C++ STL string constructor

#include <iostream>
#include <string>
using namespace std;

intmain()
{
    string str("12345678");
    char ch[]="abcdefghijklmn";
    //Define empty string a
    string a;
    //Constructor, copy all str to str_1;
    string str_1(str);
    //Constructor, starting from the second element of the string str, copying five elements and assigning them to str_2
    string str_2(str,2,5);
    //Assign the first five elements of ch to str_3
    string str_3(ch,5);
    //Assign a string consisting of five A's to str_4
    string str_4(5,'A');
    //Copy all the contents of str to str_5
    string str_5(str.begin(),str.end());
    cout<<str<<endl<<a<<endl<<str_1<<endl<<str_2<<endl<<str_3<<endl<<str_4<<endl<<str_5<<endl;
    return 0;
}