Using typedefs as friends

Hi all,

I'm having a problem using a typedef'd name in a friend class declaration. What I'm trying is variations on the following:

Code:

---


struct Type
{
  typedef OtherType other_type;
};

class A
{
public:
  typedef typename Type::other_type other_type;
  // ...
private:
  friend typename other_type;
};

---

Surely there's a way to make A::other_type a friend class, or am I mistaken? I can't help feel I'm missing something obvious here. Any pointers would be much appreciated.

You shouldn't have the "typename" keyword anywhere. This is not a template.

Quote:

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Originally Posted by brewbuck

You shouldn't have the "typename" keyword anywhere. This is not a template.

---

Oops, of course. That wasn't the problem though. I just got it working, but I can't say I understand why one works (Works) and not the other (Broken).

Code:

---

template<typename Type>
struct Traits
{
  typedef OtherType other_type;
};

template<typename SomeType>
class Broken
{
  typedef typename Traits<SomeType>  traits;
  typedef typename traits::other_type other_type;
  friend class other_type;
};

template<typename SomeType>
class Works
{
  typedef typename Traits<SomeType> traits;
  friend class traits::other_type;
};

---

At least, 'Broken' doesn't work on GCC 4.1, but I guess it's a language not a compiler issue. Oh well, thanks anyway.

Ok, I've experimented a bit, but I'm still hazy on the correct way to do this. I'd appreciate some advice, as my searches have been mostly fruitless.

Code:

---

template<typename T>
struct traits
{
  typedef T type;
};
 
template<typename V>
struct some_struct
{
  typedef traits<V>      traits_type;
  typedef traits<V>::type friend_type;
 
  friend class traits_type::type;    // works with GCC, not with MSVC
  friend typename traits_type::type; // works with MSVC, not with GCC
  friend typename friend_type;      // doesn't work
  friend class friend_type;          // doesn't work
};

---

Can anyone explain which, if any, compiler has it right? (note: MSVC means 7.1 and 8.0) Is this the wrong way to go about this?

P.S. I did find this:

Quote:

---

According to Section 7.1.5.3 paragraph 2 of Standard C++:

3.4.4 describes how name lookup proceeds for the identifier in an elaborated-type-specifier. ... If the identifier resolves to a typedef-name or a template type-parameter, the elaborated-type-specifier is ill-formed. [Note: this implies that, within a class template with a template type-parameter T, the declaration

friend class T;

is ill-formed. ]

---

IIUC, this means the "don't work" lines above are expected to break, but I'm not so sure about the other two...

A friend declaration MUST name a class. You cannot access this class type via a typedef. MSVC is wrong in this case.