Constant integer trait in simple class

Question

Given the following simple declaration, is it possible to give the class a constant integer trait specifying the number of components (2 in this case)? Vec3 and Vec4 would have 3 and 4 respectively. I just want this as a compile-time constant for instantiating other templates in various ways. It doesn't have to be there at runtime.

template<class T>
struct Vec2
{ 
    typedef T value_type;
    typedef unsigned index_type;

    struct 
    {
        T x, y; 
    };
};

Answer 1

The most portable way would be to add an enum constant:

template<class T> struct Vec2
{
    enum { num_components = 2 };
};
template<class T> struct Vec3
{
    enum { num_components = 3 };
};

Then just use V::num_components where needed.

If you are into C++11 then you can also use static const int num_components = 2; instead of an anonymous enum, but if you need compatibility with old compilers, then the enum idiom will save you some headaches.

Answer 2

A solution would have you specifying policies to do exactly that

// base class to store members
template<size_t N>
struct Coords; 

template<>
struct Coords<2> {
    double x, y; 
}; 

template<>
struct Coords<3> {
    double x, y, z; 
}; 

template<>
struct Coords<4> {
    double x, y, z, w; 
}; 

// members depend on your base class
template<class T, size_t N>
struct Vec : Coords<N>
{
    using num = integral_constant<size_t, N>; 
};

Now the following exists per type (note that num is a type so it doesn't occupy any space) :

Vec<2> -> x, y        and   num::value = 2 (`constexpr` - compile time usable)        
Vec<3> -> x, y, z     and   num::value = 3              //  
Vec<4> -> x, y, z, w  and   num::value = 4              //

Unless you specifically want the named members style I'd suggest using a meber that contains all values, like

double _coords[N]; // N is a compile time constant so you can have arrays

because such a solution would be more scalable, generic and easier to code.

PS I'm using double in place of T to simplify the examples.