mpl::if_

[nobody]

There is an alternative implementation of mpl::if.
This implementation makes code clearer.

We can write:
typename if_
<
  X,
  Y
>
::else_if
<
  Z,
  U
>
::else_
<
  V
>::type

The implementation can be found here:
http://cbear.berlios.de/meta/if.hpp

[Dave Abrahams]

I disagree; it's far too verbose, and thus less clear, when you consider the need for the template keyword, which you left out.

  typename if_
  <
    X,
    Y
  >
  ::template else_if
  <
    Z,
    U
  >
  ::template else_
  <
    V
  >::type

[anonymous]

Replying to dave:

I disagree; it's far too verbose, and thus less clear, when you consider the need for the template keyword, which you left out.

Ok, lets compare:

1 condition:

// Boost
typedef Mpl::if_<C, T, E>::type X;

// cbear
typedef Meta::if_<C, T, E>::type X;

5 conditions:

// Boost
template<class C1, class C2, class C3, class C4, class C5>
struct X: 
        Mpl::if_<C1, T1, 
        typename Mpl::if_<C2, T2, 
        typename Mpl::if_<C3, T3, 
        typename Mpl::if_<C4, T4, 
        typename Mpl::if_<C5, T5, 
        E
        >::type 
        >::type 
        >::type 
        >::type 
        >
{
};

// cbear
template<class C1, class C2, class C3, class C4, class C5>
struct X:
        Meta::if_<C1, T1>::
        template else_if<C2, T2>::
        template else_if<C3, T3>::
        template else_if<C4, T4>::
        template else_if<C5, T5>::      
        template else_<E>
{
};

enable if T is integer:

// Boost
template<class T>
T foo(T t, typename boost::enable_if<boost::is_integral<T> >::type* dummy = 0);

// cbear
template<class T>
T foo(T t, typename Meta::if_<boost::is_integral<T> >::type* dummy = 0);

enable if T is integer (returns T) or pointer (returns int):

// Boost
template<class T>
typename Mpl::lazy_enable_if<
        Mpl::or_<boost::is_integral<T>, boost::is_pointer<T> >, // !!!
        Mpl::if_<boost::is_integral<T>, T, int> >::type
foo(T t);

// cbear
template<class T>
typename Meta::if_<boost::is_integral<T>, T>::
template else_if<boost::is_pointer<T>, int>::type
foo(T t);

I would agree with some other arguments. But not with 'it's far too verbose, and thus less clear'. That's false!

[Dave Abrahams]

Sorry, I hadn't looked at your implementation, so I assumed that you had to use else_ rather than a third argument. I'm beginning to like your suggestion. We'll have to see what Aleksey says.

[Aleksey Gurtovoy]

It's a nice extension, I like it. Let's get it in.

[sergey.junk@…]

Note: it also replaces lazy_enable_if (if_), and enable_if (if_c).

[Dave Abrahams]

Replying to sergey.junk@comrades.id.ua:

Note: it also replaces lazy_enable_if (if_), and enable_if (if_c).

I know, and I like the way it does that, too.

[anonymous]

Replying to dave:

Replying to sergey.junk@comrades.id.ua:

Note: it also replaces lazy_enable_if (if_), and enable_if (if_c).

I know, and I like the way it does that, too.

In case berlios.de dosn't work, see ​http://cbear.svn.sourceforge.net/viewvc/cbear/trunk/cbear.berlios.de/meta/if.hpp?revision=8&view=markup&pathrev=13

[NN]

This is a very good alternative. I wish boost had this feature.