A question about memory leak (from A Guide for Symbian C++ developers)

[AllenZhu]

class CElementsEngine : public CBase
{
private:
CElementList* iElementList;
CConsoleBase& iConsole;
};

CElementsEngine::~CElementsEngine()
{
delete iElementList;
}



What about the constructor? You might think this would suffice:
CElementsEngine::CElementsEngine(CConsoleBase& aConsole)
: iConsole (aConsole)
{
iElementList = new (ELeave) CElementList();
}



However, such constructors are not used in Symbian OS, and for good reason. To see why, you need to consider what would happen if the allocation of the new CElementList were to fail, causing a leave. Well, presumably this constructor would have been invoked elsewhere something like this:

CElementsEngine* myEngine = new (ELeave) CElementsEngine(*console);


Remember that this line does two important things: First, the new (ELeave) operator allocates memory for the new CElementsEngine instance (and all its nested data). Assume for the moment that this allocation is successful. Next it calls the C++ constructor shown above. If the allocation of the CElementList in the CElementsEngine constructor were to fail, say, due to running out of memory, there would be a problem. When this leave occurs, there is no pointer pointing to the area of memory successfully allocated for the CElementsEngine object, so this memory will be orphaned, and a memory leak will occur.


how to lead a memory leak? I can't understand.
if running out of memory, OS can't allocate any memory. Not to mention a memory leak.

[kishore84]

But what abt the memory which have been sucessfully alocated for CElementsEngine. Who will free this memory if a leave occurs.

Here i am explaining u in a simple exp

Class String:public CBase
{
public:
   string();
private:
  char* str1;
  char* str2;
};

string::string()
{
  
  str1 = new char[10];// this allocates sucessfully //Who will free this memory
  str2 = new char[888888888]; // it fails and panic raises
}

[/code]

void main()
{
  string* obj = new(ELeave) string(); // this line will panic
  //str1 = new char[10];// this allocates sucessfully //Who will free this memory now // who is the owner 
}

Therefore in symbian we use lightweight constructors. and make free this memory by cleanupstack methods.

I suggest u to serch wiki for twophase construction and cleanupstack...

Hope this will help to understand..