CODEWITHSUNDEEP
c++templatestraits
suitendaal
suitendaal

Reputation: 193

Trait instance as template parameter for different trait

I am creating a trait with a template, which has as template parameter a trait instance. The code is more complicated, but for this question I keep the code simple. The code is the following:

#include <iostream>


template<int i>
struct A
{
    static const int value = i;
};


template<typename a>
struct B
{
    static const int value = a::value;
};


int main(int argc, char *argv[]) {
  std::cout << A<3>::value << std::endl; // 3
  std::cout << B< A<3> >::value << std::endl; // 3
  return 0;
}

This works, but now I want to change typename to something like A<int i> to make sure B can only be called when you pass an instance of A<int i> as template parameter. If I do this I get the following error:

test.cpp:11:17: error: template argument 1 is invalid template<\A<\int i> a> ^ test.cpp:14:30: error: 'a' has not been declared static const int value = a::value; ^

How do I do this?

Upvotes: 1

Views: 122

Answers (1)

max66
max66

Reputation: 66210

How do I do this?

Using specialization

template <typename>
struct B;

template <int I>
struct B<A<I>>
 { static const int value = A<I>::value; }; // or also value = I;

Upvotes: 1

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