为什么输出为0

#include<bits/stdc++.h>
using namespace std;

int main(){
/*
freopen (".in","r",stdin);
freopen (".out","w",stdout);
*/
string s;
s[0]='1';
s[1]='2';
cout<<s.size();





/*
fclose(stdin);
fclose(stdout);
*/
	return 0;
}