ctype_base

class ctype_base {
public:
    enum mask;
    static const mask space, print, cntrl,
        upper, lower, digit, punct, xdigit,
        alpha, alnum, graph;
    };

The class serves as a base class for facets of template class ctype. It defines just the enumerated type mask and several constants of this type. Each of the constants characterizes a different way to classify characters, as defined by the functions with similar names declared in the header <ctype.h>. The constants are:

• space (function isspace)
• print (function isprint)
• cntrl (function iscntrl)
• upper (function isupper)
• lower (function islower)
• digit (function isdigit)
• punct (function ispunct)
• xdigit (function isxdigit)
• alpha (function isalpha)
• alnum (function isalnum)
• graph (function isgraph)

You can characterize a combination of classifications by ORing these constants. In particular, it is always true that alnum == (alpha | digit) and graph == (alnum | punct).