Re: about SFINAE usage
* feverzsj:
my apology for all these confused stuff...
this is an example from [C++ Templates: The Complete Guide],which i
thought should be typical enough.
anyway the error code is as below:
template<typename T>
class ClassChecker{
typedef char One;
typedef struct{char _c[2];} Two;
static One test(int C::*); // an ordinary member function
instead of member template
static Two test(...); // **
public:
enum{YES = sizeof(test(0))==1}; //**
enum{NO = !YES};
};
template<typename T>
void CheckClass()
{
if( ClassChecker<T>::YES )
cout<<"type '"<<typeid(T).name()<<"' IS a Class
type"<<endl;
else
cout<<"type '"<<typeid(T).name()<<"' IS NOT a Class
type"<<endl;
}
and the compiler[vc9] gives:" ...error C2645: no qualified name for
pointer to member (found ':: *')..."
Again you leave out the relevant information.
First, because the first message you get with your compiler is, I suspect,
error C2653: 'C' : is not a class or namespace name
unless you're compiling some code that's *different* from the one you posted,
which I would readily believe you're doing...
Second, because you don't supply the calling code, which is where you
instantiate ClassChecker with some non-class type, causing the error,.
Yeah, I know, sometimes one is just blind on both eyes, happens to everybody,
and has certainly happened enough times with me, but you really should make a
very strong effort to make sure that you supply the relevant information, and
that means, *not* applying preconceived ideas about what's relevant.
When you're asking for an explanation it's because you do not understand it at all.
So the assumption that you do understand enough to decide what's relevant is
unwarranted (I'd use stronger words but I already did and that's enough :-) ).
Anyways, if you correct the code to use type T instead of undefined C, then when
instantiating ClassChecker with e.g. 'int' type T, then you have told the
compiler to instantiate the One version of 'test', which it can't do because
there's no such thing as e.g. 'int::*'. Without templating you do not have
SFINAE. With templating the compiler simply ignores the possible instantiation,
because that's what SFINAE is all about, ignoring possible instantiations.
Cheers & hth.,
- Alf
PS: Don't forget, when you *do not* understand something, don't assume that you
understand enough to decide that something is irrelevant. And please make sure
that the code you're compiling (and/or running) is the same as the posted code.
--
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