C++：赋值运算符和多重继承

#include <iostream>
using namespace std;


class Socket
{
public:
    Socket(int flags): _flags(flags){ cout << "Socket constructor" << " _flags pointer: " << &_flags << endl;}

    Socket& operator = (const Socket& socket) 
    {
        cout << "Socket assignment operationer. _flags = " << _flags << ", _flags pointer : " << &_flags << ", socket._flags = " << socket._flags << ", socket._flags pointer : " << &(socket._flags) << endl; 
        _flags = socket._flags;
        return *this;
    }

    int getFlags() {
        return _flags;
    }

private:
    int _flags = 0;
};

class StreamSocket: public Socket
{
public:
    StreamSocket(): Socket(1){ cout << "StreamSocket(1) constructor" << endl; }
    StreamSocket(int flags): Socket(flags) {cout << "StreamSocket(flags) constructor" << endl;}

    StreamSocket& operator = (const Socket& socket) 
    {
        cout << "StreamSocket assignment operationer" << endl;
        Socket::operator = (socket);
        return *this;
    }
};

class SecureStreamSocket: public StreamSocket
{
public:
    SecureStreamSocket(): StreamSocket(2){ cout << "SecureStreamSocket(2) constructor" << endl;}

    SecureStreamSocket& operator = (const Socket& socket)
    {
        StreamSocket::operator = (socket);
        cout << "SecureStreamSocket assignment operationer" << endl; 
        return *this;
    }
};

class ZkStreamSocket: public StreamSocket
{
};

class ZkSecureStreamSocket : public ZkStreamSocket, public SecureStreamSocket
{
public:
    using SecureStreamSocket::SecureStreamSocket;
};

/*
*   总结：
*       1：ZkSecureStreamSocket的内存分布包含了ZkStreamSocket和SecureStreamSocket两块，因此也包含了两块Socket和它的成员变量_flags；
*   2：由于赋值运算符不属于函数重写，其调用依赖于左值类型。同时，在调用赋值运算符的时候找不到参数匹配的赋值运算符，因此各个子类会生成
*   默认赋值运算符并且在默认赋值运算符中调用父类的赋值运算符。所以我们能看到最终调用到的都是Socket::operator =(Socket)（只要左值类
*   不是右值类的派生类）；3：在test1和test2的对比中，多重继承的右值对象根据左值类型将内存中的对应的Socket::_flags在Socket类中拷贝
*   给左值。
*/
int main()
{
    /*几个类中的赋值运算符看起来既不属于函数重载也不属于函数重写*/
    /*test 1： https://stackoverflow.com/questions/8894090/derived-class-inherit-base-class-assignment-operator*/
    /*反编译查看代码流程：https://www.cnblogs.com/skynet/archive/2010/09/05/1818636.html*/
    ZkStreamSocket socket; // 菱形继承
    cout << socket.getFlags() << endl;   // 1
    socket = ZkSecureStreamSocket();     // 改成ZkStreamSocket结果是一样的
    cout << socket.getFlags() << endl;   // 1

    /*test 2*/
    // SecureStreamSocket socket;  // 菱形继承, 更改左值类型， 和上面结果对比，根据左值类型（基类作用域）进行子类给基类赋值（SecureStreamSocket和ZkStreamSocket是菱形继承的基类）？
    // cout << socket.getFlags() << endl;   // 2
    // socket = ZkSecureStreamSocket();
    // cout << socket.getFlags() << endl;   // 2

    /*test 3: https://www.codenong.com/cs106790463/*/
    // StreamSocket socket;   // 非菱形继承, 子类和父类互相赋值
    // cout << socket.getFlags() << endl;   // 1
    // socket = SecureStreamSocket();
    // cout << socket.getFlags() << endl;   // 2
    // return 0;
}

poco中在Socket部分代码中使用的是PIMPL（桥接）,Socket类的Impl变量是“桥”，所有的其他派生类通过构造函数的方式将自己的Impl配置给Socket，其UML大概如下：