Re: [Boost-users] [MPL]...if_ and enable_if

Subject: Re: [Boost-users] [MPL]...if_ and enable_if
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2012-03-09 13:24:24

AMDG

On 03/09/2012 07:04 AM, Robert Jones wrote:
> I'm having a bit of punt into the murky world of metaprogramming, so this
> might be a bit naive...!
> 
> I've been using Boost.enable_if for a while, and now I've just encountered
> Boost.MPL.if_
> and wonder if enable_if is simply a special case of MPL.if_.
> 
> Given
> 
> struct empty { };
> template <typename T> struct my_condition : /* boost true_ or false_ */;
> 
> is
> 
> template <typename T> typename enable_if<my_condition<T>, int>::type f( ) {
> /* .... */ }
> 
> the same as
> 
> template <typename T> typename eval_if<my_condition<T>, identity<int>,
> empty>::type f( ) { /* ... */ }
> 
> Or is there some subtlety I'm missing (which is quite likely!)
> 

They aren't the same.  You're violating the preconditions of
mpl::eval_if.  This is illegal and only happens to work because
mpl::eval_if is implemented using metafunction forwarding
for most compilers.

In Christ,
Steven Watanabe