Re: [Boost-bugs] [Boost C++ Libraries] #799: mpl::if_

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #799: mpl::if_
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2007-09-05 08:22:20

#799: mpl::if_
-------------------------------+--------------------------------------------
  Reporter:  nobody            |       Owner:  nobody     
      Type:  Feature Requests  |      Status:  reopened   
 Milestone:                    |   Component:  None       
   Version:  None              |    Severity:  Showstopper
Resolution:                    |    Keywords:             
-------------------------------+--------------------------------------------
Changes (by anonymous):

  * status:  closed => reopened
  * resolution:  invalid =>

Comment:

 Replying to [comment:1 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:
 {{{
 #!cpp
 // Boost
 typedef Mpl::if_<C, T, E>::type X;

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

 5 conditions:
 {{{
 #!cpp
 // 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:
 {{{
 #!cpp
 // 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):
 {{{
 #!cpp
 // 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!

-- 
Ticket URL: <http://svn.boost.org/trac/boost/ticket/799#comment:2>
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