目的比較

相同之處

• All of them are defined as structural patterns as GOF design pattern
• All of them uses Composition and delegation to fulfill there intent
  • Adapter uses composition to forward calls from target interface to adaptee interface
  • Decorator also uses same technique before adding new behavior
  • Facade is composed of all sub components
• Both Adapter and Facade pattern, forwards requests to a different interface, which can be either adaptee or any component interface from sub-system

相異之處

• Unlike Decorator, Facade doesn't add any new behavior, it just call existing methods from interface
• Adapter pattern converts interface, Decorator & Facade patterns don't
• Decorator just implements original object's interface, so that it can be passed to a method, which accepts an original object

UML Diagram of 3 Patterns

1. Adapter pattern
   
2. Decorator pattern
   
3. Facade pattern
   

[補充]: 組合 (Composition) vs 委派 (Delegation)

組合:

• 用於建立一個新類別，該類別包含一或多個現有類別的實例作為其成員變量。組合可以實現「有一個」（Has-a）的關係
  • 建立對象: 在新類別中建立一個或多個現有類別的實例
  • 方法封裝: 新類別可以封裝或者擴展被包含對象的功能
  • 附加功能: 根據需要，新類別可以添加額外的方法和屬性
• 範例

// "Engine" 類別
class Engine {
public:
    void start() {
        std::cout << "Engine started" << std::endl;
    }
};

// "Car" 類別包含 "Engine"
class Car {
private:
    Engine engine;
public:
    Car() : engine() {}
    void start() {
        engine.start();
    }
};

int main() {
    Car car;
    car.start();
}

委派:

• 是一種將某個操作的責任轉交給另一個對象的技巧。這通常用於實現代碼重用和靈活性
  • 建立接口: 為需要委派的功能建立一個接口
  • 實現接口: 被委派類別會實現這個接口
  • 轉交責任: 在主類別中，建立被委派類別的一個實例，並將該實例用作執行特定任務
• 範例

// "Printer" 介面
class Printer {
public:
    virtual void print_page(const string& content) = 0;
};

// "InkjetPrinter" 實現 "Printer"
class InkjetPrinter : public Printer {
public:
    void print_page(const string& content) override {
        cout << "Printing from Inkjet: " << content << endl;
    }
};

// "LaserPrinter" 實現 "Printer"
class LaserPrinter : public Printer {
public:
    void print_page(const string& content) override {
        cout << "Printing from Laser: " << content << endl;
    }
};

// "Document" 使用委派
class Document {
private:
    Printer* printer;
public:
    Document(Printer* p) : printer(p) {}
    
    void print(const string& content) {
        printer->print_page(content);
    }
};

int main() {
    InkjetPrinter inkjet;
    LaserPrinter laser;
    Document doc1(&inkjet);
    Document doc2(&laser);
    doc1.print("Hello, world!");
    doc2.print("Hello, world!");
    
    // 輸出為:
    // Printing from Inkjet: Hello, world!
    // Printing from Laser: Hello, world!
}

• 小結
  • 組合: 適用於「Has-a」關係，並希望直接使用或擴展被包含對象的功能
  • 委派: 適用於需要代碼重用，或在運行時能動態更改行為

Reference

1. https://ahmedabdelkarim.wordpress.com/2019/11/21/facade-vs-adapter-vs-decorator-design-pattern/
2. https://javarevisited.blogspot.com/2015/01/adapter-vs-decorator-vs-facade-vs-proxy-pattern-java.html#axzz8EEMvOEuO