c++ 享元模式（flyweight）

桃扇骨 2022-03-31 04:20 183阅读 0赞

举个围棋的例子，围棋的棋盘共有361格，即可放361个棋子。现在要实现一个围棋程 序，该怎么办呢？首先要考虑的是棋子棋盘的实现，可以定义一个棋子的类，成员变量包括棋子的颜色、形状、位置等信息，另外再定义一个棋盘的类，成员变量中 有个容器，用于存放棋子的对象。下面给出代码表示：

棋子的定义，当然棋子的属性除了颜色和位置，还有其他的，这里略去。这两个属性足以说明问题。

#include <iostream>
    #include <vector>
    #include <string>
    
    
    enum PieceColor {BLACK,WHITE};
    using namespace std;
    
    class Pos
    {
    public:
        Pos(int x,int y):m_x(x),m_y(y)
        {
        
        }
        int getX()
        {
            return m_x;
        }
        int getY()
        {
            return m_y;
        }
    private:
        int m_x;
        int m_y;
    };
    
    class Piece
    {
    public:
        Piece(PieceColor color,Pos pos):m_color(color),m_pos(pos){};
        ~Piece() {}
        virtual void Draw() {}
    protected:
        PieceColor m_color;
        Pos m_pos;
        
    };
    
    class WhitePiece : public Piece
    {
    public:
        WhitePiece(PieceColor color, Pos pos):Piece(color,pos){}
        ~WhitePiece();
        virtual void Draw() {
            cout << "draw a white piece" << endl;
        }
    };
    
    class BlackPiece:public Piece
    {
    public:
        BlackPiece(PieceColor color,Pos pos):Piece(color,pos){}
        ~BlackPiece();
        virtual void Draw(){
    
        cout << "draw a black piece" <<endl;
    
        }
    };
    
    class PieceBoard
    {
    public:
        PieceBoard(string black,string white):m_blackName(black),m_whiteName(white)
        {
        
        }
        ~PieceBoard() {
            Clear();
        }
        void SetPiece(PieceColor color,Pos pos)
        {
            Piece * piece = NULL;
            if(color == BLACK)
            {
                piece = new BlackPiece(color,pos);
                std::cout << m_blackName << "在位置(" << pos.getX() << ","<<pos.getY() <<")" <<endl ;
                piece->Draw();
                m_pieceArray.push_back(piece);
            }
            else
            {
                piece = new WhitePiece(color,pos);
                std::cout << m_whiteName << "在位置(" << pos.getX() << ","<<pos.getY() <<")" <<endl;
                piece->Draw();
                m_pieceArray.push_back(piece);
            }
        }
        void Clear()
        {
            int size = m_pieceArray.size();
            for(int i = 0; i < size ; i++)
                delete m_pieceArray[i];
        }
    private:
        std::vector<Piece*> m_pieceArray;
        std::string m_blackName;
        std::string m_whiteName;
    };

主函数：

#include "flyweight.h"
    int main()
    {
        PieceBoard pieceBoard("A","B");
        pieceBoard.SetPiece(BLACK,Pos(4,4));
        pieceBoard.SetPiece(WHITE,Pos(16,6));
    
        system("pause");
        return 0;
    }

可以发现，棋盘的容器中存放了已下的棋子，而每个棋子包含棋子的所有属性。一盘棋往往需要含上百颗棋子，采用上面这种实现，占用的空间太大了。如何改进呢？用享元模式。其定义为：运用共享技术有效地支持大量细粒度的对象。

在围棋中，棋子就是大量细粒度的对象。其属性有内在的，比如颜色、形 状等，也有外在的，比如在棋盘上的位置。内在的属性是可以共享的，区分在于外在属性。因此，可以这样设计，只需定义两个棋子的对象，一颗黑棋和一颗白棋， 这两个对象含棋子的内在属性；棋子的外在属性，即在棋盘上的位置可以提取出来，存放在单独的容器中。相比之前的方案，现在容器中仅仅存放了位置属性，而原 来则是棋子对象。显然，现在的方案大大减少了对于空间的需求。

关注PieceBoard 的容器，之前是vector<Piece\*> m\_vecPiece，现在是vector<PiecePos> m\_vecPos。这里是关键。

棋子的新定义，只包含内在属性：

#include <iostream>
    #include <vector>
    #include <string>
    
    
    enum PieceColor {BLACK,WHITE};
    using namespace std;
    
    class Pos
    {
    public:
        Pos(int x,int y):m_x(x),m_y(y)
        {
        
        }
        int getX()
        {
            return m_x;
        }
        int getY()
        {
            return m_y;
        }
    private:
        int m_x;
        int m_y;
    };
    
    class Piece
    {
    public:
        Piece(PieceColor color):m_color(color){};
        ~Piece() {}
        virtual void Draw() {}
    protected:
        PieceColor m_color;
        
    };
    
    class WhitePiece : public Piece
    {
    public:
        WhitePiece(PieceColor color):Piece(color){}
        ~WhitePiece();
        virtual void Draw() {
            cout << "draw a white piece" << endl;
        }
    };
    
    class BlackPiece:public Piece
    {
    public:
        BlackPiece(PieceColor color):Piece(color){}
        ~BlackPiece();
        virtual void Draw(){
    
        cout << "draw a black piece" <<endl;
    
        }
    };
    
    class PieceBoard
    {
    public:
        PieceBoard(string black,string white):m_blackName(black),m_whiteName(white)
        {
        
        }
        ~PieceBoard() {
            Clear();
        }
        void SetPiece(PieceColor color,Pos pos)
        {
            Piece * piece = NULL;
            if(color == BLACK)
            {
                piece = new BlackPiece(color);
                std::cout << m_blackName << "在位置(" << pos.getX() << ","<<pos.getY() <<")" <<endl ;
                piece->Draw();
                m_blackPosArray.push_back(pos);
            }
            else
            {
                piece = new WhitePiece(color);
                std::cout << m_whiteName << "在位置(" << pos.getX() << ","<<pos.getY() <<")" <<endl;
                piece->Draw();
                m_whitePosArray.push_back(pos);
            }
        }
        void Clear()
        {
            //int size = m_pieceArray.size();
            //for(int i = 0; i < size ; i++)
            //    delete m_pieceArray[i];
        }
    private:
        std::vector<Pos> m_whitePosArray;
        std::vector<Pos> m_blackPosArray;
        std::string m_blackName;
        std::string m_whiteName;
    };

主函数：