How to hide dependencies of implementation of template class?

Question

In regular C++ classes, we can have a class A as follows:

in A.h:

class A {
    public:
        void method();
    // rest of class declaration
};

in A.cpp:

#include "A.h"
#include "implementation_dependencies.h"

void A::method(){
    // use dependencies to implement method
}

But if A is a template class, this isn't possible. How can we still achieve information hiding with C++ template classes?

The use of extern template in C++11 (or global function prior to C++11 with the same effect) proves useful for this, but what to do when the template class needs to be available for all types?

Like a smart pointer for example: I can't put the definition of the class inside A.h since that would expose "implementation_dependencies.h" to anyone including "A.h".

Answer 1

From c++20, you can do this with modules:

// A.cpp
export module A;

export 
 template <typename>
 class A {
  // ...
};

#include "implementation_dependencies.h"

export template <typename T>
void A<T>::method() {
  // use dependencies, but don't export them from this module
}