Bitwise Shift Operators

Bitwise shift operators shift the bits in the left-hand operand integer the specified number of bits given in the right-hand operand, in the specified direction. A shift by one bit left multiplies the number by two, a shift one bit to the right divides by 2. In the following a 2-byte integer is bit-shifted:

    unsigned short s1 = 0x0010; 
    unsigned short s2 = s1 << 8; 
    std::cout << std::hex << std::showbase; 
    std::cout << s2 << std::endl; 
    // 0x1000  
    s2 = s2 << 3; 
    std::cout << s2 << std::endl; 
    // 0x8000

In this example, the s1 variable has the fifth bit set (0x0010 or 16). The s2 variable has this value, shifted left by 8 bits, so the single bit is shifted to the 13th bit, and the bottom 8 bits are all set to 0 (0x10000 or 4,096). This means that 0x0010 has been multiplied by 2⁸, or 256, to give 0x1000. Next, the value is shifted left by another 3 bits, and the result is 0x8000; the top bit is set.

The operator discards any bits that overflow, so if you have the top bit set and shift the integer one bit left, that top bit will be discarded:

    s2 = s2 << 1; 
    std::cout << s2 << std::endl; 
    // 0

A final shift left by one bit results in a value 0.

It is important to remember that, when used with a stream, the operator << means insert into the stream, and when used with integers, it means bitwise shift.