No type named 'type' in CTRP derived class

Question

I\'ve been experimenting with the Curiously Recurring Template Pattern for a generic single-argument functor and have two implementations: one using a template template para

Answer 1

When the line

using Ftype = typename Functor::type;

is processed in the base class, the definition of Functor is not available. Hence, you can't use Functor::type.

One way to get around this limitation is to define a traits class.

// Declare a traits class.
template  struct FunctorTraits;

template
class FunctorInterface_2 {
   private:
      const Functor &f_cref;
   public:

      // Use the traits class to define Ftype
      using Ftype = typename FunctorTraits::type;

      FunctorInterface_2() : f_cref(static_cast(*this)) {}
      Ftype operator() ( Ftype val ) const { return f_cref(val); }
}; // FunctorInterface_2 (no type in Functor!)

// Forward declare Cube to specialize FunctorTraits
template struct Cube;

// Specialize FunctorTraits for Cube
template  struct FunctorTraits>
{
   using type = T; 
};

template
struct Cube : public FunctorInterface_2> {
   using type = T; 
   T operator() ( T val ) const { return val*val*val; }
}; // Cube

Working code: https://ideone.com/C1L4YW