Question

I have an object with an operator defined like this:

P& operator +(const P &rhs) {
    return P(x + rhs.x, y + rhs.y, z + rhs.z);
}

It does not have custom copy or assignment operators.

After I assign directly the result of an addition inside the vector, garbage appears inside it.

P p1(1.0, 0.0, 0.0);
P p2(0.0, 0.0, 0.0);

vector<P> v(1);
v[0] = p1 + p2; // v[0] now contains garbage.

If I do it through a variable, everything is as expected.

vector<P> u(1);
P q = p1 + p2;
u[0] = q; // u[0] contains correct value.

What can be the reason for such behavior? What is the difference between the two cases?

Answer 1

You return a reference to a temporary. This is a bad idea, since the temporary goes out of scope at the end of the function (in this case, the operator+ function). Declare your operator as:

P operator +(const P &rhs) const

instead.