Bit Flag Basics

Bit Flag Basics

The main use of C++ bitwise operators is to use bit flags in integer variables, which is very efficient in both storage space and execution time. A vanilla “int” can store 32 bit flags, and a “long” can store 64 bits. The basic bit operations in C++ use these bitwise operators:

• Check a bit — bitwise-AND (&)
• Set a bit — bitwise-OR (|)
• Toggle a bit — bitwise-XOR (^)
• Clear a bit — bitwise-AND with complement (& with ~)

Here are some example macros for examining the bits in a 32-bit integer, which should be of “unsigned int” type:

    // Bit Flags in Integers
    #define AUSSIE_ONE_BIT_SET(x, b)   \
      (( ((unsigned)(x)) & ((unsigned)(b))) != 0 )
    #define AUSSIE_ANY_BITS_SET(x, b) \
      (( ((unsigned)(x)) & ((unsigned)(b))) != 0 )
    #define AUSSIE_ALL_BITS_SET(x, b) \
      ((((unsigned)(x))&((unsigned)(b))) == ((unsigned)(b)))
    #define AUSSIE_NO_BITS_SET(x, b)  \
      (( ((unsigned)(x)) & ((unsigned)(b))) == 0 )

The corresponding macros to set and clear these bit flags are:

    #define AUSSIE_SET_BITS(x, b)    \
      (( ((unsigned)(x)) | ((unsigned)(b))))
    #define AUSSIE_CLEAR_BITS(x, b)  \
      (( ((unsigned)(x)) & (~((unsigned)(b)))))
    #define AUSSIE_TOGGLE_BITS(x, b) \
      (( ((unsigned)(x)) ^ ((unsigned)(b))))

Yikes! What a mess! But all those parentheses are necessary to avoid precedence issues with preprocessor macros.

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