Some Error collection when std for C++

#include<iostream.h>
#include<string.h>
using namespace std;
int main(){
     int a(10);
     cout<<a;
}
will be run ok but the code as fallows will be ...

#include<iostream.h>
#include<string.h>//here need to #include<string>
using namespace std;
int main(){
     int a(10);
     cout<<a;
     string mystr = "c++ std error";
     cout<<mystr;//notice here. error occur!!
}