getXbyVal() = 3;  // ... just like this fails.
    std::cout << b.a.x;
    b.getAbyRef() = A{7};
    std::cout << b.a.x;

}

So my question is two folds:

• what in b.getAbyVal() = A{7}; is different from b.getXbyVal() = 3; so that the former compiles and the latter doesn't (beside the fact that the types are A and int)?
• changing void operator=(A const& other) { x = other.x; } to void operator=(A const& other) & { x = other.x; } makes b.getAbyVal() = A{7}; fail to compile. Why is this the case?

Answer 1

what in b.getAbyVal() = A{7}; is different from b.getXbyVal() = 3; so that the former compiles and the latter doesn't (beside the fact that the types are A and int)?

Surprisingly, the difference in types is exactly what makes one compile correctly, and other to fail.

A has an assignment operator defined for it, so compiler dutifully invokes it on the return value (only to discard the whole object later). But the code you wrote supports this. From the compiler view, some other interesting things might have happened in your assignment operator, despite the fact that the object will be eradicated (side effects in formal parlance).

With int as a return value, compiler knows there are no side effects of assigning value to int, so assigning any value to object which is to be eradicated immediately does not make any sense.