Re: How to specialize a class template in a different namespace than the current one?

From:

Victor Bazarov <v.bazarov@comcast.invalid>

Newsgroups:

comp.lang.c++

Date:

Thu, 21 Oct 2010 10:14:46 -0400

Message-ID:

<i9phso$erp$1@news.eternal-september.org>

On 10/21/2010 9:24 AM, Patrik Kahari wrote:

A common pattern is to have users register their classes with a
framework. By having users specialize a templated class, located in
the framework namespace, with their user types. To make this easier
for users a macro function is often used. The problem becomes that
users can not use this macro from within any user namespaces. This
makes the macro less useful. Is there a way around this?

If a class template is declared in a namespace, its specializations need
to be in the same namespace, otherwise they are not its specializations.
It's quite a limitation, but considering the rules for name lookup,
it's inevitable, I think.

Perhaps specializations aren't what you need. If you can explain what
you're trying to accomplish with the specializations, an alternative
solution could be worked out...

<CODE>

namespace LibNs1 {
namespace LibNs2 {
    template<typename T>
    struct RegsiterClass {};
}
}

//Works but expects to be called from root namespace which is
cumbersome for users
#define REGISTER_CLASS_WITH_LIB_V1(T) \
namespace LibNs1 { \
namespace LibNs2 { \
template<> \
struct RegsiterClass< T> {}; \
}} \

//does not work
#define REGISTER_CLASS_WITH_LIB_V2(T) \
template<> \
struct ::LibNs1::LibNs2::RegsiterClass< T> {}; \

namespace UserNs1 {
namespace UserNs2 {
namespace UserNs3 {
    struct UserTypeA {};
    REGISTER_CLASS_WITH_LIB_V2(UserTypeA); /* error:
'LibNs1::LibNs2::RegsiterClass<UserNs1::UserNs2::UserNs3::UserTypeA>' :
symbol cannot be defined within namespace 'UserNs3' */

    struct UserTypeB {};
    REGISTER_CLASS_WITH_LIB_V2(UserTypeB);

    struct UserTypeC {};
    REGISTER_CLASS_WITH_LIB_V2(UserTypeC);
}}}

//Works but cumbersome for users
REGISTER_CLASS_WITH_LIB_V1(UserNs1::UserNs2::UserNs3::UserTypeA);
REGISTER_CLASS_WITH_LIB_V1(UserNs1::UserNs2::UserNs3::UserTypeB);
REGISTER_CLASS_WITH_LIB_V1(UserNs1::UserNs2::UserNs3::UserTypeC);

</CODE>

For example this issue can be seen with the boost::serialize
BOOST_CLASS_EXPORT macro.

I am not familiar with that macro.

V
--
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