a compiation issue with gcc

a compiation issue with gcc

[Damanjit Singh]

The following code  does not compile with gcc (ggc -c simple.cpp) and
gives the following errors ( where as the code compiles in windows
VC++ )-
Could someone tell me the reason for this, and if there is an
available workaround.
Thanks,

#include "iostream.h"
#include "stdio.h"
template <class T,void (*DestroyCallback)( T )> class B
{
private:	
	int a;
};		
template <class T> class A
{
private:
	static void ASFreeWrapper( T Ptr )
	{
		free( Ptr );	
	}
public:
	typedef B<T,&ASFreeWrapper> Type;

};
int main()
{
	A<int *> a;
	return 0;
}

/* Here is the error reported by gcc -
simple.cpp:9: warning: all member functions in class `A<T>' are private
simple.cpp: In instantiation of `A<int*>':
simple.cpp:21:   instantiated from here
simple.cpp:16: invalid use of undefined type `class A<int*>'
simple.cpp:9: declaration of `class A<int*>'
*/

Re: a compiation issue with gcc

[Peter Doerfler]

Hi Damanjit.

As far as I understand there is some confusion with pointers to member 
functions esp. if they are static depending on the compiler.

I managed to get your code compiled by removing the & in 
>         typedef B<T,&ASFreeWrapper> Type;
Can't recall which is correct but I think the version without &.

and of course including malloc.h for free().

This is with gcc4.0.1 on a x86 system.
On my gentoo version of gcc3.4.3 I get an ICE, but I guess that is a known 
issue. Does anybody know?

If you are interested: I found 
http://www.codeproject.com/cpp/FastDelegate.asp
an informative read. Goes into detail what the standard says about member 
function pointers and what different compilers actually implement.

Hope this helps
- Peter

On Thursday 19 May 2005 14:12, Damanjit Singh wrote:
> #include "iostream.h"
> #include "stdio.h"
> template <class T,void (*DestroyCallback)( T )> class B
> {
> private:        
>         int a;
> };              
> template <class T> class A
> {
> private:
>         static void ASFreeWrapper( T Ptr )
>         {
>                 free( Ptr );    
>         }
> public:
>         typedef B<T,&ASFreeWrapper> Type;
>
> };
> int main()
> {
>         A<int *> a;
>         return 0;
> }

On Thursday 19 May 2005 14:12, Damanjit Singh wrote:
> The following code  does not compile with gcc (ggc -c simple.cpp) and
> gives the following errors ( where as the code compiles in windows
> VC++ )-
> Could someone tell me the reason for this, and if there is an
> available workaround.
> Thanks,
>
> #include "iostream.h"
> #include "stdio.h"
> template <class T,void (*DestroyCallback)( T )> class B
> {
> private:
> 	int a;
> };
> template <class T> class A
> {
> private:
> 	static void ASFreeWrapper( T Ptr )
> 	{
> 		free( Ptr );
> 	}
> public:
> 	typedef B<T,&ASFreeWrapper> Type;
>
> };
> int main()
> {
> 	A<int *> a;
> 	return 0;
> }
>
> /* Here is the error reported by gcc -
> simple.cpp:9: warning: all member functions in class `A<T>' are private
> simple.cpp: In instantiation of `A<int*>':
> simple.cpp:21:   instantiated from here
> simple.cpp:16: invalid use of undefined type `class A<int*>'
> simple.cpp:9: declaration of `class A<int*>'
> */

Re: a compiation issue with gcc

[Eljay Love-Jensen]

Hi Damanjit,

I see a few unrelated problems with your code.

1)
#include "iostream.h" is not C++ compliant (it's deprecated at best).  Use #include <iostream>.  Since it wasn't used in your code example, I removed it.

2)
#include "stdio.h" is not C++ compliant (it's deprecated at best).  Use #include <cstdio>.  Since it wasn't used in your code example, I removed it.

3)
With GCC, you should compile C++ code with the g++ front end, not with gcc.

Now on to the meat of the problem.

4)
You appear to be replicating std::auto_ptr, which is part of Standard C++ in the C++ <memory> header.  But maybe you have a simplified example for illustrative purposes, so just consider this an FYI.

5)
You were missing the header for free().  I added #include <cstdlib>.  Instead of using <cstdlib>'s malloc / free for your heap memory management, I strongly recommend using C++ new / delete.  But that may be an aside from your actual problem at hand.

6)
You are passing in a function pointer as a template type parameter.  I vaguely recall there can be issues with function pointers (especially ones that are static-at-the-translation-unit level, or static-at-the-class-level).

As an alternative for you consideration (which will be more digestible / less problematic amongst a variety of C++ compilers at different levels of C++ compliance), you could use a functor object.  Functor objects tend to be a lot more C++ friendly in general, anyway -- but may or may not be suitable for your needs.

That could look something like this:
--------------------------------------
#include <cstdlib> // free

template <class T, class DestroyFunctor>
class B
{
private:
    T a;
};

template <class T>
class A
{
private:
    void operator ()(T Ptr)
    {
        free(Ptr);
    }

public:
    typedef B<T, A<T> > Type;
};

int main()
{
    A<int*> a;
    return 0;
}
--------------------------------------

HTH,
--Eljay