CRTP and template template?
-
25-06-2021 - |
Вопрос
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 ?
Решение
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];
// ...
};
Другие советы
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
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> > {
};
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