# C++ structure and class --- #### What is struct/class * What: Encapsulate multiple data to one. * Why: Give the structure a meaning. * How: ``` cpp struct People { std::string name; int age; int height; int weight }; ``` --- #### The expression of the struct/class * Keyword: `struct/class` * Member `variables/functions` * Access specifiers: `public/private/protected` * 表示外部的code是否可以訪問這個作用域 * 如果表示為private的函式跟變數，除了自己以外不能使用 * 如果表示為public的函式或變數則為公開變數，可以供人使用 * Remember add the semicolon at the end. --- #### The expression of the struct/class ``` cpp // use class to define class People { // Access specifiers public: // member functions std::string getName() {return name;} std::string getAge() {return age;} void setName(std::string n){name=n;} void setAge(int a){age=a;} void introduceMySelf() { std::cout<<"My name is "<<getName() <<", and age is "<<getAge()<<"\n"; } private: // member variables std::string name; int age; }; // <-- Add the semicolon ``` --- ### Example ``` cpp int main() { std::string name; int age; std::cin>>name>>age; People Ann; // Ann.name = name, compile error because name is a private member // Ann.age = age, compile error because age is a private member Ann.setName(name); Ann.setAge(age); Ann.introduceMySelf(); // My name is Ann, and age is 18 } ``` --- #### What is difference between class and struct. * In C language, there is only struct and no member functions in it. * In C++ language, only one difference. * The default access specifier is private for class and public for struct. --- #### LifeTime and Initialization * 所有的變數都有它的生命週期 * 全域變數會存活在整個程式中，生命週期無限大 * 區域變數會存活在某一個區間 * 兩個括號之間{} * 或者一個function之間 --- #### LifeTime and Initialization * 區域變數會存活在某一個區間 * 兩個括號之間{} * 或者一個function之間 ``` cpp int main() { int a = 0; // 在宣告時創造，在離開main時結束 if (a==0) { People Ann; // 在進入if後創造它，並在離開if時結束 Ann.setAge(18); std::cout<<Ann.getName()<<"\n"; // ↑--------------↑--- // getName回傳的std::string只存在於<<之間 } { People Ann; // 在進入這個{}後創造它，並在離開{}時結束 } } ``` --- #### LifeTime and Initialization * 所有的變數都有它的生命週期 * 全域變數會存活在整個程式中，生命週期無限大 * 區域變數會存活在某一個區間 * 兩個括號之間{} * 或者一個function之間 * 任何變數在使用前都要先思考要怎麼初始化，以及怎麼解構 * class has constructor/desturctor --- #### LifeTime and Initialization  --- #### Initialization * 有兩種初始化的寫法 * 第一種直接在宣告member variable的時候給定初始值 * Constructor ```cpp= class People { int age=0; std::string name=""; } ``` --- #### Initialization * Constructor * 於class中特殊的寫法，使用跟class相同名稱當作開頭 * 從: 到 {} 中間為initialize list(初始化列表),以','分隔 * 初始化列表內的順序要跟member一致 * {}內可想像成function body，做一些初始化會做的事情 ```cpp= class People { public: People():age{0}, name{""}{} private: int age; std::string name; } ``` --- #### Initialization * Constructor 可接受多個變數 ``` cpp= class People { public: People(std::string n, int a):age{a}, name{n}{} private: int age; std::string name; } int main() { People Ann{"Ann", 18}; return 0; } ``` --- #### specifal constructor and destructor * default constructor * copy constructor * copy assignment * move constructor * move assignment * destructor --- #### specifal constructor and destructor ```cpp= int main() { People Ann; // Default constructor People Ann2 = Ann; // Copy constructor People Ann3; // Default constructor Ann3 = Ann; // Copy Assignment; Poeple Ann4 = std::move(Ann3) // move constructor Ann3 = std::move(Ann); // move assignment; } // Call destructor of Ann/Ann2/Ann3/Ann4 before leave main ``` --- #### specifal constructor and destructor ``` cpp= class People { public: // Default constructor People():age{0}{} // Copy constructor, use const People& in the parameters. People(const People& p):age{p.age}{} // Move constructor, like copy but use People&& People(People&& p):age{p.age}{} // Copy Assignment, redefine the = for People // Return type must be People& // Return *this at the end. People& operator=(const People& p){age=p.age; return *this;} // Move Assignment, like copy assignment but parameter use People&& People& operator=(People&& p){age=p.age; return *this;} // Destructor ~People(){} private: int age; } ``` --- #### Example * [Link](https://godbolt.org/z/1PPPPbE97) ``` cpp= class People { public: People():name{"Default"}{std::cout<<"I'm born!\n";} ~People(){std::cout<<"I'm "<<name<<", and I leave!\n";} void setName(std::string n){name=n;} private: std::string name; }; ``` ```cpp= int main() { People Ann; Ann.setName("Ann"); { People Terry; Terry.setName("Terry"); } } ``` --- #### Example 2 * 請問這邊會印出甚麼東西來. [Link](https://godbolt.org/z/dYnEbKzvv) ``` cpp= class People { public: People() {std::cout<<"Ctor\n";} People(const People& p) {std::cout<<"Copy Ctor\n";} People(People&& p) {std::cout<<"Move Ctor\n";} People& operator=(const People& p) {std::cout<<"Copy Ass\n"; return *this;} People& operator=(People&& p) {std::cout<<"Move Ass\n"; return *this;} ~People() {std::cout<<"Dtor\n";} }; int main() { People p1; People p2 = p1; People p3 = std::move(p2); p3 = p1; p3 = std::move(p1); } ``` --- #### Constructor and Destructor * 為什麼需要這麼複雜的constructor跟destructor呢 * 在未來會學到pointer，或者你用到別人設計的class時會用到 * pointer需要更為複雜的初始化跟解構 * 別人了class可能需要複雜的初始化以及解構 * Rule of 5 or Rule of 0 * 意思就是說，要馬全部的constructor都寫出來。要馬都不寫。 * 現階段妳可以先不寫這些東西 --- #### You can define everything outside the class * Why: 你的class應當簡潔有力，把定義包含其中會顯得混亂 * How: 在定義時加入 \<ClassType>:: ```cpp= class People { public: People(); People(int a); ~People(); int getAge(); void setAge(int a); private: int age; }; int main(){ //... } People::People():age{0} {} People::People(int a):age{a}{} People::~People(){} int People::getAge(){return age;} void People::setAge(int a){age=a;} ``` --- #### Exercise * 設計一個簡單的點餐系統。 1. 請輸入Steak, Fish, Chicken, IceCream, Cake的價格 2. 輸入預定人名以及想要預定的時間 3. 輸入想要吃的主餐是甚麼(Steak, Fish, Chicken, None擇一) 4. 輸入想吃的甜點是甚麼(IceCream, Cake, None擇一 5. 輸入是否有會員(Yes/No, 有會員的話打九折) 6. 輸出人名，時間，主餐，甜點，是否有折價，總價格 --- #### Exercise * [Link](https://godbolt.org/z/K3qrr1E4b) * 輸入如下 *  * 輸出如下 *  * [ANS](https://godbolt.org/z/x981v9sE3)