How to make a template a friend of T?

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Let's say I have a class C and a container template T_CONTAINER.
I have a function in date that I only want to expose to T_CONTAINER.
How can I do this? Note that I am using Visual C++ 2010 as a compiler.

I've had to make the functions public, which is less than desirable.

I've tried:

C++

class C;
#include "T_CONTAINER.H"
class C {
     friend T_CONTAINER< C >;
}

Posted 18-May-11 4:44am

T2102

Updated 18-May-11 5:08am

Sandeep Mewara

v2

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Niklas L · Answer 1 · 2011-05-18T05:18:00

Solution 1

Tried the following which works.

C++

template <typename T>
class A
{
    T *_t;
public:
    A(T *t) : _t(t) {}
    void accessT()
    {
        _t->privateAccess();
    }
};
class C {
    void privateAccess()
    {
        cout << "private" << endl;
    }
    friend A<C>;
};
int main()
{
    C c;
    A<C> a(&c);
    a.accessT();
    return 0;
}

With the friend declaration, the template class can access the private methods of class C. I don't know where you went wrong.

Edit: Most container classes have some default template arguments for allocators and such. Make sure they match in your friend declaration.

Posted 18-May-11 5:18am

Niklas L

Updated 18-May-11 5:28am

v3

Comments

T2102 18-May-11 11:52am

It actually looks like it's solely limited to the copy constructor. I want to avoid implicit copy construction whenever possible.

Niklas L 18-May-11 12:05pm

I added a copy constructor to C, and it still works. Then I added a copy constructor to A, but C can still touch his private parts, so to speak. What did you do to have it not working?

T2102 18-May-11 12:33pm

My template class contains an STL template class S. I added both A<c> and S<c> as friends just in case. When the copy constructor is private, I get the error C::C cannot access private member declared in class C. When the copy constructor is public, it compiles correctly.

T2102 18-May-11 12:33pm

Note that the error is pointing to xmemory line 48

T2102 18-May-11 12:37pm

I am bow guessing that an implementation detail in STL is causing the problem, but I am not sure.