Template Method
动机（Motivation）
- 在软件构建过程中，对于某一项任务，它常常有稳定的整体操作结构，但各个子步骤却有很多改变的需求，或者由于固有的原因（比如框架与应用之间的关系）而无法和任务的整体结构同时实现。
- 如何在确定稳定操作结构的前提下，来灵活应对各个子步骤的变化或者晚期实现需求？
模式定义
定义一个操作中的算法的骨架 (稳定) ，而将一些步骤延迟 (变化) 到子类中。
Template Method使得子类可以不改变(复用)一个算法的结构即可重定义(override 重写)该算法的
某些特定步骤。
——《 设计模式》 GoF
要点总结
- Template Method模式是一种非常基础性的设计模式，在面向对象系统中有着大量的应用。它用最简洁的机制（虚函数的多态性）
为很多应用程序框架提供了灵活的扩展点，是代码复用方面的基本实现结构。 - 除了可以灵活应对子步骤的变化外， “不要调用我，让我来调用你” 的反向控制结构是Template Method的典型应用。
- 在具体实现方面，被Template Method调用的虚方法可以具有实现，也可以没有任何实现（抽象方法、纯虚方法），但一般推荐将它们设置为protected方法。
代码结构
…
├── build.sh
├── clearBuild.sh
├── CMakeLists.txt
├── src
├── exam1
│ ├── include
│ │ ├── CaffeineBeverage.hpp
│ │ ├── Coffee.hpp
│ │ └── Tea.hpp
│ └── main.cpp
└── exam2
├── examStu.cpp
├── include
│ └── examStu.h
└── main.cpp
源码例子1
CaffeineBeverage.hpp
#ifndef CAFFEINEBEVERAGE_HPP
#define CAFFEINEBEVERAGE_HPP
#include <iostream>
class CaffeineBeverage {
public:
virtual ~CaffeineBeverage(){
}
void prepareRecipe() {
boilWater();
brew();
pourInCup();
if(customerWantsCondiments())
addCondiments();
}
virtual void brew() = 0;
virtual void addCondiments() = 0;
void boilWater() {
std::cout << "Boiling water\n";
}
void pourInCup() {
std::cout << "Pouring into cup\n";
}
virtual bool customerWantsCondiments() {
return true;
}
};
#endif
Coffee.hpp
#ifndef COFFEE_HPP
#define COFFEE_HPP
#include "CaffeineBeverage.hpp"
#include <iostream>
class Coffee : public CaffeineBeverage {
public:
~Coffee(){
}
void brew() {
std::cout << "Dripping Coffee through filter\n";
}
void addCondiments() {
std::cout << "Adding Sugar and Milk\n";
}
bool customerWantsCondiments() {
char c;
std::cout << "Do you want to add condiments?\n";
std::cin >> c;
if(c == 'y')
return true;
else
return false;
}
};
#endif
Tea.hpp
#ifndef TEA_HPP
#define TEA_HPP
#include "CaffeineBeverage.hpp"
#include <iostream>
class Tea : public CaffeineBeverage {
public:
~Tea(){
}
void brew() {
std::cout << "Steeping the tea\n";
}
void addCondiments() {
std::cout << "Adding Lemon\n";
}
bool customerWantsCondiments()
{
char c;
std::cout << "Do you want to add condiments?\n";
std::cin >> c;
if (c == 'y')
return true;
else
return false;
}
};
#endif
main.cpp
#include "Tea.hpp"
#include "Coffee.hpp"
int main() {
Tea* tea = new Tea();
Coffee* coffee = new Coffee();
std::cout << "\nMaking tea...\n";
tea->prepareRecipe();
std::cout << "\nMaking coffee...\n";
coffee->prepareRecipe();
delete tea;
delete coffee;
}
源码例子2
examStu.h
#ifndef _EXANSTU__
#define _EXANSTU__
#include <iostream>
#include <string>
#include <vector>
#include <list>
using namespace std;
class FileOperation
{
public:
FileOperation()=default;
~FileOperation(){
cout << "~FileOperation"<<endl;};
void DownloadFiles()
{
if (!DownloadFileBegin()) return ;
if (!DownloadFileProcess()) return ;
if (!DownloadFileEnd()) return ;
}
protected:
virtual bool DownloadFileBegin( ){
return true;}
virtual bool DownloadFileProcess() {
return true;}
virtual bool DownloadFileEnd(){
return true;}
private:
};
class WebDownLoadFiles :public FileOperation
{
public:
WebDownLoadFiles()=default;
~WebDownLoadFiles(){
cout << "~WebDownLoadFiles"<<endl;}
protected:
bool DownloadFileBegin() override {
cout << "WebDownloadFileBegin"<<endl; return true; }
bool DownloadFileProcess() override {
cout << "WebDownloadFileProcess"<<endl; return true; }
bool DownloadFileEnd() override {
cout << "WebDownloadFileEnd"<<endl; return true; }
};
class PhoneDownLoadFiles :public FileOperation
{
public:
PhoneDownLoadFiles()=default;
~PhoneDownLoadFiles(){
cout << "~PhoneDownLoadFiles"<<endl;}
protected:
bool DownloadFileBegin( ) override {
cout << "PhoneDownloadFileBegin"<<endl; return true; }
bool DownloadFileProcess( ) override {
cout << "PhoneDownloadFileBegin"<<endl; return true; }
bool DownloadFileEnd( ) override {
cout << "PhoneDownloadFileBegin"<<endl; return true;}
};
#endif
main.cpp
#include <iostream>
#include <string>
#include <memory>
#include "examStu.h"
using namespace std;
int main()
{
std::shared_ptr<FileOperation> pWebDownLoadFiles = std::make_shared<WebDownLoadFiles>();
pWebDownLoadFiles->DownloadFiles();
std::shared_ptr<FileOperation> pPhoneDownLoadFiles = std::make_shared<PhoneDownLoadFiles>();
pPhoneDownLoadFiles->DownloadFiles();
return 0;
}