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Posted 20 Dec 2001

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Easier bitwise Operations

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4.46/5 (11 votes)
25 Dec 2001CPOL2 min read
A collection of templates that simplify the comprehension of bitwise operations

Introduction

I've been developing software using C/C++ for a very long time yet I still have to think twice when I encounter bitwise operations, let alone when I have to explain them to somebody else! For example:

C++
unsigned const STAT_ONE = 0x0001;
unsigned const STAT_TWO = 0x0002;

unsigned status = 0x0001;
if(status & (STAT_ONE | STAT_TWO)) // Does this evaluate to true or false?
   DoSomething();

We all know that the preceding statement checks if both STAT_ONE and STAT_TWO bits are set then DoSomething() will be executed... or is it if either STAT_ONE or STAT_TWO bits are set? Let's see:

C++
0001 or   // STAT_ONE
0010      // STAT_TWO
----
0011 and  // STAT_ONE | STAT_TWO
0001      // status
----
0001      // status & (STAT_ONE | STAT_TWO)

Because the result is non-zero, then DoSomething() will be executed. I think this can be very tricky, especially if the expressions get too complex. Isn't the intent of the following code easier to understand and less error-prone?

C++
unsigned const STAT_ONE = 0x0001;
unsigned const STAT_TWO = 0x0002;

unsigned status = 0x0001;
if(isAnyBitSet(status, STAT_ONE | STAT_TWO)) // This evaluates to true, bit
                                             // STAT_ONE is set and bit STAT_TWO
                                             // isn't. Same as previous example
   DoSomething();
C++
if(areAllBitsSet(status, STAT_ONE | STAT_TWO)) // This evaluates to false,
                                               // bit STAT_ONE is set but bit
                                               // STAT_TWO isn't
   DoSomething();

Also it's very convenient to access a bit's value by its position like this:

C++
if(isBitSetByPos(status, 5))
   bitClearByPos(status, 5);

All the functions provided in the BitTools.h header are inlined so there is no size or run-time speed tradeoffs to worry about.

Templates

Bitmask-based Functions

  • C++
    template <class T, class U>
    	bool isAnyBitSet(T value, U mask)

    Returns true if any of the bits in mask is set in value. Defined as: (value & mask) != 0

    C++
    1010 and  // value
    0110      // mask
    ----
    0010      // Non-zero: true
    
  • C++
    template <class T, class U>
    	bool areAllBitsSet(T value, U mask)

    Returns true if all the bits in mask are set in value. Defined as: (value & mask) == mask

    C++
    1010 and  // value
    1110      // mask
    ----
    1010      // 1010 == 1010: true
    
  • C++
    template <class T, class U>

    bool areAllBitsClear(T value, U mask)

  • Returns true if all the bits in mask are cleared in value. Defined as: (value & mask) == 0
    C++
    1010 and  // value
    0101      // mask
    ----
    0000      // Zero: true
    
  • C++
    template <class T, class U>
    	T setBits(T value, U mask)

    Returns value with the mask bits set. Defined as: value | mask

    C++
    1000 or   // value
    0110      // mask
    ----
    1110
    
  • C++
    template <class T, class U>
    	T setBitsExcept(T value, U mask)

    Returns value with all the bits set except the mask bits. Defined as: value | ~mask

    C++
    1001 not  // mask
    ----
    0110 or   // ~mask
    0001      // value
    ----
    0111
    
  • C++
    template <class T, class U>
    	T clearBits(T value, U mask)

    Returns value with the mask bits cleared. Defined as: value & ~mask

    C++
    1001 not  // mask
    ----
    0110 and  // ~mask
    1111      // value
    ----
    0110
    
  • C++
    template <class T, class U>
    	T clearBitsExcept(T value, U mask)

    Returns value with all the bits cleared except the mask bits. Defined as: value & mask

    C++
    0010 and  // value
    0110      // mask
    ----
    0010
    
  • C++
    template <class T, class U>
    	T setClearBits(T value, U add, U remove)

    Returns value with the add bits set and the remove bits cleared. Defined as: (value | add) & ~remove

    C++
    1101 or   // value
    0101      // add
    ----
    0111
    
    0001 not  // remove
    ----
    1110 and  // ~remove
    0111      // value | add
    ----
    0110
    
  • C++
    template <class T, class U, class V>
    	T setBits(T value, U mask, V set)

    Returns value with the mask bits set or cleared depending on the value of set.

Position-based Functions

  •  

    C++
    template <class T>
    	T setBitByPos(T value, unsigned char n)

    Returns value with the nth bit set. Defined as value | (1 << n)

  •  

    C++
    template <class T>
    	T clearBitByPos(T value, unsigned char n)

    Returns value with the nth bit cleared. Defined as value & ~(1 << n)

  •  

    C++
    template <class T>
    	bool isBitSetByPos(T value, unsigned char n)

    Returns true if value has the nth bit set. Defined as (value & (1 << n)) != 0

  •  

    C++
    template <class T>
    	bool isBitClearByPos(T value, unsigned char n)

    Returns true if value has the nth bit cleared. Defined as (value & (1 << n)) == 0

Conclusion

The templates contained in BitTools.h provide an easier and less error-prone way of expressing bitwise operations with no performance or size penalties compared to hand written code. I hope you find these functions as useful and easy to use as I did.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

 
QuestionNice Article Thank You Pin
Member 118470888-Jan-19 23:16
Member 118470888-Jan-19 23:16 
GeneralNone Pin
Jonathan de Halleux25-Feb-03 21:49
Jonathan de Halleux25-Feb-03 21:49 
GeneralWhy templates Pin
3-Jan-02 10:37
suss3-Jan-02 10:37 
GeneralRe: Why templates Pin
Nemanja Trifunovic3-Jan-02 11:12
Nemanja Trifunovic3-Jan-02 11:12 
GeneralRe: Why templates Pin
Eddie Velasquez3-Jan-02 14:52
Eddie Velasquez3-Jan-02 14:52 
GeneralASSERTs Pin
Mike Klimentiev2-Jan-02 10:20
Mike Klimentiev2-Jan-02 10:20 
GeneralRe: ASSERTs Pin
Eddie Velasquez3-Jan-02 2:36
Eddie Velasquez3-Jan-02 2:36 
GeneralRe: ASSERTs Pin
Mike Klimentiev3-Jan-02 8:06
Mike Klimentiev3-Jan-02 8:06 
GeneralRe: ASSERTs Pin
Eddie Velasquez4-Jan-02 2:23
Eddie Velasquez4-Jan-02 2:23 
GeneralRe: ASSERTs Pin
Mike Klimentiev4-Jan-02 8:41
Mike Klimentiev4-Jan-02 8:41 
GeneralRe: ASSERTs Pin
Eddie Velasquez4-Jan-02 8:53
Eddie Velasquez4-Jan-02 8:53 
GeneralStandard C MACRO version Pin
Daniel May26-Dec-01 16:27
Daniel May26-Dec-01 16:27 
GeneralRe: Standard C MACRO version Pin
Eddie Velasquez27-Dec-01 3:19
Eddie Velasquez27-Dec-01 3:19 
GeneralRe: Standard C MACRO version Pin
Daniel May27-Dec-01 3:23
Daniel May27-Dec-01 3:23 
GeneralRe: Standard C MACRO version Pin
Eddie Velasquez27-Dec-01 4:11
Eddie Velasquez27-Dec-01 4:11 
GeneralI think this is much more intuitive. Pin
Henry Jacobs22-Dec-01 3:01
Henry Jacobs22-Dec-01 3:01 
GeneralI think this is less error prone Pin
Igor Okulist21-Dec-01 14:59
Igor Okulist21-Dec-01 14:59 
GeneralRe: I think this is less error prone Pin
Eddie Velasquez21-Dec-01 15:13
Eddie Velasquez21-Dec-01 15:13 
GeneralRe: I think this is less error prone Pin
Philippe Lhoste2-Jan-02 23:28
Philippe Lhoste2-Jan-02 23:28 
GeneralRe: I think this is less error prone Pin
Eddie Velasquez3-Jan-02 2:40
Eddie Velasquez3-Jan-02 2:40 
GeneralRe: I think this is less error prone Pin
Philippe Lhoste3-Jan-02 3:21
Philippe Lhoste3-Jan-02 3:21 
GeneralRe: I think this is less error prone Pin
Igor Okulist3-Jan-02 5:33
Igor Okulist3-Jan-02 5:33 
GeneralRe: I think this is less error prone Pin
Eddie Velasquez3-Jan-02 7:14
Eddie Velasquez3-Jan-02 7:14 

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