C++ Primer, Fifth Edition

9.2.6. Container Size Operations

With one exception, the container types have three size-related operations. The size member (§ 3.2.2, p. 87) returns the number of elements in the container; empty returns a bool that is true if size is zero and false otherwise; and max_size returns a number that is greater than or equal to the number of elements a container of that type can contain. For reasons we’ll explain in the next section, forward_list provides max_size and empty, but not size.