Pointer problem

This is a discussion on Pointer problem within the C++ Programming forums, part of the General Programming Boards category; Hello, I am trying to do a ZeroMemory on a pointer to structure, but it fails on runtime. Code: rt3DModel ...

  1. #1
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    Pointer problem

    Hello, I am trying to do a ZeroMemory on a pointer to structure, but it fails on runtime.

    Code:
    rt3DModel *pModel;
    ...
    ...
    ...
    ZeroMemory(pModel, sizeof(rt3DModel));
    Debugger shows at the ZeroMemory line (VC++6)

    What might be wrong here? I dont use new here.
    Thanks
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  2. #2
    &TH of undefined behavior Fordy's Avatar
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    If your not using new, then how is that memory allocated?

  3. #3
    Code Monkey Davros's Avatar
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    The pModel pointer will be pointing to an invalid address.

    You could do:
    Code:
    rt3DModel pModel; // on stack
    
    ...
    
    ZeroMemory(&pModel, sizeof(rt3DModel));
    What does zero memory do?

  4. #4
    Just a Member ammar's Avatar
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    ZeroMemory?In what headerfile is this function.
    What's the problem?

  5. #5
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    ZeroMemory ammar is a macro of memset.

    Ok guys, I created it as new and it worked. But I have a stupid question here:
    lets stay i have a structure with several variables, k? And i make an instance of that class:
    rt3DModel model1;
    rt3DModel model2;

    Will both model1 and model2 point to the same values? I mean if model1.something = 5, then will it be model2.something = 5 too (if i set model1.something=5 first)

    I think not, but asking to make sure.
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  6. #6
    Just a Member ammar's Avatar
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    Of course Not, because when you creat an instance you creat all the variables related to that belonges to it.

    Maybe model1.something is equal to model2.something if has a value by default or if you changed it to be equal.

    Is that what you mean?

  7. #7
    Code Monkey Davros's Avatar
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    No model1 & model2 are seperate entities. model1.something could equal 2, while model2.something could equal 5.

    What you could do is this:

    Code:
    rt3DModel model;
    
    // Extra pointer to model
    rt3DModel* modelPtr = &model;
    Now modelPtr points at the address of model object. In effect there is only one object (model) plus a pointer to it.

    So when you set model.something = 5, when modePtr->something is read, it will be equal to 5 also.

    Does this make sense?

  8. #8
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    Ohhh right, so I dont have to update the modelPtr everytime.
    Thanks for making this clear
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