C++ typecast overloading

Question

Supposed I have a class like this:

template<class T>
class Vector2 {
public:
    Vector2(const T a, const T b) : x(a), x(b) {}
    T x;
    T y;
}

I want to be able to do something like this:

const Vector2<double> d(31.4, 48.2);  // note the const!!!

Vector2<int> i = static_cast<Vector2<int>>(d);

// i.x == 31
// i.y == 48

I have tried overloading a generic operator but it seems to break when trying to convert from a const value. Help?

Answer 1

Provide an additional constructor that's taking another template parameter U :

template<class T>
class Vector2 {
public:
    template <class U>
    Vector2(const Vector2<U> & other) : x(other.x), y(other.y){}

    // other code ommited
};

After all, you're trying to use Vector2<T>::Vector2(const Vector2<U> &) , where U = double and T = int .

Note that this has nothing to do with your original vector being const . Instead, you're trying to construct a value of type Vector2<int> with a value of another type Value2<double> . Those are distinct types, and therefore you need to provide a constructor.

Answer 2

A possibility would be to write a cast operator which does what you want:

template<class T>
class Vector2 {
public:
    Vector2(const T a, const T b) : x(a), y(b) {}
    T x;
    T y;

    template<typename U>
    operator Vector2<U>() const { return Vector2<U>( (U)x, (U)y ); }
 // ^^^^^^^^ cast operator
};

int main()
{
    const Vector2<double> d(31.4, 48.2);  // note the const!!!

    Vector2<int> i = static_cast<Vector2<int>>(d);

    return 0;
}

An additional constructor as shown in the answer of Zeta is the much more elegant solution.