C++ Primer, Fifth Edition

Bitwise NOT Operator

The bitwise NOT operator (the ~ operator) generates a new value with the bits of its operand inverted. Each 1 bit is set to 0; each 0 bit is set to 1:

Here, our char operand is first promoted to int. Promoting a char to int leaves the value unchanged but adds 0 bits to the high order positions. Thus, promoting bits to int adds 24 high order bits, all of which are 0-valued. The bits in the promoted value are inverted.