Re: Template class as static property problem

From:

Michal M <mich.mierzwa@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 5 May 2011 10:51:42 -0700 (PDT)

Message-ID:

<5f3450fa-fde9-4423-929e-c307c7bb009c@e35g2000yqc.googlegroups.com>

On 5 Maj, 19:08, Fred <fred.l.kleinschm...@boeing.com> wrote:

On May 5, 9:54 am, Michal M <mich.mier...@gmail.com> wrote:

Hi

I have been trying to write a class which could serve as a cache
manager for objects.

file Cache.h
-------------------
class CachedObject;

template<typename T>
class Cache {
public:
        unsigned int cache_size;
        unsigned int cache_counter;
        CachedObject **objects_cache;
        Cache() {
                this->cache_size = 5;
                objects_cache = new CachedObject*[thi=

s->cache_size];

        };
        CachedObject *GetInstance() {
                //return new T();
                if (this->cache_counter > 0) {
                        return objects_cache[--=

cache_counter];

                } else {
                        return new T();
                }
        }
        void AddCacheOrFree(CachedObject *o) { delete o; }
        virtual ~Cache() {delete [] objects_cache;}

};

file CachedObject.h
----------------------------
#include "Cache.h"

class CachedObject {
public:
        static Cache<CachedObject> myCache;
        CachedObject() {};
        void Destroy() {
                CachedObject::myCache.AddCacheOrFree(th=

is);

        }
        virtual ~CachedObject() {};

};

file main.cpp
------------------
#include "CachedObject.h"

int main(int argc, char* argv[]) {
        CachedObject *myCachedObject = CachedObject::myCache.=

GetInstance();

myCachedObject->Destroy();
return 0;

}

It does not compile and gives linker error "main.o:main.cpp:(.text
+0xf): undefined reference to `CachedObject::myCache'".
If I uncomment //return new T(); in GetInstance it compiles without
errors.
I can't figure out what I am doing wrong. Could you help?

Well, you have declared myCache as a static variable in CachedObject,
but have never defined it. In some .C file you need:
CachedObject::myCache;

--
Fred

Short, simple, accurate... Thank you!