Calling a template method from another template method in C++? - c++

Question

I am currently having an issue with templated methods. I have this public class implementing a template method:
namespace Private { class InternalClass; }
namespace Public
{
class PublicClass
{
public:
PublicClass();
virtual ~PublicClass();
template<class T>
bool Add(bool primary);
private:
Private::InternalClass* _pInternal;
};
template<class T>
bool PublicClass::Add(bool primary) { return _pInternal->Add<T>(primary); }
}
The internal class is implemented that way:
namespace Private
{
class InternalClass
{
public:
InternalClass();
virtual ~InternalClass();
template <class T>
bool Add(bool primary);
};
template<class T>
bool InternalClass::Add(bool primary) { return false; }
}
As this internal class header won't be available with the provided sources, I must class forward it within the PublicClass header and I add the include to PrivateClass.h inside the PublicClass.cpp file.
1) Any idea why I would be getting the following error:
error : member access into incomplete type 'Private::InternalClass' / note: forward >declaration of 'Private::InternalClass'
2) What would be the best way of hiding my PublicClass::Add() implementation?
UPDATED
Reason for error at 1) is because of this as stated by Cornstalks.
For 2), how can I hide my implementation without including PrivateClass.h within the PublicClass header file?

Answer 1

You have encountered a very interesting problem - you want to implement the PImpl idiom where the privately implemented class has a template method. Well, this can be solved, meaning you CAN hide the template's implementation, but only when you know which types will be used to instantiate your Add<T> method in your program.
Say, your template will work only with types AClass and BClass. Then you can split your files as follows (comments are inlined):
File public.h:
#ifndef PUBLIC_H
#define PUBLIC_H
// Forward declaration ! It's sufficient in this case !
namespace Private { class InternalClass; }
// Declare all classes your Add<T> method should work with
struct AClass {};
struct BClass {};
namespace Public
{
class PublicClass
{
public:
PublicClass() {}
virtual ~PublicClass() {}
template <typename T>
bool Add(bool primary); // DO NOT implement this method, just declare
private:
Private::InternalClass* _pInternal;
};
// "Explicit instantiation declarations", for each type the method will work with:
extern template bool PublicClass::Add<AClass>(bool primary);
extern template bool PublicClass::Add<BClass>(bool primary);
}
#endif
File public.cpp:
#include "public.h"
// NOTE: this is hidden in CPP file, noone will see your implementation
namespace Private
{
class InternalClass
{
public:
InternalClass() {}
virtual ~InternalClass() {}
template <typename T>
bool Add(bool primary);
};
// Magic! Here is the actual implementation of your private method
template <typename T>
bool InternalClass::Add(bool primary)
{
return false;
}
}
namespace Public
{
// Original definition moved to CPP file !
template <typename T>
bool PublicClass::Add(bool primary)
{
return _pInternal->Add<T>(primary);
}
// And again list the allowed types, this time using "explicit instantiation definitions"
template bool PublicClass::Add<AClass>(bool primary);
template bool PublicClass::Add<BClass>(bool primary);
}
File main.cpp:
#include "public.h"
int main()
{
Public::PublicClass pc;
pc.Add<AClass>(true); // works !
pc.Add<BClass>(false); // works !
// pc.Add<int>(true); linker error as expected,
// becuase there is no explicit instantiation for Add<int>
return 0;
}