Template class casting in constructor

Question

Lets say I have a class like so:

template< typename T, int nDimensions = 2 >
class Vec
{
private:
    std::array< T, nDimensions > elements_;
}

Then I typedef out a few different types.

typedef Vec< int, 2 > Vec2i;
typedef Vec< int, 3 > Vec3i;
typedef Vec< float, 2 > Vec2f;
typedef Vec< float, 3 > Vec3f;

What would the constructor be if I wanted to convert from one type to another?

Vec2i something(10,20); //10,20
Vec2f somethingElse(something); //10.0f,20.0f

Same goes for different sizes:

Vec3f somethingmore(something); //10.0f,20.0f,0.0f

So far I have:

template<typename F>
    Vec(const F& other)
    {
        for (int i = 0; i < nDimensions; i++)
        {
            this->elements_[i] = static_cast<F>(other[i]); //I know this is wrong.
        }
    }

I cant figure out a good way to get the base type of the other class to do the static casts on each element, nor a good way to get the others nDimension size so I can do proper bounds checking.

Answer 1

What would the constructor be if I wanted to convert from one type to another?

The most generic constructor would be:

template <typename T2, int nDimension2>
Vec(Vec<T2, nDimension2> const& copy) { ... }

That will need appropriate logic to make sure that you don't access memory using out of bounds indices.

A less generic constructor would be:

template <typename T2>
Vec(Vec<T2, nDimension> const& copy) { ... }

Here, you can use std::copy to copy the elements.

template <typename T2>
Vec(Vec<T2, nDimension> const& copy) { std::copy(copy.elements_.begin(),
                                                 copy.elements_.ennd(),
                                                 this->elements_.begin()); }