11.3 Bitwise OR ()

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11.3 Bitwise OR ()

The inclusive OR operator (also known as just the OR operator) compares its two operands. If one or the other bit is a 1, the result is 1. Table 11-4 lists the truth table for the OR operator.

Table 11-4. Bitwise OR operator

Bit1	Bit2	Bit1 Bit2

	1	1
1		1
1	1	1

Here's an example of bitwise OR performed on a byte:

i1=0x47       01000111
      i2=0x53       01010011
______________________________
         0x57        01010111

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11.4 The Bitwise Exclusive OR (^)

The exclusive OR (also known as XOR) operator results in a 1 when either of its two operands is a 1, but not both. The truth table for the exclusive OR operator is listed in Table 11-5.

Table 11-5. Bitwise exclusive OR operator

Bit1	Bit2	Bit1 ^ Bit2

	1	1
1		1
1	1

Here's an example of bitwise exclusive OR performed on a byte:

i1=0x47       01000111 
^      i2=0x53       01010011
______________________________
         0x14        00010100

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11.5 The Ones Complement Operator (NOT) (~)

The NOT operator (also called the invert operator or bit flip) is a unary operator that returns the inverse of its operand, as shown in Table 11-6.

Table 11-6. NOT operator

Bit	~Bit
	1
1

Here's an example of NOT performed on a byte:

c=      0x45      01000101
________________________________
~c=     0xBA      10111010

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11.6 The Left and Right Shift Operators (<<, >>)

The left shift operator moves the data left a specified number of bits. Any bits that are shifted out the left side disappear. New bits coming in from the right are zeros. The right shift does the same thing in the other direction. For example:

	c=0x1C	00011100
c << 1	c=0x38	00111000
c >> 2	c=0x07	00000111

Shifting left by one ( x << 1) is the same as multiplying by 2 ( x * 2 ). Shifting left by two ( x << 2 ) is the same as multiplying by 4 ( x * 4 , or x * 22 ). You can see a pattern forming here. Shifting left by n places is the same as multiplying by 2 ⁿ . Why shift instead of multiply? Shifting is faster than multiplication, so:

i = j << 3;     // Multiply j by 8 (2**3)

is faster than:

i  = j * 8;

Or it would be faster if compilers weren't smart enough to turn "multiply by power of two" into "shift."

Many clever programmers use this trick to speed up their programs at the cost of clarity. Don't do it. The compiler is smart enough to perform the speedup automatically. This means that putting in a shift gains you nothing at the expense of clarity.

The left shift operator multiplies; the right shift divides. So:

q = i >> 2;

is the same as:

q = i / 4;

Again, this clever trick should not be used in modern code.

11.6.1 Right Shift Details

Right shifts are particularly tricky. When a variable is shifted to the right, C++ needs to fill the space on the left side with something. For signed variables, C++ uses the value of the sign bit. For unsigned variables , C++ uses zero. Table 11-7 illustrates some typical right shifts.

Table 11-7. Right shift examples

	Signed character	Signed character	Unsigned character
Expression	9 >> 2	-8 >> 2	248 >> 2
Binary value >> 2	0000 1010 ₂ >> 2	1111 1000 ₂ >> 2	1111 1000 ₂ >> 2
Result	??00 0010 ₂	??11 1110 ₂ >> 2	??11 1110 ₂ >> 2
Fill	Sign bit (0)	Sign bit (1)	Zero
Final result (binary)	0000 0010 ₂	1111 1110 ₂	0011 1110 ₂
Final result (short int)	2	-2	62