#include <iostream>
#include <string>
 
void show_pos(const std::string& str, std::string::size_type found) {
    if (found != std::string::npos) {
        std::cout << "[" << found << "] = \'" << str[found] << "\'\n";
    } else {
        std::cout << "not found" "\n";
    }
}
 
int main()
{
    std::string str { "abc_123" };
    char const* skip_set { "0123456789" };
    std::string::size_type str_last_pos { std::string::npos };
 
    show_pos(str, str.find_last_not_of(skip_set)); // [3] = '_'
 
    str_last_pos = 2;
    show_pos(str, str.find_last_not_of(skip_set, str_last_pos)); // [2] = 'c'
 
    str_last_pos = 2;
    show_pos(str, str.find_last_not_of('c', str_last_pos)); // [1] = 'b'
 
    const char arr[] { '3', '4', '5' };
    show_pos(str, str.find_last_not_of(arr)); // [5] = '2'
 
    str_last_pos = 2;
    std::string::size_type skip_set_size{4};
    show_pos(str, str.find_last_not_of(skip_set,
                                       str_last_pos,
                                       skip_set_size)); // [2] = 'c'
 
    show_pos(str, str.find_last_not_of("abc")); // [6] = '3'
 
    str_last_pos = 2;
    show_pos(str, str.find_last_not_of("abc", str_last_pos)); // 找不到
}