how to define a static member in a template class

wanziforever · 10-06-2009, 06:44 PM

i have trouble to define a static member in a template, and i search from google, but didn't find any similar case, my case is a little complex, i paste the code below.

code in xxx.h

Code:

template <class T> class FreeListManager
{
 public:
  FreeListManager(int s=0);
  ~FreeListManager();
  T* acquire();
  void release(T*);

 protected:
  static list<T*> freeList;
  static list<T*> usedList;
  // We have size parameters separated from list, since
  // size() function of list container is inefficient.

};

code in xxx.cpp

Code:

template<class T> list<T*>
   FreeListMaager<T>::freeList;
template<class T> list<T*>
   FreeListManager<T>::usedList;

//there is no T defined

for the xxx.cpp file, actually, it is confused me a lot, and it seem to be not what i want, what is the type of the entry in freeList like? i got coredump when use the freeList.pushback. and i ever tried to define the two list static member like

Code:

template<class CLASSA> list<CLASSA*>
   FreeListManager<CLASSA>::freeList;
template<class CLASSA> list<CLASSA*>
   FreeListManager<CLASSA>::usedList;

template<class CLASSB> list<CLASSB*>
   FreeListManager<CLASSB>::freeList;
template<class CLASSB> list<CLASSB*>
   FreeListManager<CLASSB>::usedList;

i got redefinition for freeList and usedList error.
i was told FreeListManager<CLASSA> and FreeListManager<CLASSB> are totally two different class, why does the redefinition error come out?

i also try

Code:

list<CLASSA*> FreeListManager<CLASSA>::freeList;
list<CLASSA*> FreeListManager<CLASSA>::usedList;

list<CLASSB*> FreeListManager<CLASSB>::freeList;
list<CLASSB*> FreeListManager<CLASSB>::usedList;

i got too few template-parameter-lists error, the code can be compiled on gcc earlier version,

can someone help me, really appreciate your help.

denny

graemef · 10-07-2009, 03:06 AM

Are you sure that you want to use static? This means that there is only one instance of the data member; controlled by the class. Should you wish to create two instances of FreeListManager with the same template parameter then you will lose the second list.

graemef · 10-07-2009, 03:26 AM

But to address your question first note that for templates the definition and the declaration need to be in the same header file.

I would suggest that you make your lists pointers, thus you will have:

Code:

static list<T*> *freeList;

Initialise it will then be:

Code:

template<class T> list<T*> *FreeListManager<T>::freeList = 0;

wanziforever · 10-07-2009, 08:07 AM

sure, that's what i want, the code was download from opendiameter project, it can be compiled by very old version gcc complier.

but i found it cannot compliled by gcc-4.1.2.

yes, i include the xxx.h file in the xxx.cpp file.

As i know, FreeListManager is a only template, and FreeListManager<CLASSA> is actual class, and FreeListManager<CLASSB> is another one, i want to define two static list member for both class,FreeListManager<CLASSA>::freeList, FreeListManager<CLASSB>::freeList, so the two static member should be not one instance, but two. Just like what i tested

Code:

template <class T> class FreeListManager
{
 public:
  static int counter;
};

template <class T> int FreeListManager<T>::counter=0;
//the following definition will report redefinition error, either
//template <CLASSA> int FreeListManager<CLASSA>::counter=0;
//template <CLASSB> int FreeListManager<CLASSB>::counter=0;
//i thought the complier only can initial the two static member once, and you can set other value later.

FreeListManager<CLASSA>::counter=2
FreeListManager<CLASSB>::counter=3

main()
{
   printf("FreeListManager<CLASSA>::counter=%d\n",FreeListManager<CLASSA>::counter);
   printf("FreeListManager<CLASSB>::counter=%d\n",FreeListManager<CLASSB>::counter);
}

##########################
the result is
FreeListManager<CLASSA>::counter=2
FreeListManager<CLASSB>::counter=3

so it seem there are two static variable FreeListManager<CLASSA> and FreeListManager<CLASSB>

wanziforever · 10-07-2009, 08:28 AM

i also tried

template<> list<CLASSA>
FreeListManager<CLASSA>::freeList;
template<> list<CLASSA>
FreeListManager<ACLASSA>::usedList;
template<> list<CLASSB>
FreeListManager<CLASSB>::freeList;
template<> list<CLASSB>
FreeListManager<CLASSB>::usedList;

i aslo got undefined reference error

graemef · 10-08-2009, 02:23 AM

for the gcc compiler "the definition and the declaration need to be in the same header file." That's NOT include the header file in a source but actually placed in the header file.