The <string> library is included in the <iostream> library, so you don't need to include <string> separately, if you already use <iostream>

Alternatively, you can use the <string> library if you don't use <iostream>.

PS: <iostream> and <string> library both need to be in namespace std.

#include <iostream>
using namespace std;

int main(){
    string a ="ff";
    return 0;
}

#include<string>
using namespace std;

int main(){
    string a ="ff";
    return 0;
}