6-Virtual Functions, Pure Virtual Functions and Abstract Classes

Virtual Function

提供多型的功能,一種修飾function的修飾字
Defined in base class and redefined in derived class.
使用pointer或reference存取derived class物件時，且pointer指向的類型是 base class 時才有此效果
dynamic binding or late binding

使用pointer或reference存取derived class物件時，且pointer指向的類型是 base class 如果衍生類別覆蓋了基礎類別中的method，通過上述指標或參照呼叫該方法時，可以有兩種結果：

呼叫到基礎類別的方法：編譯器根據指標或參照的類型決定，稱作 early binding 呼叫到衍生類別的方法：執行時系統根據物件的實際類型決定，稱作 dynamic binding or late binding

early binding

首先使用指針建立物件

#include <iostream>
using namespace std;

class Student{
public:
    void Hello(){
        cout << "I'm a student.\n";
    }
};

class ChemsitryStudent : public Student{
public:
    void Hello(){
        cout << "I'm a student major in chemistry.\n";
    }
};

int main()
{
    Student *p_Amy = new ChemsitryStudent;
    p_Amy -> Hello();
}

p_Amy會指向class為Student的物件,故output出

I'm a student.

dynamic binding or late binding

將bass class的class Student中的void Hello()加上修飾字virtual 此時由於base class中的 Hello() 被宣告為 virtual ,此時會告訴compilier Hello() 這個method會進行 dynamic binding

class Student{
public:
    virtual void Hello(){  //add virtual
        cout << "I'm a student.\n";
    }
};

第一種情況derived class 有 implement

class ChemsitryStudent : public Student{
public:
    void Hello(){
        cout << "I'm a student major in chemistry.\n";
    }
};

int main()
{
    Student *p_Amy = new ChemsitryStudent;
    p_Amy -> Hello();
}

此時程式執行時會連到ChemsitryStudent的Hello() output

I'm a student major in chemistry.

第二種情況derived class 沒有 implement

class ChemsitryStudent : public Student{
public:
};

int main()
{
    Student *p_Amy = new ChemsitryStudent;
    p_Amy -> Hello();
}

此時程式執行時連不到ChemsitryStudent的Hello(),故會連結到Student的virtual void Hello() output

I'm a student major in chemistry.

Pure Virtual Functions

知道子類別一定會衍生至此父類別,且子類別一定會有此function但表現的型態不同,故父類別不實作此function 將實作function的工作交給derived class,而且derived class一定要實作

virtual return_type function() = 0; virtual void Hello() = 0;

class Student{
public:
    virtual void Hello() = 0 ;
};

class ChemsitryStudent : public Student{
public:
    void Hello(){
        cout << "I'm a student major in chemistry.\n";
    }
};

int main()
{
    Student *p_Amy = new ChemsitryStudent;
    p_Amy -> Hello();
}

output I'm a student major in chemistry.

Abstract Classes

含有pure virtual function的class Student 無法實作Abstract Class的物件,必須實作其subclass

多型的表現

class Student{
public:
    virtual void Hello() = 0 ;
};

class MathStudent : public Student{
public:
    void Hello(){
        cout << "I'm a student major in Math.\n";
    }
};

class ChemsitryStudent : public Student{
public:
    void Hello(){
        cout << "I'm a student major in chemistry.\n";
    }
};

int main()
{
    Student *p_Amy = new MathStudent;
    Student *p_Jim = new ChemsitryStudent;
    Student *p_Name[]={p_Amy,p_Jim};

    for(int i = 0 ; i <=2 ; i++){
        p_Name[i] -> Hello();
    }
}

雖然pointer皆指向 Student 這個class,但是表現出來的method不同

I'm a student major in Math.
I'm a student major in chemistry.

Pin-Jiun / Programming-Language-CPP