[boost] Re: implicit_cast

From: Gennaro Prota (gennaro_prota_at_[hidden])
Date: 2002-11-26 05:34:47

On Thu, 21 Nov 2002 19:15:21 -0500, David Abrahams
<dave_at_[hidden]> wrote:

>Thoughts?

This is one... A nice thing about the problem you are talking about is
that any function having a parameter of type T is in fact a general
"detector" of convertibility to T (It's also worth noting that it
detects convertibility of an *expression*, so it is fundamentally
different from usual templates like is_convertible<T, U> and the
like). Well, once you have it it's easy to enforce either
convertibility _and_ non-convertibility. As an example of the latter,
this is a trivial way to cause a compile-time error if 'expr' is
implicitly convertible to the type T. 

// untested code following
//

typedef char  (&no_type)[1];
typedef char (&yes_type)[2];

template <typename T>
struct identity { typedef T type;};

template <typename T>
yes_type implicit_cast (typename identity<T>::type x) {
    return x;
}

template <typename T, std::size_t sz >
struct check_helper{
 /*
  *   empty
  */
};

template <typename T>
struct check_helper<T, sizeof(no_type)> {
  typedef T type;
};

template <typename T>
no_type implicit_cast (...);

#define EXPLICIT_CAST(dst_type, expr)                  \
          ( static_cast<  check_helper<dst_type,       \
            sizeof(implicit_cast<dst_type>(expr)) >    \
            :: type>(expr)  )

So it looks like we can separate the two 'directions' in which
static_cast works in two distinct facilities. Maybe this has already
been discussed here though, I don't know (Actually there's at least
one limitation: you can't use a local type for T, but... c'est la
vie!)

Also, a static assertion facility (for expressions) can be easily
implemented:

#define ASSERT_NON_IMPLICITLY_CONVERTIBLE(T, expr)             \
          typedef char PASTE (name_, __LINE__)                 \
            [                                                  \
             sizeof(no_type) == sizeof(implicit_cast<T>(expr)) \
            ];

and maybe this is what someone may be interested in (rather than the
two macros for static-casting).

Genny.