transform std::string to lower case or upper case.

result.txt

$ ./a.out 
HELLO WORLD!
hello world!

str_transform.hpp

#include <string>
#include <algorithm>

namespace std {
const auto str_tolower = [](std::string s) {
    std::transform(s.begin(), s.end(), s.begin(), [](unsigned char c) { return std::tolower(c); });
    return s;
};
const auto str_toupper = [](std::string s) {
    std::transform(s.begin(), s.end(), s.begin(), [](unsigned char c) { return toupper(c); });
    return s;
};
}  // namespace std

test.cc

#include <iostream>
#include <string>
#include "str_transform.hpp"

int main() {
    std::string lowercase{"hello world!"};
    std::string uppercase{"HELLO WORLD!"};

    std::cout << std::str_toupper(lowercase) << std::endl;
    std::cout << std::str_tolower(uppercase) << std::endl;

    return 0;
}