C++设计模式-享元（Flyweight）

一、意图

二、适用性

三、结构

四、参与者

五、代码

C++设计模式-享元（Flyweight）

一、意图

运用共享技术有效地支持大量细粒度的对象。

二、适用性

一个应用程序使用了大量的对象。
完全由于使用大量的对象，造成很大的存储开销。
对象的大多数状态都可变为外部状态。
如果删除对象的外部状态，那么可以用相对较少的共享对象取代很多组对象。
应用程序不依赖于对象标识。由于Flyweight对象可以被共享，对于概念上明显有别的对象，标识测试将返回真值。

三、结构

四、参与者

Flyweight

描述一个接口，通过这个接口flyweight可以接受并作用于外部状态。

ConcreteFlyweight

实现Flyweight接口，并为内部状态（如果有的话）增加存储空间。ConcreteFlyweight对象必须是可共享的。它所存储的状态必须是内部的；即，它必须独立于ConcreteFlyweight对象的场景。

UnsharedConcreteFlyweight

并非所有的Flyweight子类都需要被共享。Flyweight接口使共享成为可能，但它并不强制共享。在Flyweight对象结构的某些层次，UnsharedConcreteFlyweight对象通常将ConcreteFlyweight对象作为子节点。

FlyweightFactory

创建并管理flyweight对象。

确保合理地共享flyweight。当用户请求一个flyweight时，FlyweightFactory对象提供一个已创建的实例或者创建一个（如果不存在的话）。

Client

维持一个对flyweight的引用。

计算或存储一个（多个）flyweight的外部状态。

五、代码

cpp 复制代码

#include<iostream>
#include<vector>
using namespace std;

class Flyweight {
public:
	Flyweight(string tempIntrinsicState) {
		this->intrinsicState = tempIntrinsicState;
	}
	string GetIntrinsicState() {
		return intrinsicState;
	}
	virtual void Operation(const string& tempExtrinsicState) = 0;
private:
	string intrinsicState;
};

class ConcreteFlyweight :public Flyweight {
public:
	ConcreteFlyweight(string tempIntrinsicState) :Flyweight(tempIntrinsicState) {}
	void Operation(const string& tempExtrinsicState) {
		cout << this->GetIntrinsicState() << endl;
		cout << tempExtrinsicState << endl;
	}
};

class UnsharedConcreteFlyweight : public Flyweight {
public:
	UnsharedConcreteFlyweight(string tempIntrinsicState) :Flyweight(tempIntrinsicState) {}
	void UnsharedConcreteOperation(const string& tempExtrinsicState) {
		cout << tempExtrinsicState << endl;
	}
};

class FlyweightFactory {
public:
	Flyweight* GetFlyweight(string state) {
		vector<Flyweight*>::iterator it = this->flyweightVector.begin();
		for (; it != this->flyweightVector.end(); it++) {
			if ((*it)->GetIntrinsicState() == state) {
				return *it;
			}
		}
		Flyweight* flyweight = new ConcreteFlyweight(state);

		this->flyweightVector.push_back(flyweight);

		return flyweight;
	}

	int GetFlyweightCount() {
		return this->flyweightVector.size();
	}
private:
	vector<Flyweight*> flyweightVector;
};

int main() {
	string extrinsicState = "ExtrinsicState";

	FlyweightFactory* flyweightFactory = new FlyweightFactory();

	Flyweight* flyweight1 = flyweightFactory->GetFlyweight("AAA");
	Flyweight* flyweight2 = flyweightFactory->GetFlyweight("AAA");
	Flyweight* flyweight3 = flyweightFactory->GetFlyweight("BBB");

	flyweight1->Operation(extrinsicState);

	cout << flyweightFactory->GetFlyweightCount() << endl;

	cout << (flyweight1 == flyweight2) << endl;

	return 0;
}