Question

I stumbled upon something similar today, and subsequently tried a few things out and noticed that the following seems to be legal in G++:

struct A {
    int val_;
    A() { }
    A(int val) : val_(val) { }
    const A& operator=(int val) { val_ = val; return *this; }
    int get() { return val_; }
};

struct B : public A {
    A getA() { return (((A)*this) = 20); } // legal?
};

int main() {
    A a = 10;
    B b;
    A c = b.getA();
}

So B::getB returns a type A, after it as assigned the value 20 to itself (via the overloaded A::operator=).

After a few tests, it seems that it returns the correct value (c.get would return 20 as one may expect).

So I’m wondering, is this undefined behavior? If this is the case, what exactly makes it so? If not, what would be the advantages of such code?

Answer 1

After careful examination, with the help of @Kerrek SB and @Aaron McDaid, the following:

return (((A)*this) = 20);

…is like shorthand (yet obscure) syntax for:

A a(*this); 
return a.operator=(20);

…or even better:

return A(*this) = 20;

…and is therefore defined behavior.