OOP

Object-Oriented Programming

什麼是OOP

Object-Oriented

Everything is an object
Objects do everything

Why?

方便管理程式碼
程式碼的擴充性強
抽象化 (Abstraction)
~~跟資奧其實沒什麼關係~~，但很重要

名詞解析

Object (物件)
Struct
Class

名詞解析

Attribute (Variable)
Method (Function)

Basics

宣告

struct Person{
	int id;
    string name;
    int height, weight;
};

宣告

struct Person{
	int id;
    string name;
    int height, weight;
};

Person teacher, students[30];

宣告

struct Person{
	int id;
    string name;
    int height, weight;
};

Person teacher, students[30];

這時候它們的 id 和各個數值會是多少？

本來的預設值

宣告

struct Person{
	int id;
    string name;
    int height, weight;
};

Person teacher, students[30];
teacher.id = 1;
teacher.name = "yungyao";
teacher.height = 180;
teacher.weight = 60;
for (int i=0;i<30;++i){
	students[i] = ...
    ...
}

是不是要寫的東西有點多？

建構子

struct Person{
	int id;
    string name;
    int height, weight;
    Person(int _id, string _name, int _height, int _weight){
    	id = _id;
        name = _name;
        height = _height;
        weight = _weight;
    }
};

Person teacher, students[30];
teacher = Person(1, "yungyao", 180, 60);
for (int i=0;i<30;++i){
	students[i] = Person(...);
}

id 需要我們手動輸入嗎？

Maybe?
但更多時候不需要！

建構子/Static (靜態變數)

struct Person{
	static int personCount = 0;
	int id;
    string name;
    int height, weight;
    Person(string _name, int _height, int _weight){
    	id = personCount;
        personCount++;
        name = _name;
        height = _height;
        weight = _weight;
    }
};

Person teacher, students[30];
teacher = Person("yungyao", 180, 60);
for (int i=0;i<30;++i){
	students[i] = Person(...);
}

Static Variable (靜態變數)

有點類似全域變數的性質
但還是屬於包括它的 struct/class

讓 object 去做事情

struct Person{
	static int personCount = 0;
	int id;
    string name;
    int height, weight;
    Person(string _name, int _height, int _weight){
        ...
    }
    
    void speakName(){
    	cout << "I am " << name << ".\n";
    }
};

...

teacher.speakName(); // I am yungyao.

讓 object 去做事情

struct Person{
	static int personCount = 0;
	int id;
    string name;
    int height, weight;
    Person(string _name, int _height, int _weight){
        ...
    }
    
    double BMI(){
    	return weight / height / height;
    }
};

...

return teacher.BMI();

mutator

struct Person{
	static int personCount = 0;
	int id;
    string name;
    int height, weight;
    Person(string _name, int _height, int _weight){
        ...
    }
    
    void gainHeight(int gainedHeight){
    	height += gainedHeight;
    }
};

...

cout << teacher.height << '\n'; // 180
teacher.gainedHeight(-20);
cout << teacher.height << '\n'; // 160

Object as attribute

struct Classroom{
	Person teacher;
    Person student[40];
    int studentCount;
    
    Classroom(int _studentCount, Person _teacher){
    	teacher = _teacher;
        studentCount = _studentCount;
        for (int i=0;i<studentCount;++i){
        	student = Student("unknown", 140, 40);
        }
    }
};

Struct / Class

(for C++)

Accessibility

Private
Public
從外部能不能看進去

不能讓 Object 外部看到的資訊

struct Person{
	static int personCount = 0;
	int id;
    string name;
    private:
    int height, weight;
    Person(string _name, int _height, int _weight){
        ...
    }
    
    double BMI(){
    	return weight / height / height;
    }
};

...

cout << teacher.height; // Compile Error: Can not access private

Struct / Class 差在哪

struct 預設是 public
class 預設是 private

繼承

子型別

人如果是一種型別
- 學生是一個子型別
- 老師是另一個子型別
- 都有名字，但只有學生有學號

母型別

很多型別間會有共同的功能
把這些功能只寫一次

子型別

只屬於特定型別的功能，在子型別裡完成

Code Example

class Person{
	string name;
    public:
    void printName(){
    	cout << name << endl;
    }
};

class Student: public Person{
	int number;
    public:
    void printNumber(){
    	cout << number << endl;
    }
};

Code Example (多型)

class Person{
	string name;
    public:
    void printName(){
    	cout << name << endl;
    }
};

class Student: public Person{
	int number;
    public:
    void printName(){
    	cout << name << ' ' << number << endl;
    }
};

用途

Project

要怎麼處理每個東西