C++ Primer Plus

The Logical NOT Operator: !

The ! operator negates, or reverses the truth value of, the expression that follows it. That is, if expression is true, then !expression is false—and vice versa. More precisely, if expression is true, or nonzero, then !expression is false. Incidentally, many people call the exclamation point bang, making !x “bang-ex” and !!x “bang-bang-ex.”

Usually you can more clearly express a relationship without using the ! operator:

if (!(x > 5))                 // if (x <= 5) is clearer

But the ! operator can be useful with functions that return true/false values or values that can be interpreted that way. For example, strcmp(s1,s2) returns a nonzero (true) value if the two C-style strings s1 and s2 are different from each other ...