[boost] Re: is_class

From: Rani Sharoni (rani_sharoni_at_[hidden])
Date: 2003-02-19 09:09:26

Gennaro Prota wrote:
> On Tue, 18 Feb 2003 19:14:50 +0200, "Rani Sharoni"
> <rani_sharoni_at_[hidden]> wrote:
>
>> This is very nice technique. Just notice that you also need to
>> handle void, functions and arrays types.
>>
>> Abstract classes, functions and array types will fail the completion
>> on the burn conversion operator since they are not allowed as
>> returns types.
>
> Yeah. The code was just to give the idea, which I could have expressed
> in English if it wasn't that my English is even worse than my C++ :-)
>
>
>> IMHO the main achievement of your technique is that, unlike
>> techniques that use conversion constructor to burn user defined
>> conversion, its works for incomplete classes (EDG and GCC complain
>> when trying to pass lvalue of incomplete class to function that take
>> ellipsis as an argument).
>
> Yes. Attempting lvalue-to-rvalue conversion of an expression of
> incomplete type makes the program ill-formed.

Sure. 5.2.2/7. It's easy to overlook this case.

> Nice :-) BTW, you can avoid the select_type machinery:
Thanks

>
>   template <typename T>
>   struct Burn {
>     operator T&() const;
>   };
>
>   // fire-prevention
>   template <typename T>
>   Burn<T> ref(T*);
You can improve it to deal with qualified function types (you can't have
pointers or references for such types):
Burn<T> ref(T(*)[1]);

>   template <typename T>
>   char* ref(...);
>
>   // outside the is_enum<> template
>   // to help gcc
>   //
>   yes is_enum_checker (unsigned long);
>   no  is_enum_checker (...);
>
>
>   template <typename T>
>   struct is_enum {
>
>    static const bool value =
>          !is_integral<T>::value
>       && !is_floating<T>::value
>       && sizeof(yes) == sizeof( is_enum_checker (ref<T>(0)) );
>   };
>
>
> But this simple stuff is already enough to knock out a lot of
> compilers :-/
>
> BTW, I've seen how complex boost::is_reference is. Why so? What
> compiler(s) cause(s) most problems?

VC6 is problematic since the usual techniques to simulate partial
specialization ICEd it.

I remember that I came up with the same VC6 trick before I saw it in boost
and I wonder who invented it.

Rani