C++的类做友元使用问题

Question

这样写为什么是错误的?已经提前声明了mygoods,但是为什么还是会报错未定义，把mygoods修改到goodgay之前定义，就不会有问题
编译错误提示：

```c++
invalid use of incomplete type 'class mygoods'
invalid use of incomplete type 'class mygoods'
invalid use of incomplete type 'class mygoods'

程序代码如下：

#include <iostream>
using namespace std;
#include <string>
class mygoods;
class goodgay
{
public:
    goodgay()
    {
        goods = new mygoods;
    }
    void usegoods()
    {
        cout<<"use public:"<<goods->gs1<<endl;
        cout<<"use public:"<<goods->gs2<<endl;
    }
    mygoods *goods;
};

class mygoods
{
    friend class goodgay;
public:
    mygoods()
    {
        gs1 = "抽纸";
        gs2 = "水杯";
    }
    string gs1;
private:
    string gs2;
};

void test01(void)
{
    goodgay gay1;
    gay1.usegoods();
}
int main(void)
{

    cout<<"hello world"<<endl;
    test01();
    system("pause");
    return 0;
}

```

Answer 1

#include <iostream>
using namespace std;
#include <string>
class mygoods;
class goodgay
{
public:
    goodgay();
    void usegoods();
    mygoods *goods;
};
 
class mygoods
{
    friend class goodgay;
public:
    mygoods()
    {
        gs1 = "抽纸";
        gs2 = "水杯";
    }
    string gs1;
private:
    string gs2;
};

goodgay::goodgay()
{
    goods = new mygoods;
}

void goodgay::usegoods()
{
    cout<<"use public:"<<goods->gs1<<endl;
    cout<<"use public:"<<goods->gs2<<endl;
}

void test01(void)
{
    goodgay gay1;
    gay1.usegoods();
}
int main(void)
{
 
    cout<<"hello world"<<endl;
    test01();
    system("pause");
    return 0;
}

另外，英文单词不晓得含义的去查一查，别瞎用。