C++ 设计模式外观模式 The Facade Pattern

介绍

Facade Pattern 为一组复杂的子系统提供了一个统一的简单接口，它是一种结构型设计模式。

它隐藏了子系统的复杂性，并向客户端提供了一个简单的接口来访问子系统。通过使用 Facade 模式，客户端可以方便地使用子系统，而不必了解子系统的内部细节。

Facade 模式由以下三个角色组成：

Facade 类：它是一个Facade类，它定义了客户端可以使用的简单接口。
Subsystem 类：它是一组复杂的子系统，它们完成了客户端请求的具体工作。
Client 类：它是一个客户端类，它通过使用 Facade 类来请求子系统。

Facade 模式的优点包括：

减少了客户端和子系统的耦合。
提高了子系统的独立性。
提高了客户端的可用性。
提高了系统的整体容错性。

在 C++ 中，Facade 模式可以使用继承和组合的方式实现。Facade 类可以继承自 Subsystem 类，也可以包含一个 Subsystem 类的对象。在 C++ 中，Facade 模式常常用于提供简单的接口来访问一组复杂的系统，从而简化客户端代码。

示例

#include <iostream>

// Subsystem class
class SubsystemA
{
public:
    void operationA()
    {
        std::cout << "Subsystem A operation" << std::endl;
    }
};

class SubsystemB
{
public:
    void operationB()
    {
        std::cout << "Subsystem B operation" << std::endl;
    }
};

// Facade class
class Facade
{
private:
    SubsystemA *subA;
    SubsystemB *subB;

public:
    Facade()
    {
        subA = new SubsystemA();
        subB = new SubsystemB();
    }

    void operation()
    {
        std::cout << "Facade operation starts" << std::endl;
        subA->operationA();
        subB->operationB();
        std::cout << "Facade operation ends" << std::endl;
    }
};

// Client class
int main()
{
    Facade *facade = new Facade();
    facade->operation();
    return 0;
}

维基百科示例

C++ Programming: Code patterns design - Wikibooks, open books for an open world

/*Facade is one of the easiest patterns I think... And this is very simple example.

Imagine you set up a smart house where everything is on remote. So to turn the lights on you push lights on button - And same for TV,
AC, Alarm, Music, etc...

When you leave a house you would need to push a 100 buttons to make sure everything is off and are good to go which could be little 
annoying if you are lazy like me 
so I defined a Facade for leaving and coming back. (Facade functions represent buttons...) So when I come and leave I just make one 
call and it takes care of everything...
*/

#include <string>
#include <iostream>

using namespace std;

class Alarm
{
public:
	void alarmOn()
	{
		cout << "Alarm is on and house is secured"<<endl;
	}

	void alarmOff()
	{
		cout << "Alarm is off and you can go into the house"<<endl;
	}
};

class Ac
{
public:
	void acOn()
	{
		cout << "Ac is on"<<endl;
	}

	void acOff()
	{
		cout << "AC is off"<<endl;
	}
};

class Tv
{
public:
	void tvOn()
	{
		cout << "Tv is on"<<endl;
	}

	void tvOff()
	{
		cout << "TV is off"<<endl;
	}
};

class HouseFacade
{
	Alarm alarm;
	Ac ac;
	Tv tv;

public:
	HouseFacade(){}

	void goToWork()
	{
		ac.acOff();
		tv.tvOff();
		alarm.alarmOn();
	}

	void comeHome()
	{
		alarm.alarmOff();
		ac.acOn();
		tv.tvOn();
	}
};

int main()
{
	HouseFacade hf;

	//Rather than calling 100 different on and off functions thanks to facade I only have 2 functions...
	hf.goToWork();
	hf.comeHome();
}