C++ 友元

#include<iostream>
#include<string>
using namespace std;
class Sales_item;//前置申明,让Test识别Sales_item
class Test
{
public:
    void GetName(Sales_item &item);

    std::string TestName;
};

class Sales_item
{
    friend void Test::GetName(Sales_item &item);
    //freind class Test;的话Test任何方法都能访问
public:
    Sales_item(const std::string &book)
        :bookName(book){}
    std::string GetBookName() const
    {
        return bookName;
    }
private:
    std::string bookName;
};

void Test::GetName(Sales_item &item)//必须放友元申明之后
{
    cout<<item.bookName<<endl;;
}

int main()
{
    Sales_item item1="bookName1";
    Test t;
    t.GetName(item1);
    
}