C++设计模式：访问者模式

一、源代码：

#include<iostream>
#include<memory>
#include<vector>

using namespace std;

class Computer;
class Mouse;
class Keyboard;
class Monitor;
class ComputerPartVisitor
{
public:
    virtual void visit(shared_ptr<Computer> computer) = 0;
    virtual void visit(shared_ptr<Mouse> mouse) = 0;
    virtual void visit(shared_ptr<Keyboard> keyboard) = 0;
    virtual void visit(shared_ptr<Monitor> monitor) = 0;
    virtual ~ComputerPartVisitor() = default;
};

class ComputerPartDisplayVisitor:public ComputerPartVisitor
{
public:
    virtual void visit(shared_ptr<Computer> computer) override
    {
        cout<<"Displaying Computer."<<endl;
    }
    virtual void visit(shared_ptr<Mouse> mouse) override
    {
        cout<<"Displaying Mouse."<<endl;
    }
    virtual void visit(shared_ptr<Keyboard> keyboard) override
    {
        cout<<"Displaying Keyboard."<<endl;
    }
    virtual void visit(shared_ptr<Monitor> monitor) override
    {
        cout<<"Displaying Monitor."<<endl;
    }
};

class ComputerPart
{
public:
    virtual void accept(shared_ptr<ComputerPartVisitor> computerPartVisitor) = 0;
    virtual ~ComputerPart() = default;
};

class Keyboard:public ComputerPart
{
public:
    virtual void accept(shared_ptr<ComputerPartVisitor> computerPartVisitor) override
    {
        computerPartVisitor->visit(make_shared<Keyboard>(*this));
    }
};

class Monitor:public ComputerPart
{
public:
    virtual void accept(shared_ptr<ComputerPartVisitor> computerPartVisitor) override
    {
        computerPartVisitor->visit(make_shared<Monitor>(*this));
    }
};

class Mouse:public ComputerPart
{
public:
    virtual void accept(shared_ptr<ComputerPartVisitor> computerPartVisitor) override
    {
        computerPartVisitor->visit(make_shared<Mouse>(*this));
    }
};

class Computer:public ComputerPart
{
public:
    virtual void accept(shared_ptr<ComputerPartVisitor> computerPartVisitor) override
    {
        vector<shared_ptr<ComputerPart>> parts = {make_shared<Mouse>(),make_shared<Keyboard>(),make_shared<Monitor>()};
        for(const auto& part:parts)
        {
            part->accept(computerPartVisitor);
        }
        computerPartVisitor->visit(make_shared<Computer>(*this));
    }
};

int main()
{
    shared_ptr<ComputerPart> computer = make_shared<Computer>();
    computer->accept(make_shared<ComputerPartDisplayVisitor>());
}

二、运行结果：

三、吐槽：

绕的头晕，只用一个C++文件，问题很多，以后尽量不要设计写这种来回嵌套的类——两个类的定义互相包含对方，不利于维护，要降低耦合度，单向依赖