error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'std::string'

#include "student.h"
#include <string>
#include <iostream>
using namespace std;

student::student(int n,char s,datetime date,int i,string c,int g): person(n,s,date,i), course(c), performance(g)
{
}

void student::GetScore()
{
cout<<person.show<<endl;
}

void student::GetCourseName()
{
cout<<course<<performance<<endl;
}

student::~student(void)
{
}

这一句报错了：cout<<course<<performance<<endl;
怎么改？

相关了解……