#include <cctype> (or older <ctype.h>)

Because these are the original C functions, bool type isn't used; instead, zero is false and non-zero is true.

Type	Method	Description
int	`isalnum(c)`	true if c is a letter or digit.
int	`isalpha(c)`	true if c is a letter.
int	`isblank(c)`	true if c is a blank or tab.
int	`isdigit(c)`	true if c is a digit.
int	`islower(c)`	true if c is a lowercase letter.
int	`isupper(c)`	true if c is an uppercase letter.
int	`isspace(c)`	true if c is whitespace character (space, tab, vertical tab, formfeed, carriage return, or newline).
int	`tolower(c)`	returns lowercase version of c if there is one, otherwise it returns the character unchanged.
int	`toupper(c)`	returns uppercase version of c if there is one, otherwise it returns the character unchanged.