Re: [Boost-users] enable_if and multiple partial specializations

On Fri, Apr 8, 2011 at 11:04 AM, Joel Falcou wrote:

On 08/04/11 17:05, Gennadiy Rozental wrote:

...

This will not really work for me. This was just an artificial example. In reality I have generic header which defines the template, another common header which defines some not very specific specialization and multiple (number changes as project grows) headers where I define more specific specializations. I can't "NOT" all the specific conditions in "scalar" level specialization - It's just not maintainable.

Any other options?

- have a first level function enable_if on the highest catching metafunction then do tag dispatching inside to other function enabled_if on specialization of said initial test.

- build sensible, non overlapping traits for your cases and use these instead of raw type traits.

enable_if cant work with overlapping conditions.

This suggestion really is your best bet. To elaborate a bit further, you don't really even want to use enable_if in this case. You really want to write something like: #include #include #include #include <iostream> struct tag_other{}; struct tag_scalar{}; struct tag_enum : tag_scalar{}; // there are many better ways of writing tag_lookup. // This is just an example. template<typename T> struct tag_lookup { typedef typename boost::mpl::if_< boost::is_scalar<T>, typename boost::mpl::if_< boost::is_enum<T>, tag_enum, tag_scalar >::type, tag_other

::type type; };

template<typename T> struct M_impl; template<> struct M_impl { static void foo() { std::cout << "generic" << std::endl; } }; template<> struct M_impl { static void foo() { std::cout << "scalar" << std::endl; } }; template<> struct M_impl { static void foo() { std::cout << "enum" << std::endl; } }; template<typename T> struct M : M_impl< typename tag_lookup<T>::type > {}; struct S {}; enum FOO { FOO1, FOO2 }; int main() { M<S>::foo(); M<int>::foo(); M<FOO>::foo(); } -- Stirling Westrup Programmer, Entrepreneur. https://www.linkedin.com/e/fpf/77228 http://www.linkedin.com/in/swestrup http://technaut.livejournal.com http://sourceforge.net/users/stirlingwestrup