Re: [boost] Interesting variadic template trick

Subject: Re: [boost] Interesting variadic template trick
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2013-11-17 23:42:46

Louis Dionne <ldionne.2 <at> gmail.com> writes:

>     template <typename ...T> void allow_expansion(T&&...);
> 
>     template <typename ...T>
>     struct or_ {
>         static constexpr bool value = !noexcept(
>             allow_expansion((T::value ? throw : 0)...)
>         );
>     };

How embarrassing; my unit test was very poor and the above trick
does not seem to work. However, the following was tested more rigorously
and works (with similar performance improvements):

    template <typename ...T>
    constexpr bool all_pointers(T*...) { return true; }

    template <typename ...T>
    constexpr bool all_pointers(T...) { return false; }

    template <typename ...T>
    struct or_ {
        static constexpr bool value = !all_pointers(
            ((typename std::conditional<T::value, int, void*>::type)0)...
        );
    };

    template <>
    struct or_<> {
        static constexpr bool value = false;
    };

Regards,

Louis Dionne