Thursday 15 January 2015

[C++] Abstract Class

只要類別含有任何一個 pure virtual function 就稱為 abstract class.

ex.
class Shape {  // abstract class
    virtual void rotate(int) = 0;  // pure virtual function
    virtual void draw() = 0;  // pure virtual function
    virtual bool is_closed = 0; // pure virtual function
    ...
};

abstract class 無法建立任何 object,只能做為其他 object 的 interface & base object

base:
ex.
class Point { /* */ };
class Circle : public Shape {
    public:
        void rotate(int) {}  // 覆蓋 Shape::rotate
        void draw();  // 覆蓋 Shape::draw
        bool is_closed() { return true; }  // 覆蓋Shape::is_closed

        Circle(Point p, int r);
    private:
        Point center;
        int radius;
};

interface:
ex. driver
class Character_device {
    public:
        virtual int open(int opt) = 0;
        virtual int close(int opt) = 0;
        virtual int read(char* p, int n) = 0;
        virtual int write(const char* p, int n) = 0;
        virtual int ioctl(int ...) = 0;
        virtual ~Character_device() {}
};
可從 Character_device 衍生出各式各樣的 driver object

有 abstract class 便能以 object 拼出完整模組化的系統。