为什么要写一个std :: string给cout导致一个未知的运算符＆lt;＆lt;错误？

Question

当我尝试从我的一个方法输出返回值时出现错误：

Error: No operator "<<" matches these operands. Operand types are: std::ostream << std::string

Main.cpp的

#include <iostream>
using namespace std;

#include "Book.h"

int main()
{
    book.setTitle("Advanced C++ Programming");
    book.setAuthorName("Linda", "Smith");
    book.setPublisher("Microsoft Press", "One Microsoft Way", "Redmond");
    book.setPrice(49.99);

    cout << book.getBookInfo(); // <-= this won't compile because of the error above.

    int i;
    cin >> i;

    return 0;
};

应返回字符串的方法：

string Book::getBookInfo()
{
    stringstream ss;
    ss << title << endl << convertDoubleToString(price) << endl;

    return ss.str();
}

Answer 1

#include <string>缺失。

Answer 2

代码是如何得到string的定义的？标头<string>也声明了流插入器。