c++ 享元模式（flyweight）

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：

#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;
}