CRTP and template template?

Question

I would like to do CRTP for template classes and I want the abstract base class to know about the template parameters of the derived classes.

I tried this but it does not work :

template<template<class T, unsigned int TDIM> class TCRTP> class NAbstract
{
    T _data[TDIM];
};

template<typename T, unsigned int TDIM> class NArray : NAbstract< NArray<T, TDIM> >
{

};

template<typename T, unsigned int TDIM> class NVector : NAbstract< NVector<T, TDIM> >
{

};

g++ 4.6.2 tells me :

main.cpp|5|error: 'T' does not name a type|
main.cpp|8|error: type/value mismatch at argument 1 in template parameter list for 'template<template<class T, unsigned int TDIM> class TCRTP> class NAbstract'|
main.cpp|8|error:   expected a class template, got 'NArray<T, TDIM>'|
main.cpp|13|error: type/value mismatch at argument 1 in template parameter list for 'template<template<class T, unsigned int TDIM> class TCRTP> class NAbstract'|
main.cpp|13|error:   expected a class template, got 'NVector<T, TDIM>'|

What is the problem and what would be the good solution for a such thing ?

Answer 1

One solution is to specialize the template partially:

template <typename> struct NAbstract;

template <typename T, unsigned int N>
struct NAbstract<NArray<T, N>>
{
    T _data[N];
    // ...
};

Alternatively:

template <template <typename, unsigned int> class Container,
          typename T, unsigned int N>
struct NAbstract<Container<T, N>>
{
    T _data[N];
    // ...
};

Answer 2

Below demonstrates your code after fixing errors:

template<class T, unsigned int TDIM, template<class, unsigned int> class TCRTP>
class NAbstract
{
  T _data[TDIM];
};

template<typename T, unsigned int TDIM>
class NArray : NAbstract< T, TDIM, NArray >
{
};

template<typename T, unsigned int TDIM>
class NVector : NAbstract< T, TDIM, NVector >
{
};

You cannot use template template parameters. In fact, compiler simply ignores them.
e.g. From your original code, T and TDIM are ignored in below line:

template<template<class T, unsigned int TDIM> class TCRTP> class NAbstract
                       ^^^              ^^^^  // <--- ignored

Answer 3

You can accomplish this with traits. Then, you don't need a template template parameter for NAbstract.

template<class TCRTP>
class NAbstract {
    typename TCRTP::value_type data_[TCRTP::DIM];
};

template<typename T, unsigned int TDIM>
struct NTraits {
    typedef T value_type;
    enum { DIM = TDIM };
};

template<typename T, unsigned int TDIM>
struct NArray : NAbstract< NTraits<T, TDIM> > {
};

template<typename T, unsigned int TDIM>
struct NVector : NAbstract< NTraits<T, TDIM> > {
};