darius darius - 4 months ago 9
C++ Question

Is there an equivalent to #if that can evaluate template arguments?

The

#if
preprocessor directive evaluates expression of "constants" known to the preprocessor. Is there anything similar that can evaluate expressions on template arguments?

In practice, I have something like this:

template<int i>
class Elem{ /*...*/};

#ifndef NUM_ELEM
#define NUM_ELEM 2
#endif

class MyClass
{

#if NUM_ELEM >= 1
Elem<1> e_1;
#endif
#if NUM_ELEM >= 2
Elem<2> e_2;
#endif
#if NUM_ELEM >= 3
Elem<3> e_3;
#endif

/*...*/
}


But I'd really like to make MyClass into a template itself:

template<int num_elem>
MyClass{

#if num_elem >= 1 //but #if can't understand num_elem
Elem<1> e_1;
#endif
#if num_elem >= 2
Elem<2> e_2;
#endif
#if num_elem >= 3
Elem<3> e_3;
#endif

/*...*/
};

Answer

The short answer is no, but if you're willing to change your requirements a little, you could do something like:

// A run of elements - Elem<n>, ..., Elem<2>, Elem<1>
template <int n> class NumElems
{
    template <int u, int v> friend class ElemGetter;

    NumElems<n-1> before;

    public:
    Elem<n> e;

    // method to retrieve an element by number
    template <int m> Elem<m> &getElem();
};

// helper class to retrieve an element.
//  'n' is the element number to retrieve
//  'm' is the number of elements
// by default, ElemGetter<n,m> defers to ElemGetter<n,m-1>.
template <int n, int m> class ElemGetter
{
    public:
    static Elem<n> &getElem(NumElems<m> &numElems)
    {
        return ElemGetter<n,m-1>::getElem(numElems.before);
    }
};

// specialisation of ElemGetter: if the element to get is the same as the
// number of elements (i.e. is the last element) then return it
// immediately.
template <int n> class ElemGetter<n,n>
{
    public:
    static Elem<n> &getElem(NumElems<n> &numElems)
    {
        return numElems.e;
    }
};

// get an element by number; defers to the ElemGetter helper.
template <int n> template <int m> Elem<m> &NumElems<n>::getElem()
{
    return ElemGetter<m,n>::getElem(*this);
}

template <> class NumElems<0>
{
};

... then you can declare your Elem member set with:

NumElems<NUM_ELEM> elems;

And you can access them by using:

Elem<2> &e = elems.getElem<2>();

Original proposed code

The original code I proposed does not actually compile, but I'll include it here as it better demonstrates intent of the above:

// Original, doesn't compile - but it would be nice if it did :/
template <int n> class NumElems : private NumElems<n-1>
{
    Elem<n> e;

    template <int m> Elem<m> &getElem()
    {
        return NumElems<n-1>::getElem<m>();
    }

    template <> Elem<n> &getElem<n>()
    {
        return e;
    }
};

template <> class NumElems<0>
{
};

Unfortunately, C++ does not allow specialisation of member template functions in this way, though it's not clear (to me) why not - the code is definitely simpler without having to create a helper class, as in the working code above.