Conditional reference declaration in template class

Question

In a template class, how to define a property alias conditionally to the template?

Example:

template<class Type, unsigned int Dimensions>
class SpaceVector
{
public:
    std::array<Type, Dimensions> value;
    Type &x = value[0]; // only if Dimensions >0
    Type &y = value[1]; // only if Dimensions >1
    Type &z = value[2]; // only if Dimensions >2
};

Is this conditional declaration possible? if yes, how?

Answer 1

Specialise the first two cases:

template<class Type>
class SpaceVector<Type, 1>
{
public:
    std::array<Type, 1> value; // Perhaps no need for the array
    Type &x = value[0];
};

template<class Type>
class SpaceVector<Type, 2>
{
public:
    std::array<Type, 2> value;
    Type &x = value[0];
    Type &y = value[1];
};

If you have a common base class then you gain a measured amount of polymorphism for common functionality.

Answer 2

If you can do without the array, you could do this:

template<class Type, std::size_t Dimension>
class SpaceVector
{
public:
    Type x;
};

template<class Type>
class SpaceVector<Type, 2> : public SpaceVector<Type,1>
{
public:
    Type y;
};

template<class Type>
class SpaceVector<Type, 3> : public SpaceVector<Type,2>
{
public:
    Type z;
};

This is more scalable if you decide to support more than three elements, but otherwise, Bathsheba's answer is probably more suitable.