Re: making friends with static members of template arguments

From:

Barry <dhb2000@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 29 May 2008 06:57:38 CST

Message-ID:

<g1lnsk$245$1@news.cn99.com>

Barry wrote:

Oncaphillis wrote:

Hi,

I'm trying to make friends with static member functions
of template arguments:

<snip>
#include <iostream>
class some_policy {
public:
   template<class FooT>
   static void bar(const FooT *f) {
     std::cerr << f->a;
   }
};

template<class PolicyT>
class foo {
public:
   foo() : a(33) {
   }
   void bar() {
     PolicyT::bar(this);
   }
protected:
   int a;

   friend
   void PolicyT::bar< foo >(const foo *);
};

int main() {
   foo<some_policy> f;
   f.bar();
}
</snip>

So the idea is to specialize some behavior of the foo class in
class PolicyT which itself contains template static member
functions depending on the concrete foo class. I do not want
to make the whole PolicyT a template class since other static
member functions should only depend on some other internal
types of foo (kind of a replacement of partial template
specialization of foo members).

Everything works fine whenever I make 'int a' public and leave
out the 'friend..' part, but making the PolicyT::bar< foo>(...) a
friend fails

g++ 4.1.2 tells me

<snip>
test.cc:26: error: type ?PolicyT? is not derived from type ?foo<PolicyT>?
test.cc:26: error: expected ?;? before ?<? token
</snip>

Hmm... that's not very enlightening. Am I on the totally wrong trip
here or just in search for the proper syntax ?

http://www.comeaucomputing.com/techtalk/templates/#friendclassT

I can't find exact information for your problem
But I think these are quite the same,

allowing "friend T;" or friend "void T::foo(...);" violates the
encapsulation, as client programmer can define his own class or policy
function to access private/protected data.

Ironically, I came into writing the code like this

template <class Policy>
struct A {
private:
   int i_;
   friend class Policy::Impl;
};

struct Policy1
{
   struct Impl {
     void f(A<Policy1> a) { (void)a.i_; }
   };
};

int main()
{
   Policy1::Impl impl;
   impl.f(A<Policy1>());
}

This seems to violate with my post earlier.
Anyway the code above compiles with Comuea Online, GCC 4.3.0
VC8, VC with version lower than 8.0 barks with the message:

'Policy::Impl' : is not a 'class'

Is this code well-formed?

--
Best Regards
Barry

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