Red Hat 6.1 C++ doesn't return allocated memory to system RES and SHR stay the same

Question

I have c++ code that I clean its containers with simple clear and delete in windows. Its working great and I can see the memory reducing nicely. But for the same code in Linux, I can't see any memory reducing. Is there any compiler flag or something that I am missing ?

I have the abstract code. Sorry I can't put more code but you can see the basic idea . It contain map's that getting big over time and there is cleanThemALL() method that suppose to empty them .

struct ObjectsA {

    std::vector  aObjects;

    ObjectsA(){}
  };


  struct sObjects {
    std::string a;       
    std::string b;      
    std::string c;   
  };


  typedef std::map<std::string, ObjectsA> ObjectsAType;
  typedef std::map<std::string, sObjects> SObjectsType;
  ObjectsAType m_aObjectsATypes;
  SObjectsType m_sObjectsType;

  typedef std::map<std::string, CacheObject*> OCacheMap;
  OCacheMap m_MyCache;

  MyObject::cleanThemALL()
  {
     m_aObjectsATypes.clear();
     m_sObjectsType.clear();;

     for (OCacheMap::iterator cahceIter = m_MyCache.begin();
          cahceIter != m_MyCache.end();
          ++cahceIter) {

         // Delete the Cache.
         delete (*cahceIter).second;
     }
  }
  malloc_trim (0);  // NEW only resident memory did decrease

I know that in AIX there is MALLOCOPTIONS=disclaim
AIX process memory is increasing with malloc free calls in loop

Is there such thing in Linux also ?

UPDATE
after using malloc_trim (0); the resident memory did decrease to 438m from 2.5 giga but the virtual memory still stayed the same why ? from:

3339 test20   0 5555m 2.5g  26m S 98.7  5.3   0:42.48 MyServer.exe 

to:

3339 test20   0 5555m 417m   26m S 98.7  5.3   0:52.48 MyServer.exe