Boost C++ Libraries: Ticket #799: mpl::if_

status, resolution, description changed; severity set

Dave Abrahams — Fri, 06 Jul 2007 01:50:54 GMT

status assigned → closed
resolution None → invalid
description modified (diff)
severity → Showstopper

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

status changed; resolution deleted

anonymous — Wed, 05 Sep 2007 08:22:19 GMT

status closed → reopened
resolution invalid

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!

owner, status changed

Dave Abrahams — Wed, 05 Sep 2007 21:07:06 GMT

owner changed from nobody to Aleksey Gurtovoy
status reopened → new

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.

status changed

Aleksey Gurtovoy — Thu, 06 Sep 2007 05:33:27 GMT

status new → assigned

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

sergey.junk@… — Thu, 06 Sep 2007 06:08:07 GMT

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

Dave Abrahams — Thu, 06 Sep 2007 10:38:54 GMT

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 — Thu, 06 Sep 2007 13:45:47 GMT

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

component changed

Dave Abrahams — Thu, 25 Oct 2007 23:11:56 GMT

component None → mpl

severity changed

Daniel James — Wed, 13 May 2009 06:46:30 GMT

severity Showstopper → Problem

owner, status changed

anonymous — Mon, 03 Jan 2011 17:53:31 GMT

owner changed from Aleksey Gurtovoy to Joel Falcou
status assigned → new

version changed; cc, milestone set

Joel Falcou — Mon, 03 Jan 2011 17:55:58 GMT

cc joel.falcou@… added
version None → Boost 1.45.0
milestone → Boost 1.46.0

NN — Thu, 06 Jan 2011 10:33:31 GMT

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