std::isupper

Checks if the given character is an uppercase character as classified by the currently installed C locale. In the default "C" locale, std::isupper returns a nonzero value only for the uppercase letters (ABCDEFGHIJKLMNOPQRSTUVWXYZ

If std::isupper returns a nonzero value, it is guaranteed that std::iscntrl, std::isdigit, std::ispunct, and std::isspace

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 uppercase letter, zero otherwise.

Notes

Like all other functions from <cctype>, the behavior of std::isupper 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_isupper(char ch)
{
    return std::isupper(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

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

Example

#include <cctype>
#include <clocale>
#include <iostream>
 
int main()
{
    unsigned char c = '\xc6'; // letter Æ in ISO-8859-1
 
    std::cout << "isupper(\'\\xc6\', default C locale) returned "
              << std::boolalpha << (bool)std::isupper(c) << '\n';
 
    std::setlocale(LC_ALL, "en_GB.iso88591");
    std::cout << "isupper(\'\\xc6\', ISO-8859-1 locale) returned "
              << std::boolalpha << (bool)std::isupper(c) << '\n';
 
}

isupper('\xc6', default C locale) returned false
isupper('\xc6', ISO-8859-1 locale) returned true

See also