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

Subject: Re: [Boost-users] enable_if and multiple partial specializations
From: Robert Jones (robertgbjones_at_[hidden])
Date: 2011-04-08 05:08:15

On Fri, Apr 8, 2011 at 9:59 AM, Gennadiy Rozental <rogeeff_at_[hidden]> wrote:

> Hi,
>
> It's late and I might be missing something, but I can't figure out how can
> I use
> enable_if to disambiguate 2 partial specializations where one is subcase of
> another one:
>
> #include <boost/utility/enable_if.hpp>
> #include <boost/type_traits/is_enum.hpp>
> #include <boost/type_traits/is_scalar.hpp>
>
> #include <iostream>
>
> template<typename T, typename Enabler=void>
> struct M {
>    static void foo() { std::cout << "generic" << std::endl; }
> };
>
> template<typename T>
> struct M<T, typename boost::enable_if<boost::is_scalar<T> >::type> {
>    static void foo() { std::cout << "scalar" << std::endl; }
> };
>
> template<typename T>
> struct M<T,typename boost::enable_if<boost::is_enum<T> >::type> {
>    static void foo() { std::cout << "enum" << std::endl; }
> };
>
> struct S {};
> enum FOO { FOO1, FOO2 };
>
> int main()
> {
>    M<S>::foo();
>    M<int>::foo();
>    M<FOO>::foo();
> }
>
> ---------------
>
> M<FOO>::foo() fails to compile due to ambiguity.
>
> Any pointers? Or How to get what I want?
>
>
I'm assuming the problem is that the scalar case is also true for the enum
case?

On that basis, could you not us an enabler that asks if it is scalar && !
enum?

- Rob.

(That's just off the cuff - I 'spect a proper expert will be along soon!)