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

Victor Bazarov <v.bazarov@comcast.invalid>
Thu, 21 Oct 2010 10:14:46 -0400
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...


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

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

//does not work
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 {};

    struct UserTypeC {};

//Works but cumbersome for users


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

I am not familiar with that macro.

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