Standard C++ Library Reference ISO/IEC (VERSION3)

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<ctype.h>
isalnum · isalpha · iscntrl · isdigit · isgraph · islower ·
isprint · ispunct · isspace · isupper · isxdigit · tolower ·
toupper
Include the standard header <ctype.h> to declare several functions that are useful for
classifying and mapping codes from the target character set. Every function that has a parameter
of type int can accept the value of the macro EOF or any value representable as type unsigned
char. Thus, the argument can be the value returned by any of the functions fgetc, fputc,
getc, getchar, putc, putchar, tolower, toupper, and ungetc. You must not call
these functions with other argument values.
Other library functions use these functions. The function scanf, for example, uses the function
isspace to determine valid white space within an input field.
The character classification functions are strongly interrelated. Many are defined in terms of
other functions. For characters in the basic C character set, here are the dependencies between
these functions:
The diagram indicates with + those functions that can define additional characters in locales
other than the "C" locale. Boldface indicates a feature added with C99.