20.5. Bitwise Operators

C++ provides extensive bit-manipulation capabilities for getting down to the so-called “bits-and-bytes” level. Operating systems, test-equipment software, networking software and many other kinds of software require that you communicate “directly with the hardware.” We introduce each of the bitwise operators, and we discuss how to save memory by using bit fields.

All data is represented internally by computers as sequences of bits. Each bit can assume the value 0 or the value 1. On most systems, a sequence of eight bits, each of which forms a byte—the standard storage unit for a variable of type char. Other data types are stored in larger numbers of bytes. Bitwise operators are used to manipulate the bits of integral operands (char, short, int and long; both signed and unsigned). Normally the bitwise operators are used with unsigned integers.

The bitwise operator discussions in this section show the binary representations of the integer operands. For a detailed explanation of the binary (also called base-2) number system, see Appendix D. Because of the machine-dependent nature of bitwise manipulations, some of these programs might not work on your system without modification.

The bitwise operators are: bitwise AND (&), bitwise inclusive OR (|), bitwise exclusive OR (^), left shift (<<), right shift (>>) and bitwise complement (~)—also known as the one’s complement. We’ve been using &, << and >> for other purposes—this is a classic example of operator overloading. The bitwise AND, bitwise inclusive OR and bitwise exclusive OR operators compare their two operands bit by bit. The bitwise AND operator sets each bit in the result to 1 if the corresponding bit in both operands is 1. The bitwise inclusive OR operator sets each bit in the result to 1 if the corresponding bit in either (or both) operand(s) is 1. The bitwise exclusive OR operator sets each bit in the result to 1 if the corresponding bit in either operand—but not both—is 1. The left-shift operator shifts the bits of its left operand to the left by the number of bits specified in its right operand. The right-shift operator shifts the bits in its left operand to the right by the number of bits specified in its right operand. The bitwise complement operator sets all 0 bits in its operand to 1 in the result and sets all 1 bits in its operand to 0 in the result. Detailed discussions of each bitwise operator appear in the following examples. The bitwise operators are summarized in Fig. 20.5.

Fig. 20.5. Bitwise operators.