asked on

class def

I receive an error when I try this:

template <typename HANDLE _Init=NULL>
class CMyClass
{
public:
CMyClass()
{
m_hHanlde = _Init;
}
protected:
HANDLE m_hHandle;
};

Now to define a specility of the class:

typedef CMyClass<-1> CMyOtherClass;

This is where the compiler gives an error: a signed int cannot be converted/casted to a void* (which is a HANDLE)

typedef CMyClass<0xFFFFFFFF> CMyOtherClass;

Doesn't work either !

Anybody knows the answer ?

grtx, RoverM

KangaRoo

In short, non-type template parameters must either be some form of const expression or a pointer/reference to an object with external linkage.

You would need a declaration:
extern HANDLE MyHandle;
a definition
HANDLE MyHandle = -1;

Parameters for your template must always heave external linkage:

extern HANDLE hGl;
void f()
{
HANDLE hLc;
CMyClass<hLc> cm1; // error
CMyClass<hGl> cm2; // ok

//....
}

KangaRoo

More precisely (from '96 Draft):
==========================================
3 A non-type template-parameter shall have one of the following (option-
ally cv-qualified) types:

--integral type, accepting an integral constant expression as an argu-
ment,

--enumeration type, accepting an integral constant expression as an
argument,

--pointer to object, accepting an address constant expression desig-
nating a named object with external linkage,

--reference to object, accepting an lvalue expression designating a
named object with external linkage,

--pointer to function, accepting an expression of type pointer to
function designating a function with external linkage,

--reference to function, accepting an lvalue expression designating a
function with external linkage,

--pointer to member, accepting an address constant expression desig-
nating a named member of a class.
==========================================

KangaRoo

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LucHoltkamp

Kangaroo, the parameter that roverm uses IS a constant expression, of course it must be a parameter that is known during compile-time, but that is the case in his example. So that is not the source of the problem.
Try to cast it explicitly to HANDLE (void*) this will solve you're problem.
Luc

KangaRoo

HANDLE is not any of the allowed types (see above). A HANDLE is typedef-ed as a void*, so only void* (HANDLE's) with external linkage can be used as parameter for the template.
Explicit casting to HANDLE will not solve the problem.

KangaRoo

>> HANDLE is not any of the allowed types
That is, it is allowed if it has external linkage, as in my example.

roverm

ASKER

Sorry, still get errorsssssss!
like 'invalid template error'

roverm

ASKER

Further, I'm trying to instanciate a new definition using TypeDef, NOT by using instanciating a class !

RoverM

ASKER CERTIFIED SOLUTION

KangaRoo

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KangaRoo

>> I'm trying to instanciate a new definition using TypeDef
You can not instantiate a tmplate specialization with a typedef, if that is what you are trying to do.

roverm

ASKER

Kangaroo:
You're right! It cannot be done!
So I'll give you the points!

D'Mzzl!
RoverM