Confused with COLOR_ constants

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    Registered User ZaidK's Avatar
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    Post Confused with COLOR_ constants

    Books(the Charles Petzold one) say that there is a need of adding +1 while using the System colours like..

    wndclass.hbrBackground = (HBRUSH) (COLOR_BTNFACE + 1)

    I don't understand the need of that + 1 ? Can anyone helped in that. I am new to VC++ Programs

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    This is not a C++ question. It is a windows programming question.

    The short answer is "because Microsoft says so in their documentation for the WNDCLASSEX structure", as per this link.

    The more precise answer is that Microsoft developers messed up in defining a few things (something that is understandable with a large team developing a very large system or API) and this in one of them. HBRUSH is a pointer type and the first COLOR_ parameter defined has a value of zero. Adding the 1 is necessary to ensure all valid COLOR_xxx values correspond to a non-NULL HBRUSH.
    Right 98% of the time, and don't care about the other 3%.

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    Ok ! I got that HBRUSH is a pointer type but what about "Adding the 1 is necessary to ensure all valid COLOR_xxx values correspond to a non-NULL HBRUSH."? Is it in the MS documentation or just a logic!

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    It is logic. Converting a value of zero to a pointer yields a null pointer - a pointer (or memory address) that is guaranteed not to point at a valid object. To dereference a pointer, it is necessary for it to be non-null. If the HBRUSH corresponding to a COLOR_xxx value is NULL, then that brush cannot be used.
    Right 98% of the time, and don't care about the other 3%.

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    Quote Originally Posted by ZaidK View Post
    Books(the Charles Petzold one) say that there is a need of adding +1 while using the System colours like..

    wndclass.hbrBackground = (HBRUSH) (COLOR_BTNFACE + 1)

    I don't understand the need of that + 1 ? Can anyone helped in that. I am new to VC++ Programs
    The system colours vary depending on the current style / theme.

    These colours are stored in an array and the IDs (like COLOR_BTNFACE) are defined integer values.

    The first element of the array is 0 (in C / C++) but you need a way to tell the OS that you ment to get an element of the array, not 'no colour' (ie NULL).

    So the +1 is used to tell the difference between (HBRUSH)NULL and (HBRUSH)0 (which means get me the system colour from the array at index zero)


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    It is logic. Converting a value of zero to a pointer yields a null pointer - a pointer (or memory address) that is guaranteed not to point at a valid object. To dereference a pointer, it is necessary for it to be non-null. If the HBRUSH corresponding to a COLOR_xxx value is NULL, then that brush cannot be used.

    Ok ! u mean to say that the first value of COLOR_(which i think is COLOR_SCROLLBAR
    ) will get treated as a NULL pointer!Got it !Thnks @grumpy @novacain

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