cpp/string/byte/isalpha

Checks if the given character is an alphabetic character as classified by the currently installed C locale. In the default locale, the following characters are alphabetic:
 * uppercase letters
 * lowercase letters

In locales other than, an alphabetic character is a character for which std or std returns non-zero or any other character considered alphabetic by the locale. In any case, std, std, std and std will return zero for this character.

The behavior is undefined if the value of is not representable as  and is not equal to eof.

Return value
Non-zero value if the character is an alphabetic character, zero otherwise.