CH14 Inheritance and Polymorphism
本文為 2021-Fall 學期旁聽台大資管系孔令傑教授開授的 Programming Design 所記錄的課程筆記。課程內容程式碼可以參閱我的 Github repo: C++ Programming-Design-2021-Fall
Inheritance
OOP的3個主要特性：
- Encapsulation（封裝）: packaging + data hiding
- Inheritance（繼承）
- Polymorphism（多型）
inheritance: 用舊的class來產生新的class，child class會有些member已經被定義在parent class裡面就不用重複定義。
使用時機：判斷兩class關係為XXX is a OOO時，就會以XXX去繼承OOO。
ex: Apple is a fruit
The first example
two-dimensional (2D) vector is a vector
child用: modifier parent_Class來繼承
class MyVector
{
protected: // to be explained
int n;
double* m;
public:
MyVector();
MyVector(int n, double m[]);
MyVector(const MyVector& v);
~MyVector()
void print() const;
// ==, !=, <, [], =, +=
};
class MyVector2D : public MyVector // 繼承
{
public:
MyVector2D();
MyVector2D(double m[]);
};
MyVector2D::MyVector2D()
{
this->n = 2;
}
MyVector2D::MyVector2D(double m[]) : MyVector(2, m)
{
}
Inheriting parent class’ members
除了private members, constructor and destructor外，parent class 定義好的member都會自動被定義在child class中。
對於parent class，protected member只能被class本身與child class拿來用，像是parent與child共同的private member的概念
Invoking parent class’ constructors
parent class的constructor不會被繼承給child，但他會在child constructor建立之前先觸發以初始化一些member variable，若沒有指定的話觸發的是parent class的default constructor。
若想要呼叫特定的parent's constructor，使用the syntax for member initializer:傳入適當argument來初始化member variable
Invoking copy constructors
若沒有定義child的copy constructor，系統會自動定義一個default copy constructor，在child的copy constructor被呼叫之前，會自動先呼叫parent的copy constructor
若我們有自己定義child的copy constructor，我們必須指定要call的parent constructor或實作自定義的功能，否則系統會自動呼叫parent's default constructor。
Invoking parent class’ destructor
當child object要被消滅時，child's destructor會先被呼叫，然後parent's destructor會被自動呼叫，所以在child's destructor裡面不能清除parent's destructor會負責清除的動態記憶體配置。
Function overriding
child可以重新定義已經存在parent裡的member function，也就是overriding，定義的時候function signature必須跟parent完全相同，否則就是child自己的新member function，在child class裡面我們也可以用::來呼叫parent's member function
class MyVector2D : public MyVector
{
public:
// ...
void print() const;
};
void MyVector2D::print() const
{
cout << "2D: ";
MyVector::print();
}
Cascade inheritance
一個child class也可以有自己的child來繼承
class NNVector2D : public MyVector2D
{
public:
NNVector2D(); // MyVector2D's
// constructor?
NNVector2D(double m[]);
void setValue(double i1, double i2);
};
NNVector2D::NNVector2D()
{
// 會呼叫自己parent's default constructor : MyVector2D
// MyVector2D的default constructor又會呼叫自己parent's default constructor
}
NNVector2D::NNVector2D(double m[]) // 沒有指定要呼叫哪個parent's constructor就會呼叫parent's default constructor
{
this->m = new double[2];
this->m[0] = m[0] >= 0 ? m[0] : 0;
this->m[1] = m[1] >= 0 ? m[1] : 0;
}
NNVector2D::NNVector2D(double m[]) : MyVector2D(m)
{
if(m[0] < 0)
this->m[0] = 0;
if(m[1] < 0)
this->m[1] = 0;
}
void NNVector2D::setValue(double i1, double i2)
{
if(this->m == nullptr)
this->m = new double[2];
this->m[0] = i1 >= 0 ? i1 : 0;
this->m[1] = i2 >= 0 ? i2 : 0;
}
-
當一個object被創建時，會從最老的constructor呼叫到最年輕的constructor，每一個constructor都可以透過
::指定往上一層要呼叫哪一個constructor(不能跳過上一層呼叫上上一 層的constructor ) -
當一個object被消滅時，會從最年輕的constructor呼叫到最老的constructor， ...
Inheritance visibility
當child要繼承parent時我們會加一個modifier如public在parent's class name前面，
這個modifier的作用在於指定從parent繼承下來的member的inheritnce visibility
- 假如一個繼承下來的member在parent裡面是protected，那child能用的
inheritnce modifier就只有protected或private - 假如一個繼承下來的member在parent裡面是protected，那child能用的
inheritnce modifier就只有private
When one inherits something from its parent, it may
narrowthevisibilityof these members.
An example
An RPG game
Given a class Character that defines some general features of an RPG character, let’s create two new classes Warrior and Wizard.
The class Character includes the name, current level, accumulated experience points, and three ability levels: power, knowledge, and luck.
- There is a public function
print():- It prints out the current status of a character.
- There is a public function
beatMonster(int exp):- It is invoked when the character beats a monster.
- There is a private function levelUp():
- The character's level will be incremented, however her abilities will remain the same because occupations should get different improvements
參考14_1.cpp
class Character
{
protected:
static const int EXP_LV = 100;
string name;
int level;
int exp;
int power;
int knowledge;
int luck;
void levelUp(int pInc, int kInc, int lInc);
public:
Character(string n, int lv, int po, int kn, int lu);
void print();
void beatMonster(int exp);
};
Character, Warrior, and Wizard
Character should not be used to create an object. We define two derived classes Warrior and Wizard: (升級後的能力值應該要依職業不同而有不同提升方式)
- Character is an
abstrct class - Warrior and Wizard are
concrete classes
Issue:
- Character作為
abstrct class不應該被用來create object，但目前仍無法限制 - 若有一個class
Team如下： 由於Warrior, Wizard是不同type，因此不能開一個array既可以存Warrior的指標, 也可以存Wizard的指標。且因為有兩個職業的關係而導致任何跟Team有關的動作都要寫兩個function，既浪費空間，操作起來又沒效率
class Team
{
private:
int warriorCount;
int wizardCount;
Warrior* warrior[10];
Wizard* wizard[10];
public:
Team();
~Team();
void addWar(string name, int lv);
void addWiz(string name, int lv);
void warBeatMonster(string name, int exp);
void wizBeatMonster(string name, int exp);
void printWar(string name);
void printWiz(string name);
};
Polymorphism
根據前面的example，我們是否可以用一個共通的type array去存Warrior和Wizard？
答案是可以，我們可以用Character array同時存Warrior和Wizard。像這樣用parent type's variable去存child type's value就稱為Polymorphism（多型）
In C++ a parent variable can store a child object.
Why a parent variable for a child value?
class Parent
{
protected:
int x;
int y;
public:
Parent(int a, int b) : x(a), y(b) {}
};
class Child : public Parent
{
protected:
int z;
public:
Child(int a, int b, int c)
: Parent(a, b) { z = c; }
};
int main
{
Parent p1(1, 2);
Child c1(3, 4, 5);
Parent p2 = c1; // OK: 5 is discarded
// Child c2 = p1; // Not OK: no v3
return 0;
}
Polymorphism with functions
void printInitial(Character c)
{
string name = c.getName();
cout << name[0];
}
int main
{
Warrior alice("Alice", 10);
Wizard bob("Bob", 8);
printInitial(alice);
printInitial(bob);
return 0;
}
Polymorphism with arrays
錯誤寫法：因此直接宣告Character array，本來會在array中創建空物件，但是Character沒有default constructor， 所以會出錯，且Character也不應該有default constructor
int main
{
Character c[3]; // error! Why?
Warrior w1("Alice", 10);
Wizard w2("Sophie", 8);
Warrior w3("Amy", 12);
c[0] = w1;
c[1] = w2;
c[2] = w3;
for(int i = 0; i < 3; i++)
c[i].print(); return 0;
}
正確寫法：先宣告Character pointer array，等有東西要存了再存進去，注意，指標陣列是3個指標指向3塊空間，不是一個指標指向一個空間，應此要一個一個delete
int main
{
Character* c[3];
c[0] = new Warrior("Alice", 10);
c[1] = new Wizard("Sophie", 8);
c[2] = new Warrior("Amy", 12);
for(int i = 0; i < 3; i++)
c[i]->print();
for(int i = 0; i < 3; i++)
delete c[i]; // not delete [] c;
return 0;
}
Invoking an overridden function
目前我們仍然無法避免創建Charater物件可以自由配創建，且會發現用 了Polymorphism後，overriding 的function都沒有被使用到。
假設我們用parent variable(pointer)存了child value，並且用parent variable去呼叫overridden function，C++預設會呼叫parent自己的function而不是overridden function。若要呼叫overriden版本的function會需要用到late binding與virtual functions.
class A
{
public:
void a() { cout << "a\n"; }
void f() { cout << "af\n"; }
};
class B : public A
{
public:
void b() { cout << "b\n"; }
void f() { cout << "bf\n"; }
};
Early binding vs. late binding
For A a = b, the system does early binding:
- 在compile的時候a就已經固定裝的是A type的形狀了，b被assign給a只是把b削成a的形狀再裝進去
For A* a = &b, the system does late binding:
- a 只是一個pointer，負責指向一塊空間，本身並沒有任何形狀
- a可以指向A object也可以指向B object
- a所指向的記憶體空間儲存的
type在run time時才會被決定
Virtual functions
當用到Polymorphism想呼叫child function時，首先一定得用Late binding，type才會是child type，再來就是要宣告parent’s member function 為 virtual，表示呼叫的優先順序比child overriden function還低
class Parent
{
protected:
int x;
int y;
public:
Parent(int a, int b) : x(a), y(b) {}
virtual void print() { cout << x << " " << y; }
};
Class Child : public Parent
{
protected:
int z;
public:
Child(int a, int b, int c) : Parent(a, b) { z = c; }
void print() { count << z; }
...
}
Abstract classes
We may set beatMonster() to be a pure virtual function(只有外殼給child繼承，裡面沒有implement內容):
class Character
{
// ...
virtual void beatMonster(int exp) = 0; // 用 = 0來宣告為pure virtual function
};
宣告pure virtual function後就會自動無法create Character object了，這時候我們稱這個class為abstract class(抽象類別)，它存在的目的就是要給child繼承