C++ Forward declaration and files design

Question

I've read lots of articles about forward declaration, but I still have a question. Let's suppose we have:

// File a.hpp (in this question I avoid writing guards in header files, for the sake of simplicity)

class A
{
    // Class B is used only by pointer, the compiler doesn't need to know the structure
    // of the class, so a forward declaration is enough
    public:
        A(void);
        void Method1(B *pB);
        void Method2(B *pB);           
};

// File a.cpp

#include "a.hpp"

A::A(void) { }

// Some methods of class B are used, so the compiler needs to know the declaration of the class, it cannot be forward declared

void A::Method1(B *pB)
{
    // Something...
    pB->SomeMethod();
    // Something ...
}

void A::Method2(B *pB)
{
    int var = pB->GetSomeMember();
    // Something ...
}

Ok, now let's suppose to have one header file for class B declaration and another one for its forward declaration:

// File b.hpp

// Class declaration
class B
{ 
/* ... */
};

// File b_fwd.hpp

// Forward declaration
class B;

What I have in mind, basing on the previous considerations, is to include "b_fwd.hpp" in a.hpp (which only needs the forward declaration of class B), and "b.hpp" in the a.cpp file (which need the declaration), as follows:

// File a.hpp

#include "b_fwd.hpp"  // Forward declaration of class B

class A
{
    public:
        A(void);
        void Method1(B *pB);
        void Method2(B *pB);           
};

// File a.cpp

#include "a.hpp"
#include "b.hpp"  // Declaration of class B

A::A(void) { }

void A::Method1(B *pB) { /* like before ... */ }

void A::Method2(B *pB) { /* like before ... */ }

I know this works, but since in class A I included (let's say) "twice" the class B, the first time forward declared and the second time "normally", this sounds a little strange to me. I want to know if this is a good practise, and if it can be done or not in projects.

Answer 1

I use this technique often with great success.

And to answer the question "why not just forward declare it?" sometimes it is difficult to forward declare. For example if a class is a template class the forward declaration must include the template parameters as well as the class name.