Question

I’m writing a math library as a practical exercise. I’ve run into some problems when overloading the = operator. When I debuged it, I noticed that the call to vertex1 = vertex2 calls the copy constructor instead.

In the header file I have:

//constructors
vector3();
vector3( vector3 &v );
vector3(float ix, float iy, float iz);

//operator overloading
vector3 operator =(vector3 p);
....

In the source file I implemented:

vector3 vector3::operator =(vector3 p)
{
    vector3 v3;
    v3.x = p.x;
    v3.y = p.y;
    v3.z = p.z;
    return v3;
}

Later on I have a crossproduct method, and I want to use it like so:

vector3 v3;
v3 = v1.crossProduct(v2);

The error message is:
error: no matching function for call to `vector3::vector3(vector3)’
but I do not want to call the copy constructor.

Answer 1

There are mistakes in your code. Your copy-constructor must take a const&. The reference will avoid making a copy (which you wouldn’t be able to do, being the copy-constructor), and it should be const since you’re not modifying it:

vector3(const vector3&);

Temporary variables can be bound to const&, but cannot be bound to a mutable reference. That is, with your code you could do:

vector3 a;
vector3 b(a);

but not:

vector3 a(some_calculation()); // some_calculation returns a vector3

Additionally, your operator= is incorrect. Like the copy-constructor, it should generally take a const&, but it should return a reference to this. That’s how chaining works:

int a, b, c;
a = b = c = 0;
// a.operator=(b.operator=(c.operator=(0)));

Returning a temporary is unorthodox, and doesn’t accomplish anything. In your case, you could assign over and over and never change the value. Weird:

vector 3 a, b;
a = b; // doesn't change a...?!

operator= needs to change this.