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Checks if the given character is an alphanumeric character as classified by the current C locale. In the default locale, the following characters are alphanumeric:

• digits (0123456789)
• uppercase letters (ABCDEFGHIJKLMNOPQRSTUVWXYZ)
• lowercase letters (abcdefghijklmnopqrstuvwxyz)

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

[edit] Parameters

[edit] Return value

Non-zero value if the character is an alphanumeric character, 0 otherwise.

[edit] Notes

Like all other functions from <cctype>, the behavior of std::isalnum 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_isalnum(char ch)
{
 return std::isalnum(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_alnums(const std::string & s)
{
 return std::count_if (s.begin(), s.end(),
 // static_cast<int(*)(int)>(std::isalnum) // wrong
 // [](int c){ return std::isalnum(c); } // wrong
 // [](char c){ return std::isalnum(c); } // wrong
 [](unsigned char c){ return std::isalnum(c); } // correct
 );
}

[edit] Example

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

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

[edit] See also

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