Deleting HDC w/ bitmap selected

This is a discussion on Deleting HDC w/ bitmap selected within the Windows Programming forums, part of the Platform Specific Boards category; Ok, this is some code that was included in a DirectX utility file thinger: Code: HDC hdcImage; //some stuff not ...

  1. #1
    Carnivore ('-'v) Hunter2's Avatar
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    Deleting HDC w/ bitmap selected

    Ok, this is some code that was included in a DirectX utility file thinger:
    Code:
    HDC                     hdcImage;
    //some stuff not involving hdcImage
    hdcImage = CreateCompatibleDC(NULL);
    //if hdcImage is null, do something
    SelectObject(hdcImage, hbm); //hbm is a HBITMAP passed to the function
    //some stuff, only involving hdcImage for a StretchBlt()
    DeleteDC(hdcImage);
    If you select an object into an HDC, aren't you supposed to store the returned object in a temp variable and then select it back in before you delete the HDC? Or does it not matter because hdcImage was created within the function?
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  2. #2
    erstwhile
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    Yes, it is proper to do so. It's also proper to DeleteObject on gdi resources when finished with them too. However, windows ensures that all such resources are killed when the process/app that created them is killed.

    I think that when you create a dc, the default bitmap is a 1x1 monochrome - but don't quote me on that.
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  3. #3
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    Originally posted by Ken Fitlike
    I think that when you create a dc, the default bitmap is a 1x1 monochrome - but don't quote me on that.
    That is in fact true. So if you want to use the DC as a back buffer or something just create a compatible bitmap with correct size and select it into the dc.

  4. #4
    Carnivore ('-'v) Hunter2's Avatar
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    Yes, it is proper to do so
    Ok, thanks. But is it absolutely necessary? (i.e. otherwise will I get some of that very reliable "undefined behaviour"?)

    (...) DeleteObject on gdi resources (...)
    I would, but it's passed as a parameter, and it wouldn't be nice to kill somebody else's pet HBITMAP, would it?

    So if you want to use the DC as a back buffer or something just create a compatible bitmap with correct size and select it into the dc.
    I already knew that (I think), but thanks anyways
    Last edited by Hunter2; 12-07-2002 at 08:16 PM.
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  5. #5
    erstwhile
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    >>and it wouldn't be nice to kill somebody else's pet HBITMAP<<



    >>But is it absolutely necessary? <<

    I would say so as a matter of good practice if nothing else. Personally I would just as a matter of course - force of habit, if you will. I like to free up gdi resources asap after using them even though I understand that the system will do so when the process/app terminates. Still everyone seems to have their pet areas of sloppy coding, so whatever blits your bitmap, eh?

  6. #6
    Carnivore ('-'v) Hunter2's Avatar
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    lol ok, I sort of get the point, but I'm still getting confused This is the psuedo-code:
    Code:
    create an HDC called hdcImage, compatible with the screen
    select a bitmap (that is passed as an argument) into it
    StretchBlt() it onto another HDC
    delete it
    My question is, should it be this instead:
    Code:
    create an HDC called hdcImage, compatible with the screen
    
    select a bitmap (that is passed as an argument) into it,
    --> and store the returned bitmap in "temp"
    
    StretchBlt() it onto another HDC
    
    --> select "temp" back in
    
    delete hdcImage
    Are the pointed-to lines needed? (i.e. otherwise would the original bitmap be deleted with it or something similarly sinister?)
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  7. #7
    btq
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    Are the pointed-to lines needed? (i.e. otherwise would the original bitmap be deleted with it or something similarly sinister?)
    this is the proper manor to this yes, else when you get rid of the dc the original bitmap isn't destroyed and thus causing memory leaks...
    one should always restore GDI-objects initial data before deleting them..

    /btq
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  8. #8
    Carnivore ('-'v) Hunter2's Avatar
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    Alright, thanks. But...
    else when you get rid of the dc the original bitmap isn't destroyed and thus causing memory leaks...
    Then does that mean that the currently selected bitmap is destroyed?
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  9. #9
    train spotter
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    No, AFAIK.

    An 'created' object will not be destroyed if currently selected into a DC. Either by a call to DestroyObject() ect on the object or the DC.

    The resource leak from you code would be minor. Probably not mattering unless the app has to run on low end machines or for extended periods.

    I would use your second example as a standard.

    Remember GDI memory <<< available RAM
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  10. #10
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    It is usually very noticable when you have a memory leak with your GDI code. Windows is sufficated and slowly dies (and you can watch it do so). Also, it isn't always necessary to delete stuff immediately however, not all computers are the same and some of them are older and/or have less memory. It is good practice to take evey one's machine into consideration.

  11. #11
    btq
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    Then does that mean that the currently selected bitmap is destroyed?
    no, it doesn't...
    and those resource leaks can be a problem...I once did an app
    that blited many many times a sec between several DC's and the
    memory leaks were horrible when not selecting the original
    bitmap into the DC before destroying it. could just sit and watch
    the app eat up all the ram


    /btq
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  12. #12
    Carnivore ('-'v) Hunter2's Avatar
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    Ok, I appreciate all the replies (and I guess I'll stick to method #2 ), but one thing is still confusing me -

    -if I don't select the original bitmap back in before deleting the HDC, it won't be deleted...

    -but the currently selected bitmap won't be deleted either

    -Then the bitmap currently selected into the HDC isn't deleted when the HDC is

    -so even if the original bitmap is selected back in, it won't be deleted, will it??

    Or is there some special consideration for the original bitmap?
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  13. #13
    Guest Sebastiani's Avatar
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    You don't delete the saved bitmap. Your sole responsibilty is to reselect it and the destroy the one you created.

    DeleteObject(SelectObject(hdc, saved));

    That applies to any and all GDI objects...except(!) for stock and system objects (HBITMAPS never come in those two flavors, but brushes, pens, fonts, etc. can). In those cases, just reselect the saved GDI object and leave the on you created alone...
    Code:
    bool fun(bool value)
    {
        return std::pow(std::exp(1), std::complex<float>(0, 1) 
        * std::complex<float>(std::atan(1)*(1 << (value + 2))))
        .real() > 0;
    }

  14. #14
    Carnivore ('-'v) Hunter2's Avatar
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    Ok, I guess I get it... basically, I mind my own business and let Windows mind its own too, right? I delete my own objects, and let Windows delete its own in its own way, and how it goes about doing that (whether it deletes it when the HDC is deleted or not) is none of my beeswax, right?
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  15. #15
    Guest Sebastiani's Avatar
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    Well put.
    Code:
    bool fun(bool value)
    {
        return std::pow(std::exp(1), std::complex<float>(0, 1) 
        * std::complex<float>(std::atan(1)*(1 << (value + 2))))
        .real() > 0;
    }

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