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

on Fri Mar 09 2012, Steven Watanabe wrote:

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::type f( ) { /* .... */ }

the same as

template <typename T> typename eval_if::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.

But there is an implementation of if_ that would actually work as a substitute for enable_if: https://svn.boost.org/trac/boost/ticket/799 -- Dave Abrahams BoostPro Computing http://www.boostpro.com