How to let a method access private members of other template class instances?

Question

This extremely minimal example will fail to compile because A<int> cannot access the private member i in A<double>

template <class T>
class A {
    int i;
  public:
    template <class U>
    void copy_i_from( const A<U> & a ){
        i = a.i;
    }
};

int main(void) {
    A<int> ai;
    A<double> ad;
    ai.copy_i_from(ad);
    return 0;
}

I know that I can make all the template instances friends of each other (see: How to access private members of other template class instances?), but since I have only one method that requires the access (like in the example) I would prefer to limit the friendship to that method. Is this possible?

To the downvoter: I'd like to know what can I do to improve the question! — DarioP, Oct 17 '14 at 10:26
Both answers and the question itself were downvoted silently; I suspect it has nothing to do with the content. — Angew is no longer proud of SO, Oct 17 '14 at 10:35

Angew is no longer proud of SO · Accepted Answer · 2014-10-17T10:18:14.697

8

Yes, it's possible. Member functions can be designated as friends normally.

template <class T>
class A {
    int i;
  public:
    template <class U>
    void copy_i_from( const A<U> & a ){
        i = a.i;
    }
  template <class F>
  template <class U>
  friend void A<F>::copy_i_from(const A<U> & a);
};

int main(void) {
    A<int> ai;
    A<double> ad;
    ai.copy_i_from(ad);
    return 0;
}

Live example (gcc one Ideone)

Note that unlike gcc, clang rejects the code. I cannot find anything in the standard that would make it invalid, though.

edited Oct 17 '14 at 10:18

answered Oct 17 '14 at 10:06

Angew is no longer proud of SO

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@jrok Works for me (er, for Ideone GGC). – Angew is no longer proud of SO Oct 17 '14 at 10:07
@PiotrS. Then why doesn't it work without the `friend` declaration? I tested that. – Angew is no longer proud of SO Oct 17 '14 at 10:08
[clang parses the declaration but seeems to ignore it.](http://coliru.stacked-crooked.com/a/f239c130f92b57f7) – jrok Oct 17 '14 at 10:09
(not the downvoter to be clear) I remember there was some example with friends when gcc was failing, I can't recall for the moment – Piotr Skotnicki Oct 17 '14 at 10:09
This worked for me with g++4.9, unfortunately not with clang++3.5, but I guess is a compiler problem. – DarioP Oct 17 '14 at 10:33

Marco A. · Answer 2 · 2014-10-17T10:41:07.043

It seems that if you want to have a friend member function, the following won't work on clang:

template <class T>
class A {
    int i;
  public:
    template <class U>
    void copy_i_from( const A<U> & a ){
        i = a.i;
    }
  template <class F>
  template <class U> friend void A<F>::copy_i_from(const A<U> & a);
};

int main(void) {
    A<int> ai;
    A<double> ad;
    ai.copy_i_from(ad);
    return 0;
}

while it works on gcc.

The issue seems to be a clang's problem with representing friend class template for which the dependent name specifier cannot be resolved in the AST: http://llvm.org/klaus/clang/commit/8b0fa5241a0416fc50dfbb7e38f20e777f191848/ (still in trunk at the time of writing this).

Therefore you could go for the member function version above although it might not work on clang until this is figured out.

A plan-B solution is to have it a free templated friend function, although it might not be what you want (accepted by both cland and gcc):

#include <iostream>
using namespace std;

template <class T>
class A {
    int i;
public:
    template<class V, class U>
    friend void copy_i_from(A<V>& t, const A<U> & a);
};

template<class V, class U>
void copy_i_from(A<V>& t, const A<U> & a){
    t.i = a.i;
}

int main(void) {
    A<int> ai;
    A<double> ad;
    copy_i_from(ai,ad);
    return 0;
}

Example

I think that this was a valuable answer even before the edit (I felt bad when you deleted it), but now is even better! — DarioP, Oct 17 '14 at 10:52

How to let a method access private members of other template class instances?

2 Answers2