Question

I have a class C which is templated on A<W> or on B<W>. Now in C, I’d like to construct an object of type A<U> or B<U>, depending on what it was instantiated with.

If that sounds a bit strange, consider this code and the comment in it:

template<class W>
struct A {
  typedef A type;
};

template<class W>
struct B {
  typedef B type;
};

template<class AB>
struct C {
  // AB is A or B. If it's A we want to construct A<double>, if it's B                                                          
  // we want to construct B<double>:                                                                                            
  typedef typename AB::type type; // A or B                                                                                     
  typename type<double> D; // ERROR
  D d;
};

int main(int argc, char** argv){
  C<A<int> > c1;
  C<B<int> > c2;
}

Is there any way to do this?

I think C would need to be templated on a nested template, but I’m not sure how to do it.

Answer 1

To do that, you need partial template specifications:

// base declaration is undefined
template< typename AorB > struct C;

// declaration for A<W>
template< typename W >
struct C< A< W > >
{
    typedef A< double > type;
};

// declaration for B<W>
template< typename W >
struct C< B< W > >
{
    typedef B< double > type;
};

A more general case that works for any template class with one type argument would be:

// base declaration is undefined
template< typename AorB > struct C;

// declaration for T<W>
template< template< typename > class T, typename W >
struct C< T< W > >
{
    typedef T< double > type;
};