Pointer validity check

This is a discussion on Pointer validity check within the Windows Programming forums, part of the Platform Specific Boards category; Hi, here's my problem: given a huge source, where object pointers are stored in STL objects (queues, maps, deques, etc.), ...

  1. #1
    Normal vector Carlos's Avatar
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    Pointer validity check

    Hi,
    here's my problem: given a huge source, where object pointers are stored in STL objects (queues, maps, deques, etc.), which would be the most easiest method to determine whether an a pointer (iterator) is really a valid one.

    The problem is as follows: there are separate threads, which store many pointers to objects as described above. Due to the complexity of the project, sometimes occures that some of these pointers are deleted, but their values are not *yet* removed from the queues / maps / ... where they were inserted.
    This leads in extreme cases - sporadically - to access violations, as if in this case the pointer is found in the STL objects, and unfortunately they pass checking for NULL pointers.

    To illustrate the problem, here's a short example:
    Code:
    // deques which stores pointers of ClassA
    typedef std::deque< ClassA* > BasePtrList;
    BasePtrList myPtrList;
    
    // Thread_1
    ClassA* myClassPtr = new(Class);
    
    // Thread_2 stores the pointer in his deque
    myPtrList.push_back( myClassPtr );
    
    // later Thread_1 deletes aClassPtr,
    delete myClassPtr;
    myClassPtr = NULL; // step 1
    /*issue command to remove this pointer from Thread2's myPtrList*/
    maintainThread2List();step 2
    
    /* after myClassPtr but before *before* the pointer is removed from Thread 2's deque - between step 1 and step 2 - Thread2 performs following action */
    for ( BasePtrList::iterator anIter = myPtrList.begin(); 
           anIter != myPtrList.end(); anIter++ )
    {
              ClassA* aClassPtr = *anIter;
              // NULL pointer check
              if ( aClassPtr )
              {
                        aClassPtr->doSomething()
              } 
    }
    I thought of a method: whenever a pointer is extracted from an STL object, we try to read/write to the pointed memory location.
    If there's any (catched) exception, we know that the pointer is not more valid, and it can be removed.

    Maybe you've got the same problem. In case you have a better idea, you'd be welcome.

    Thanks in advance!

    Have a nice code.
    Last edited by Carlos; 12-10-2003 at 04:49 AM.

  2. #2
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    An idea off on a slight tangent -

    You don't want to recode, but you may want to consider smart pointers from the boost library, since you are going to have to change at least some of your code anyway. Smart pointers excel at solving this kind of problem.

    www.booost.org

    I have not used them in the Windows environment, but others have.

  3. #3
    C++ Developer XSquared's Avatar
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    You've got one too many 'o' s in your link, Jim.

    http://www.boost.org
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

    You. Fetch me my copy of the Wall Street Journal. You two, fight to the death - Stewie

  4. #4
    the hat of redundancy hat nvoigt's Avatar
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    This sounds as if you need syncronization in your threads.

    Have a look at Critical Sections, semaphores, mutexes and locks in general. You need to make sure that no other thread can do stuff between step 1 and 2.
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

  5. #5
    Normal vector Carlos's Avatar
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    Originally posted by nvoigt
    This sounds as if you need syncronization in your threads.

    Have a look at Critical Sections, semaphores, mutexes and locks in general. You need to make sure that no other thread can do stuff between step 1 and 2.
    The code was just a rough approach of what happens.
    The stuff is more sophisticated. The critical sections are of course guarded(using locks).
    Last edited by Carlos; 12-10-2003 at 06:22 AM.

  6. #6
    Registered User Codeplug's Avatar
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    Allocate and de-allocate your objects from a "memory pool" object instead of the heap. Then you can query the memory pool object: "Is this pointer still out, or has it been free'd?"

    You'll need to make the memory pool object thread safe of course.

    Best suggestion I can give without knowing all the gory details

    gg

  7. #7
    Normal vector Carlos's Avatar
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    Originally posted by Codeplug
    Allocate and de-allocate your objects from a "memory pool" object instead of the heap. Then you can query the memory pool object: "Is this pointer still out, or has it been free'd?"
    gg
    Good suggestion, CodePlug. I think this one would take some more extra work than my approach (pointer validity check), but it's also more elegant.

    Thank you.

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