如何分离子模板类的定义和声明

Question

If I have a template class defined as:

#ifndef A_HPP
#define A_HPP

template<class T>
class A
{
public:
  int doSomething(int in, bool useFirst);

private:
  template<int CNT>
  class B
  {
  public:
    int doSomething(int in);
  };

  B<2> first;
  B<3> second;
};

#include "a_imp.hpp"

#endif

Now I can go about having a declaration for A::doSomething in an implementation header file like this

#ifndef A_IMP_HPP
#define A_IMP_HPP

template<class T>
int A<T>::doSomething(int in, bool useFirst)
{
  if (useFirst)
    return first.doSomething(in);
  return second.doSomething(in);    
}

#endif

However I do not know how I go about making the declaration for the child class's method. Is it even possible or do I have to do one of the other two ways I can think of doing this, namely defining the methods in the main header or declare the class outside of A.

Please note that I am using C++11 so if this is only doable in that it will still work for me, though a C++98 solution would be good for other people.

Answer 1

You can do it in the following way:

template <class T>
template <int CNT>
int A<T>::B<CNT>::doSomething(int in)
{
    // do something
}

Answer 2

Not sure if this is what you're asking, but I guess you need something like this:

 template<class T>
 template<int CNT>
 int A<T>::B<CNT>::doSomething(int in)
 {
    return /*...*/;
 }

Note that the template keyword appears twice, first for the template parameters of A , then for the parameters of nested class B .