Difference between revisions of "cpp/string/byte/isalpha"

Revision as of 11:56, 31 July 2020

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 ABCDEFGHIJKLMNOPQRSTUVWXYZ
• lowercase letters abcdefghijklmnopqrstuvwxyz

In locales other than "C", an alphabetic character is a character for which std::isupper() or std::islower() returns non-zero or any other character considered alphabetic by the locale. In any case, std::iscntrl(), std::isdigit(), std::ispunct() and std::isspace() will return zero for this character.

The behavior is undefined if the value of ch is not representable as unsigned char and is not equal to EOF.

Parameters

Return value

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

Notes

Like all other functions from <cctype>, the behavior of std::isalpha is undefined if the argument's value is neither representable as unsigned char nor equal to EOF. To use these functions safely with plain chars (or signed chars), the argument should first be converted to unsigned char:

bool my_isalpha(char ch)
{
    return std::isalpha(static_cast<unsigned char>(ch));
}

Similarly, they should not be directly used with standard algorithms when the iterator's value type is char or signed char. Instead, convert the value to unsigned char first:

int count_alphas(const std::string& s)
{
    return std::count_if(s.begin(), s.end(),
                      // static_cast<int(*)(int)>(std::isalpha)         // wrong
                      // [](int c){ return std::isalpha(c); }           // wrong
                      // [](char c){ return std::isalpha(c); }          // wrong
                         [](unsigned char c){ return std::isalpha(c); } // correct
                        );
}

Example

#include <iostream>
#include <cctype> 
#include <clocale>
 
int main()
{        
    unsigned char c = '\xdf'; // German letter ß in ISO-8859-1
 
    std::cout << "isalpha(\'\\xdf\', default C locale) returned "
               << std::boolalpha << (bool)std::isalpha(c) << '\n';
 
    std::setlocale(LC_ALL, "de_DE.iso88591");
    std::cout << "isalpha(\'\\xdf\', ISO-8859-1 locale) returned "
              << std::boolalpha << (bool)std::isalpha(c) << '\n';
 
}

isalpha('\xdf', default C locale) returned false
isalpha('\xdf', ISO-8859-1 locale) returned true

See also