Why does returning a reference to a local variable work in this case?

I know there are a ton of questions asking something similar, but I couldn’t find anything for my exact situation (operator overload, pass by copy).

As far as I know, returning a reference to a local variable and then using it should result in undefined behaviour. But it seems to be working in my case. Is it simply a case of undefined behaviour can also mean it will work sometimes, or am I overlooking something?

// complex.cc
    Complex& operator/(Complex lhs, const Complex& rhs)
    {
        return lhs /= rhs;
    }
// inside main
    complex::Complex a{3, -6};
    complex::Complex b{1, -5};
    complex::Complex res{a / b};
    std::cout << a << '\n' << b << '\n' << res << std::endl;

As I understand it, shouldn’t there be a copy of a created, when I pass it to the overloaded operator, which then returns a reference to the modified copy? Is that understanding wrong, or why does it work when I print it?

>Solution :

But it seems to be working in my case. Is it simply a case of undefined behaviour can also mean it will work sometimes, or am I overlooking something?

You are not. The behavior is undefined, which means that it may look like it’s working.

As I understand it, shouldn’t there be a copy of a created, when I pass it to the overloaded operator, which then returns a reference to the modified copy?

That’s also correct – and the copy goes out of scope when the function returns so the returned reference is a dangling reference.