Thread: Memory leak in this case?

  1. #1
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    Memory leak in this case?

    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.

    Code:
    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

  2. #2
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by George2 View Post
    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.

    Code:
    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
    An interface obtained through QueryInterface always has its refcount increased. The correct way to counter this is to cause the refcount to be decreased again when the interface pointer is no longer needed.
    Don't ever explicitly delete COM objects! You should always allow the last Release call to free them.

    In this case you should do this:
    Code:
    CComPtr<IUnknown> spCX;
    QueryInterface(IID_IUnknown, &spCX);
    That's it. Now when the CComPtr is destroyed, Release gets called automatically.
    Note that this is using ATL, which I use all the time at work. You may have to include some header file(s) that you aren't currently using.
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  3. #3
    C++まいる!Cをこわせ!
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    CComPtr is hidden deep within atlbase.h
    Very nice class, CComPtr.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  4. #4
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    Thanks iMalc,


    Question answered. Cool!!

    Quote Originally Posted by iMalc View Post
    An interface obtained through QueryInterface always has its refcount increased. The correct way to counter this is to cause the refcount to be decreased again when the interface pointer is no longer needed.
    Don't ever explicitly delete COM objects! You should always allow the last Release call to free them.

    In this case you should do this:
    Code:
    CComPtr<IUnknown> spCX;
    QueryInterface(IID_IUnknown, &spCX);
    That's it. Now when the CComPtr is destroyed, Release gets called automatically.
    Note that this is using ATL, which I use all the time at work. You may have to include some header file(s) that you aren't currently using.

    regards,
    George

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