Re: [Boost-users] boost-users: boost enable_if

From: David Abrahams (dave_at_[hidden])
Date: 2007-10-01 17:59:24

on Mon Oct 01 2007, "chun ping wang" <cablepuff-AT-gmail.com> wrote:

> hmm that is what i want for now... (i might change it later). I want to know to keept it in back
> of my head. Thanks.
>
> On 10/1/07, David Abrahams < dave_at_[hidden]> wrote:
>
>     on Wed Sep 26 2007, "chun ping wang" < cablepuff-AT-gmail.com> wrote:
>    
>     > what i want to do is if type "T" happens to be a integeral type or a class type that
>     inherits
>     > from boost::operators than let the function compile otherwise a
>     > compile time error appears.
>    
>     I could tell you how, but it seems ill-advised.  I really doubt you
>     want to fail compilation if someone passes you a class type that acts
>     just like int, but that isn't written using boost::operators.  Are you
>     sure that's what you want?

// untested
template <class T, class U>
char (& is_op_test(operators<T,U> const&) )[2]

template <class T>
char is_op_test(T const&);

template<class T>
T const& make();

template <class T>
struct is_operators
  : mpl::bool_<(sizeof(is_op_test(make<T>())) == 2)>
{};

template <class T>
typename enable_if<mpl::or_<boost::is_integral<T>, is_operators<T> >, return_type>::type
f(T x)
{
   ...
}

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com