Lecture 10.md

Lecture 10

Singleton

1. What is the basic function (功能) of singleton?

• Ensure that a class can only have one instance with global point of access

2. Please try to implement a singleton example where the instance is dynamically created (动态创建) at its first visit.

class Singleton{
private:
	static Singleton *instance;
	Singleton(){}
	Singleton(Singleton &&) = delete;
	Singleton(const Singleton &) = delete;
	Singleton & operator=(Singleton &&) = delete;
	Singleton & operator=(const Singleton &) = delete;
	~Singleton(){}
public:
	Singleton *getInstance(){
		if(instance == nullptr)
			instance = new Singleton;
		return instance;
	}
};

3. Please list some typical situations where singleton is useful or necessary.

• Classes for a unique system device, e.g., printer
• Program configuration class for loading the configuration file
• For a GUI application, only one toolbox is popped up when clicking on the menu many times
• When the code is very large and complicated, a function may be called for many times. Singleton is necessary then.

4. Please plot a typical UML class diagram of singleton pattern.

Adapter

5. What is the basic function (功能) of adapter?

• The adapter pattern converts the interface of a class into another interface to satisfy the need.

6. Please give an example using adapter design pattern.

class BST;
class PQ{
private:
    BST a;
public:
    void push(Object o) { a.insert(o); }
    void pop() { a.erase(o); }
    Object top() { return a.getMin(); }
};

7. Please list some typical situations where adapter is useful or necessary.

• Use libraries to solve a problem while the interfaces aren’t completely compatible
• Convert the interface of a Document Object Model of an XML document into a tree structure that can be displayed.
• Multiple implementations are to be used. Make them use the same interface.
• Creating different collection-save-structure like hash-map, rb-tree, and so on, a total Adapter could solve all the things. If the developer want to change the implementation. He could only modify one line or two.

8. Please provide an example to illustrate advantages of the OOP principle “Program to an interface, not an implementation”.

Noodles *orderNoodles(string type) {
    Noodles *noodles = 0;
    // changes
   if (type == "cheese") {
    noodles = new CheeseNoodles();
   } else if (type == "veggie") {
    noodles = new VeggieNoodles();
   } //……
   //no changes
   //cut, box, and sell
   noodles->sellNoodles(); 
    
    return noodles;
}

• Noodles is the interface class; CheeseNoodles and VeggieNoodles are implementation classes; Program to interface; noodles->sellNoodles(); This avoids any negative effects of future modifications on implementation classes. E.g., adding a new type of noodles, Or removing a type of noodles that does not sell well.

9. Please provide an example to illustrate advantages of the open-closed principle.

• Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification. Conform with the incremental development requirement.
• Example: Factory method satisfy the open-closed principle.

10. Please try to explain the relation between “Program to an interface” and the open-closed principle.

• “Program to an interface”意味着我们首先要设计接口，然后我再去实现它。
• “开放封闭原则”是指我们应该使代码具有可扩展性，最好不要修改代码框架。

11. Please plot a typical UML class diagram with “object adapter” design pattern.

Simple Factory

12. What is the basic function (功能) of simple factory?

• Create pointer to objects of a children class of a basic class.
• Avoid any negative effects of future modifications on implementation classes.

13. Please give an example using simple factory.

class Juice {
public:
  virtual void sell_juice() = 0;
};

class Apple_juice {
public:
  void sell_juice() { cout << “Sell apple juice.” << endl; }
};

class Banana_juice {
public:
  void sell_juice() { cout << “Sell banana juice.” << endl; }
};

enum Juice_type {APPLE, BANANA};
class Juice_factory {
  Juice* create_juice(Juice_type type) {
  Juice* juice = 0;
  switch(type) {
    case APPLE:
      juice = new Apple_juice();
      break;
case BANANA:
      juice = new Banana_juice();
      break;
    default:
      break;
}
return juice;
}
};

14. Please list some typical situations where simple factory is useful or necessary.

• 工厂模式有一种非常形象的描述，建立对象的类就如一个工厂，而需要被建立的对象就是一个个产品；在工厂中加工产品，使用产品的人，不用在乎产品是如何生产出来的。从软件开发的角度来说，这样就有效的降低了模块之间的耦合。1.在程序中，需要创建的对象很多，导致对象的new操作多且杂时，需要使用简单工厂模式；2.由于对象的创建过程是我们不需要去关心的，而我们注重的是对象的实际操作，所以，我们需要分离对象的创建和操作两部分，如此，方便后期的程序扩展和维护。

15. Please plot a typical UML class diagram with simple factory pattern.