MANIPULATING BITS

BITWISE OPERATORS

In cases where conservative use of memory is imperative, data often need to be bit coded,
a technique to represent information as individual bits. Some examples of bit coded data
can be found in file access rights or the status-word of a stream.

To access bit coded data, you need to be able to read or modify individual bits. C++
has six bitwise operators to perform these tasks:

Logical bitwise operators

& - AND

^ - exclusive OR

| - inclusive OR

~ - NOT

Bitwise shift operators

<< - Left shift

\>> - right shift

operands for bitwise opertor must be integer
, if bit value is 1 , it will be interpreted as true , else false

The result type of a bitwise operation will be the integral type defined by the operand type.

Arithmetic Type Conversions

If the operands of bitwise operator are different , then the operands are converted .

Example : If one operand is Int and other is long , then the long is converted to Int .

The difference between & or | and logical && and || operator is later doesn't effect the individual bits but interprets the whole value of their operands as boolean returning a boolean value.

The expression 1 && 2 returns the value true, whereas 1 & 2 has the value 0.

BITWISE SHIFT OPERATORS

The shift operators << and >> shift the bit pattern of their left operand a certain number of bit positions. The number of positions is defined by the right operand.

In the case of a left shift, 0 bits are padded. The bits dropped on the left are lost.

In the case of a right shift, 0 bits are padded from the left

example

short x = 0x08;     // 0000 1000
x =  x << 1;        // after shifting 0001 0000
cout << hex << x << endl;   // output  80

Applications

Shift operators allow you to perform efficient multiplication and division with 2^n
Shifting a number n places left (or right) is equivalent to a multiplication (or division) by 2^n.

BIT MASKS

#define MASK 0x20
char c = 'A';
c = c | MASK;

Deleting Bits

The bitwise AND operator is normally used to delete specific bits. A so-called mask is used to determine which bits to delete.

Setting and Inverting Bits

You can use the bitwise OR operator | to set specific bits. The example above shows how to change the case of a letter. In ASCII code, the only difference between a lowercase and an uppercase letter is the fifth bit.

Bitwise Operators in Compound Assignments

The binary bitwise operators &, |, ^, <<, and >> can be used in compound assignments.

Examples: x >>= 1;
          x ^= 1;