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];
    // ...
};