Re: [boost] Is there BOOST_ENABLE_IF macro now?

Subject: Re: [boost] Is there BOOST_ENABLE_IF macro now?
From: paul Fultz (pfultz2_at_[hidden])
Date: 2013-08-18 16:16:33

> From: TONGARI J 
>Hi folks,
>
>It's over 2 years since this post:
>http://boost.2283326.n4.nabble.com/New-powerful-way-to-use-enable-if-in-C-0x-td3442723.html
>
>The macro looks sweet and I wonder if we have this macro in Boost in case
>that I missed it.
>

This is nice for C++11 and conversion operators, I was not aware of this
trick. However, it would be nice if boost provided requires macros to make it
easier to specify traits for enable_if. See here for an implementation:

    https://github.com/pfultz2/Zen/blob/master/zen/requires.h 

So functions can be defined like this:

    template<class T, class U>
    ZEN_FUNCTION_REQUIRES(is_arithmetic<T>, is_arithmetic<U>)
    (T) max(T x, U y)
    {
        return x > y : x ? y;
    }

All the traits our inclusive by default, but the `exclude` keyword can be
used to exclude the trait, like this:

    template<class T, class U>
    ZEN_FUNCTION_REQUIRES
    (
        is_arithmetic<T>, 
        is_arithmetic<U>, 
        exclude is_same<T, bool>, 
        exclude is_same<U, bool>
    )
    (T) max(T x, U y)
    {
        return x > y : x ? y;
    }

It can be used in classes like this:

    template<class T, ZEN_REQUIRES(is_integral<T>)>
    class A { ... };

Or this:

    template <class T, class Enable = void> 
    class A { ... };

    template <class T>
    class A<T, ZEN_CLASS_REQUIRES(is_integral<T>)> { ... };

    template <class T>
    class A<T, ZEN_CLASS_REQUIRES(is_float<T>)> { ... };

This will all work on C++03 compilers as well. It could also be extended
fairly easily to support the trick from Matt Calabrese. Plus, the
`FUNCTION_REQUIRES` seems like it could be easily converted to use the
`requires` and concept overloading when it gets added to the language as well.

What do you think?

Thanks,
Paul Fultz II