Re: [boost] Re: implicit_cast

From: David Abrahams (dave_at_[hidden])
Date: 2002-11-29 13:55:49

Gennaro Prota <gennaro_prota_at_[hidden]> writes:

> On Fri, 22 Nov 2002 13:48:01 +0200, "Peter Dimov" <pdimov_at_[hidden]>
> wrote:
>
>>From: "David Abrahams" <dave_at_[hidden]>
>>>
>>> Here's what I think might be a correct implementation:
>>>
>>>      template <class T, class U> T implicit_cast(U const& x) { return x; }
>>>      template <class T, class U> T implicit_cast(U& x) { return x; }
>>
>>The correct implementation IIRC is
>>
>>template<class T> T implicit_cast(typename identity<T>::type x) { return
>>x; }
>
>
> I do know we all hate macros but this could be taken into
> consideration if we want something usable in constant expressions too.
> The idea is to use a static_cast after checking that the conversion
> could have also been done implicitly:
>
> template <typename T>
> struct identity { typedef T type;};
>
> template <typename T>
> T implicit_cast (typename identity<T>::type x) {
>     return x;
> }
>
>
> template <typename T, std::size_t sz >
> struct check_helper{
>   typedef T type;
> };
>
> #define IMPLICIT_CAST(dst_type, expr)                  \
>           ( static_cast<  check_helper<dst_type,       \
>             sizeof(implicit_cast<dst_type>(expr)) >    \
>             :: type>(expr)  )
>

I think I'd prefer to see:

  // make sure the static_cast below is a valid expression
  BOOST_STATIC_ASSERT(sizeof(implicit_cast<dst_type>(expr)));
  ... static_cast<dst_type>(expr) ...

I know you have to write expr twice, but it still seems much clearer
to me.

-- 
                       David Abrahams
   dave_at_[hidden] * http://www.boost-consulting.com
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