Re: memory leak when use _com_ptr_t Options

From:
dertopper@web.de
Newsgroups:
microsoft.public.vc.language
Date:
Wed, 10 Dec 2008 04:47:37 -0800 (PST)
Message-ID:
<01963b9f-92a4-454d-b346-6d8cfa645d90@y1g2000pra.googlegroups.com>
On 8 Dez., 17:17, Life as I know it <82sux...@gmail.com> wrote:

I wrote a class to use C# com in C++ project, but everytime i new A()
and delete A; it will cause memory leak the memeory increase but no
return.
i have debuged into the _com_ptr_t::release function when i call
delete A but from the memory check tool the memory doesn't returned.

#import ".....\bridge.tlb"
class A
{
  A();
  ~A();
  Func1();
  m_Ptr;

}

A()
{
  m_Ptr = NULL;
  Bridge::_ObjPtr obj(__uuidof(Bridge::Obj));
  m_Ptr = (void *) obj.Detach();

}

~A()
{
  Bridge::_ObjPtr obj;
  obj.Attach((Bridge::_ObjPtr *)m_Ptr );

}

Func1()
{
  Bridge::_ObjPtr obj;
  obj.Attach((Bridge::_ObjPtr *)m_Ptr );
  ...
  obj.function();//do someing with the obj
  ...
  obj.Detach();

}

Is anybody have any ideas? what's wrong with me code?


Almost everything is wrong with your code. Let's go through it one by
one:

class A
{
  A();
  ~A();
  Func1();
  m_Ptr;
}


This won't compile. Please post only complete (compilable) examples.
For the rest of the post I assume that m_Ptr is some flavour of
_com_ptr_t.

A()
{
  m_Ptr = NULL;


That's not an error, but you don't need to set _com_ptr_t to NULL,
it's constructor will do that automatically.

  Bridge::_ObjPtr obj(__uuidof(Bridge::Obj));
  m_Ptr = (void *) obj.Detach();


I assume that Bridge::_ObjPtr is also a _com_ptr_t. In that case you
have your first error:
The variable obj holds one reference count to the newly created object
of the CoClass Bridge::Obj. If you Detach() this pointer, and assign
it to another _com_ptr_t, the assigned _com_ptr_t object will
nevertheless increase the reference count. This way you only one
_com_ptr_t, that holds a reference (when you call Detach on
_com_ptr_t, the smart pointer no longer holds the reference count),
but the reference count of your object is two!

}
~A()
{
  Bridge::_ObjPtr obj;
  obj.Attach((Bridge::_ObjPtr *)m_Ptr );


That's probably an attempt to compensate for the leaking reference
count of the contructor. Note that if you use smart pointers, you
don't have to write _any_ code in the destructor of the class that
uses the smart pointer for releasing the reference.

}

Func1()
{
  Bridge::_ObjPtr obj;
  obj.Attach((Bridge::_ObjPtr *)m_Ptr );
  ...
  obj.function();//do someing with the obj
  ...
  obj.Detach();


You must no call Detach on the smart pointer, or else you'll get a
"reference leak". As a good rule of thumb you should never call Attach
or Release on smart pointers (except when you try to do deep
optimizationsor).

Your code should look like this:

class A
{
  A();
  // No need for destructor
  void Func1();
  _com_ptr_t<ISomeInterface> m_Ptr;
}

A()
{
  HRESULT hr = m_Ptr.CreateInstance (__uuidof(Bridge::Obj));
  // TODO: error handling for failures of CreateInstance.
}

void Func1()
{
  Bridge::_ObjPtr obj;
  obj.function();//do someing with the obj
}

Regards,
Stuart

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