C / C++ / MFC

21-Mar-08 1:11

_kbhit()

CPallini21-Mar-08 1:30

Re: Read a character or continue?

21-Mar-08 1:30

*?#!

You were faster.
Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

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Anthony Appleyard21-Mar-08 1:42

Anthony Appleyard

21-Mar-08 1:42

Thanks.

Re: Read a character or continue?

CPallini21-Mar-08 1:24

Memory leak in this case?

21-Mar-08 1:24

This one [^] looks promising.
Smile | :)

George_George20-Mar-08 23:56

20-Mar-08 23:56

Hello everyone,

Suppose I have a component (class) CX implements IUnknown interface, and I have retrieved a pointer of CX by IUnknown pointer (IUnknown*), that is.

IUnknown* pCX;
QueryInterface (IID_IUnknown, &pCX);

//...

delete pCX; // memory and resource leak here?

Even if I declare the component CX's destructor as virtual, if I delete through IUnknown pointer to "release" the object, there is still potential memory and resource leak, because in IUnknown interface, destructor is the compiler provided default one, which is non-virtual and public, right?

thanks in advance,
George

CPallini21-Mar-08 0:01

21-Mar-08 0:01

NEVER do that. COM has its own mechanism to perform cleanup, you must not break the toy.
Smile | :)

George_George21-Mar-08 0:13

21-Mar-08 0:13

Thanks CPallini,

I should ask the question in another way. Is the default destructor virtual or not?

regards,
George

CPallini21-Mar-08 0:28

21-Mar-08 0:28

Default destructor is not virtual.
Smile | :)

George_George21-Mar-08 0:41

21-Mar-08 0:41

Thanks CPallini,

Question answered.

regards,
George

Rajkumar R21-Mar-08 0:05

21-Mar-08 0:05

George_George wrote:
because in IUnknown interface, destructor is the compiler provided default one, which is non-virtual and public, right?

yes, But why don't you use Release() that's the COM needs.

George_George21-Mar-08 0:14

21-Mar-08 0:14

Thanks Rajkumar,

I should ask the question in another way. The default destructor is non-virtual and public, right?

regards,
George

Rajkumar R21-Mar-08 0:38

21-Mar-08 0:38

George_George wrote:
The default destructor is non-virtual and public, right?

and your previous one
"destructor is the compiler provided default one, which is non-virtual and public, right?"

George_George wrote:
I should ask the question in another way

I can understand that the above two has no difference, you can try something different that shows the difference of your question. Smile | :)

George_George21-Mar-08 0:45

21-Mar-08 0:45

Thanks Rajkumar,

Sorry English is not my native language. I think you have answered my question, the default destructor is not virtual and it is public, right?

regards,
George

Rajkumar R21-Mar-08 0:49

21-Mar-08 0:49

George_George wrote:
I think you have answered my question

yes.

George_George21-Mar-08 0:55

21-Mar-08 0:55

Thanks Rajkumar,

Question answered.

regards,
George

Nemanja Trifunovic21-Mar-08 6:49

Nemanja Trifunovic

21-Mar-08 6:49

George_George wrote:
if I delete through IUnknown pointer to "release" the object, there is still potential memory and resource leak, because in IUnknown interface, destructor is the compiler provided default one, which is non-virtual and public, right?

Right, except that strictly speaking you get "undefined behavior" which may or may not be a memory leak (with VC++, I bet it is a memory leak).

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George_George21-Mar-08 20:46

21-Mar-08 20:46

Thanks Nemanja,

Question answered.

regards,
George

peterchen21-Mar-08 9:38

peterchen

21-Mar-08 9:38

The correct way is:

<br />
IUnknown* pCX;<br />
HRESULT hr = QueryInterface (IID_IUnknown, &pCX);<br />
if (!FAILED(hr))<br />
{<br />
  // ...<br />
  pCX->Release();<br />
}<br />

delete cannot achive many of what COM interfaces are about:

reference counting:
Many clients might use the same object. You must not delete it before everyone else is done with it.
Use Release() to indicate "I am done with the object". The object will delete itself when it is no longer needed.

language independence
The object bight be implemented in VB, or Delphi, or one of many other languages. delete cannot correctly release an object that is implemented in another language.
Even if the object was implemented in C++, delete is likely to fail: a virtual destructor helps only when the object was allocated by the same memory manager.

location independence
COM allows to use objects in a separate process or even on another computer without really knowing about it. How is C++' delete going to delete an object on a server in hawaii? Some such relocations may be activated by registry configuration, independent of your code.

You need to follow the Reference Counting Rules[^] of COM.
Using smart pointers make that somewhat easier.

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George_George21-Mar-08 20:39