Re: Ask for help about type function in "C++ Templates: The Complete Guide"

cdgray wrote:
:: Hi all,
::
:: There's a type function that determine whether a type is a class in
:: the book. I do not understand it and it can not pass the
:: compilation.
::
:: Here's the code:
::
:: // traits/isclasst.hpp
::
:: template<typename T>
:: class IsClassT {
:: private:
:: typedef char One;
:: typedef struct { char a[2]; } Two;
:: template<typename C> static One test(int C::*);
:: template<typename C> static Two test(...);
:: public:
:: enum { Yes = sizeof(IsClassT<T>::test<T>(0)) == 1 }; <===
:: Error reported here on VS2005 & GCC
:: enum { No = !Yes };
:: };

This is a known compiler bug. It works if you remove the redundant
"IsClassT<T>::" part.

::
:: What I confused is why the class type will match the member pointer
:: version instead of ellipsis version, as the ellipsis can match any
:: variables in any quantity. Not every class must have a int member,
:: in the preceding code, the size of MyClass is 1, but it has no such
:: member.

You only form a null pointer to a member. That is ok, as you are not
actually using it.

Bo Persson

--
      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]