How to convert std::string to lower case?

Question

I want to convert a std::string to lowercase. I am aware of the function tolower(), however in the past I have had issues with this function and it

Answer 1

I wrote this simple helper function:

#include  // tolower

string to_lower(string s) {        
    for(char &c : s)
        c = tolower(c);
    return s;
}

Usage:

string s = "TEST";
cout << to_lower("HELLO WORLD"); // output: "hello word"
cout << to_lower(s); // won't change the original variable.