Specialization of a template within template

[Nikolai Borissov]

Is it possible to specialize a templated class defined within another
templeted class, like below:

template<typename U>
struct A
{ template<typename T>
struct B;
};

// Attempt to specialize internal class B for type int
template<typename U>
struct A<U>::B<int> {};

typedef A<long>::B<int> MyType; // this causes a failure

Online Comeau C++ fails without a message.
What is the right syntax, if any, for such specializations ?

Thanks,

Nikolai Borissov

[David B. Held]

"Nikolai Borissov" <n.********@sympatico.ca> wrote in message
news:iV********************@news20.bellglobal.com. ..

Is it possible to specialize a templated class defined within
another templeted class, like below:
[...]

I've been told that it isn't legal C++, but looking at 14.7 makes
me think otherwise. I don't have my copy of C++T handy, so
I can't look there. You might want to ask in c.l.c++.m, as some
more knowledgeable readers tend to visit there.

Dave

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[Rob Williscroft]

Nikolai Borissov wrote in news:iVGmb.10683$7t3.492363
@news20.bellglobal.com:

Is it possible to specialize a templated class defined within another
templeted class, like below:

template<typename U>
struct A
{ template<typename T>
struct B;
};

// Attempt to specialize internal class B for type int
I changed this to:
template <typename U> template <>
struct A<U>::B<int> {};

gcc 3.2:
test.cpp:14: enclosing class templates are not explicitly specialized
test.cpp:15: template parameters not used in partial specialization:
test.cpp:15: `U'

msvc 7.1: test.cpp:14: error C3212: 'A<U>::B<int>' : an explicit
specialization of a template member must be a member of an explicit
specialization
test.cpp:15: see declaration of 'A<U>::B<int>'

online EDG: a template declaration containing a template parameter list
may not be followed by an explicit specialization declaration
template <typename U> template <>
template<typename U>
struct A<U>::B<int> {};

typedef A<long>::B<int> MyType; // this causes a failure

Online Comeau C++ fails without a message.
What is the right syntax, if any, for such specializations ?

I don't think there is any.

I'd suggest switching the problem around:

template <typename T>
struct B_inner
{
template<typename U>
struct A_outer {};
};
/* Specialize this as meny times as we want
*/
template <>
struct B_inner< int >
{
template <typename U>
struct A_outer
{
};
};

template<typename U>
struct A
{
template <typename T>
struct B : public B_inner< T >::template A_outer< U >
{
};
};

typedef A<long>::B<int> MyType; // this causes a failure

HTH

Rob.
--
http://www.victim-prime.dsl.pipex.com/