From: Gennaro Prota (gennaro_prota_at_[hidden])
Date: 2003-02-17 15:03:21
On Mon, 17 Feb 2003 10:02:58 +0100, Daniel Frey
<daniel.frey_at_[hidden]> wrote:
>I started to implement my own type-traits to see if I can do it and to
>learn something. While doing so, my design lead me to some dependencies:
>is_enum needed is_class
Daniel, my apologies in advance if this is just a stupid comment. I'm
again jumping in without actually following the thread (not enough
time), however if I had to implement is_enum the first property that I
would try exploiting is the built-in convertibility (of an rvalue) to
integral types. If you immediately burn the one user-defined
conversion allowed in any implicit conversion sequence there's no need
to exclude classes, I think. The basic idea is:
template <typename T>
struct is_integral {
static const bool value = false;
};
#define THIS_IS_INTEGRAL(t) \
template <> struct is_integral<t> { \
static const bool value = true; \
} \
/**/
template <typename T>
struct is_floating {
static const bool value = false;
};
#define THIS_IS_FLOATING(t) \
template<> struct is_floating<t> { \
static const bool value = true; \
}
...
/* add cv-qualified specs here */
...
/**/
THIS_IS_INTEGRAL(bool);
THIS_IS_INTEGRAL(char);
THIS_IS_INTEGRAL(wchar_t);
THIS_IS_INTEGRAL(signed char);
THIS_IS_INTEGRAL(short);
THIS_IS_INTEGRAL(int);
THIS_IS_INTEGRAL(long);
THIS_IS_INTEGRAL(unsigned char);
THIS_IS_INTEGRAL(unsigned short);
THIS_IS_INTEGRAL(unsigned int);
THIS_IS_INTEGRAL(unsigned long);
THIS_IS_FLOATING(float);
THIS_IS_FLOATING(double);
THIS_IS_FLOATING(long double);
template <typename T>
struct is_enum {
struct Burn {
operator T() const;
};
typedef char yes;
typedef char(&no)[2];
static yes is_it(unsigned long);
static no is_it(...);
template <typename U>
static no check_rvalue(U&);
static yes check_rvalue(...);
static T get_T();
static const bool value =
!is_integral<T>::value
&&
!is_floating<T>::value
&&
sizeof(yes) == sizeof( is_it( Burn() ) )
&&
sizeof(yes) == sizeof( check_rvalue( get_T() ) )
;
};
The above is absolutely untested and off the top of my head so take it
with great great prudence :-) It was just to say that, if I'm not
mistaken, is_enum doesn't need is_class. Maybe.
Genny.