template parameter can't be friend - and we hack it to be - c++

On Wed, 20 Jun 2007 09:33:14 CST, Ralph Zhang wrote:

>gcc will complain if you try to compile this:
>
>template <class T>
>class TT
>{
> friend class T;
>};
>
>I googled and find people saying template parameters can't be friends.

In C++03, yes.

>So I do this:
>
>// Add one more indirection
>template <class T>
>struct FriendMaker
>{
> typedef T Type;
>};
[...]

>It compiles and works!

Then, please, report the bug to the g++ guys :-)

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Ralph Zhang wrote:
> gcc will complain if you try to compile this:
>
> template <class T>
> class TT
> {
> friend class T;
> };
>
> I googled and find people saying template parameters can't be friends.

Yes, it's expressly prohibited.

> So I do this:
>
> // Add one more indirection
> template <class T>
> struct FriendMaker
> {
> typedef T Type;
> };
>
> template <class T>
> class Ty
> {
> friend class FriendMaker<T>::Type;

Comeau does not compile it this way, saying "Type" cannot be used
in elaborate type specifier. But it compiles if I change it to

friend typename FriendMaker<T>::Type;

Can just be a fluke...

> private:
> int t;
> };
>

Missing:

#include <iostream>

> class A
> {
> public:
> void f(Ty<A>& ty)
> {
> std::cout << ty.t;
> }
> };
>
> int main()
> {
> Ty<A> t;
> A a;
> a.f(t);
> }
>
> It compiles and works! Although I've not thought about how this can be
> useful, i think someone might need it some day.

Curious. Thanks for sharing!

V
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Victor Bazarov wrote:

>> template <class T>
>> class Ty
>> {
>> friend class FriendMaker<T>::Type;
>
> Comeau does not compile it this way, saying "Type" cannot be used
> in elaborate type specifier. But it compiles if I change it to
>
> friend typename FriendMaker<T>::Type;
>
> Can just be a fluke...

If it is a recent version of Comeau you should disable C++0x extensions.

--
\|||/ Gennaro Prota - For hire
(o o) https://sourceforge.net/projects/breeze/
--ooO-(_)-Ooo----- (to mail: name . surname / yaho ! com)

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Gennaro Prota wrote:
> Victor Bazarov wrote:
>
>>> template <class T>
>>> class Ty
>>> {
>>> friend class FriendMaker<T>::Type;
>>
>> Comeau does not compile it this way, saying "Type" cannot be used
>> in elaborate type specifier. But it compiles if I change it to
>>
>> friend typename FriendMaker<T>::Type;
>>
>> Can just be a fluke...
>
> If it is a recent version of Comeau you should disable C++0x
> extensions.

Ah... Right. So, without C++0x extensions enabled, it won't compile,
but *with them* you just specify

friend T;

and no need for any FriendMaker. Thanks!

V
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