is digit in c++ code example

Example 1: isdigit c++

str = "10"
  
//Evaluates to True
isdigit(str[0])

Example 2: c++98 check if character is integer

std::string s = "1234798797";
std::istringstream iss(s);

int num = 0;

if (!(iss >> num).fail()) {
    std::cout << num << std::endl;
}
else {
    std::cerr << "There was a problem converting the string to an integer!" << std::endl;
}