Examining flags when passed to a function?

This is a discussion on Examining flags when passed to a function? within the C++ Programming forums, part of the General Programming Boards category; The syntax of my question is likely to be totally incorrect! This is what I meant: When a function recieves ...

  1. #1
    Anal comment spacer DominicTrix's Avatar
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    Question Examining flags when passed to a function?

    The syntax of my question is likely to be totally incorrect! This is what I meant:

    When a function recieves a single variable that contains a set of flags seperated by the bitwise operator, how does it read those values and how are those values defined? i.e. I would like to write my own function that will take a set of flags as an argument.

    e.g.
    // Declaration
    int Function(DWORD flags);

    // Usage
    i = Function(FLAG_X | FLAG_Y | FLAG_Z);

    // Function Code:
    ?????????!

    Hope this makes sense, and thanks in advance

    dt
    "The most important thing about acting is honesty. If you can fake that you've got it made" - George Burns

  2. #2
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    Here is one solution.

    Code:
    enum TypeX {tX1 = 0x0001, tX2 = 0x02,  tX3 = 0x0003}
    
    void DetermineType(const int nType)
    {	
       for(int type = tX1; type <= tX3; type = (type << 1))
      {
          if (nType & type)
             // Type match.  Do something.
       }
    }
    Kuphryn

  3. #3
    S Sang-drax's Avatar
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    Umm kuphryn, you cannot have 3 as a flag.
    A flag must be a single bit, therefore valid flags are powers of 2.
    1
    2
    4
    8
    16
    32
    ... and so on.
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

  4. #4
    Guest Sebastiani's Avatar
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    let's say you want to use a 32 bit (4 byte ) integer to hold some flags. This just means to lay out the integer in binary format and and use each individual bit as an on/off, true/false, or similar binary flag.


    00000000 00000000 00000000 00000000

    Above is an integer with all bits 'cleared'.

    01000000 00000000 00000000 00000000

    Now we have set the second bit. To set a bit, we can | (OR) it to the number. We can do this in two ways. First, we can choose the zero-based index of the bit:


    #define TRUE 1
    int offset = 1; // second bit position

    flag |= (TRUE << offset);

    Would result in the second bit being set.

    Second, you can simply use an integer value:

    int value = 2;

    flag |= value;

    To determine if a bit is set:

    if(flag & (TRUE << offset))
    // then bit is set

    or:

    if(flag & value)
    // then bit is set

    To clear a bit & it with it's bit complement (~):

    flag &= ~(TRUE << offset);

    or:

    flag &= ~value;


    Hope that helps.
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

  5. #5
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    Correct. Modify the enum accordingly.

    Kuphryn

  6. #6
    Anal comment spacer DominicTrix's Avatar
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    Brilliant, thanks guys, I think I get it now :-)

    dt
    "The most important thing about acting is honesty. If you can fake that you've got it made" - George Burns

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