string::front

Front & char (); 
const char & Front () const; 
Function: Returns the first character string object, you can change its value

#include <string>
#include <iostream>

using namespace std;

int main()
{
string s1("test");
cout << s1.front() << endl;
s1.front() = 'T';
cout << s1 << endl;
string s2;
cout << s2.front() << endl;
}