std:: isdigit

Checks if the given character is one of the 10 decimal digits: ( 0123456789 ).

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 a numeric character, zero otherwise.

Notes

isdigit and isxdigit are the only standard narrow character classification functions that are not affected by the currently installed C locale. although some implementations (e.g. Microsoft in 1252 codepage ) may classify additional single-byte characters as digits.

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

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

Example

#include <cctype>
#include <climits>
#include <iostream>
 
int main(void)
{
    for (int i = 0; i <= UCHAR_MAX; ++i)
        if (std::isdigit(i))
            std::cout << static_cast<unsigned char>(i);
    std::cout << '\n';
}

0123456789

See also