C++ Primer, Fifth Edition

Logical NOT Operator

The logical NOT operator (!) returns the inverse of the truth value of its operand. We first used this operator in § 3.2.2 (p. 87). As another example, assuming vec is a vector of ints, we might use the logical NOT operator to see whether vec has elements by negating the value returned by empty:

// print the first element in vec if there is oneif (!vec.empty())    cout << vec[0];

The subexpression

!vec.empty()

evaluates as true if the call to empty returns false.