享元模式

定义：运用共享技术，有效的实现支持大量细粒度对象的复用 

类图：

     

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。

                                        

常用场景

1.当系统中有大量的细粒度对象实例，而且这些对象实例中有一些属性是重复的情况下，考虑使用。

文本编辑器，输入法之类的常用应用。

优点

1.提高了系统的效率，减小了内存的消耗。

2.减少了重复代码。

3.减少了系统的复杂度。

缺点

1.维护共享对象和查找所需的共享对象需要花费很多时间，因为共享模式将对象进行了细粒度话，而不是在一个对象中产生。

代码

//piece's color 
enum PieceColor {BLACK, WHITE};  

//position  
struct PiecePos  
{  
    int x;  
    int y;  
    PiecePos(int a, int b): x(a), y(b) {}  
};  

//define a piece   
class Piece  
{  
protected:  
    PieceColor m_color;  
    PiecePos m_pos;      
public:  
    Piece(PieceColor color, PiecePos pos): m_color(color), m_pos(pos) {}  
    ~Piece() {}  
    virtual void Draw() {}  
};  

class BlackPiece: public Piece  
{  
public:  
    BlackPiece(PieceColor color, PiecePos pos): Piece(color, pos) {}  
    ~BlackPiece() {}   
    void Draw() { cout<<" make a black piece"<<endl;}  
};  
class WhitePiece: public Piece  
{  
public:  
    WhitePiece(PieceColor color, PiecePos pos): Piece(color, pos) {}  
    ~WhitePiece() {}  
    void Draw() { cout<<" make a white piece"<<endl;}  
};  

class PieceBoard  
{  
private:  
    vector<Piece*> m_vecPiece; //The pieces already on board 
    string m_blackName; //the black name   
    string m_whiteName; //the white name   
public:  
    PieceBoard(string black, string white): m_blackName(black), m_whiteName(white){}  
    ~PieceBoard() { Clear(); }  
    void SetPiece(PieceColor color, PiecePos pos) //new a piece to the board   
    {  
        Piece * piece = NULL;  
        if(color == BLACK) //from the black   
        {     
            piece = new BlackPiece(color, pos); //new a piece
            cout<<m_blackName<<" the position is("<<pos.x<<','<<pos.y<<")";  
            piece->Draw(); //draw the black piece on board   
        }  
        else  
        {     
            piece = new WhitePiece(color, pos);  
            cout<<m_whiteName<<" the position is("<<pos.x<<','<<pos.y<<")";  
            piece->Draw();  
        }  
        m_vecPiece.push_back(piece);  //store the piece
    } 

    void Clear() //free the memory 
    {  
        int size = m_vecPiece.size();  
        for(int i = 0; i < size; i++)  
            delete m_vecPiece[i];  
    }  
};  

int main()  
{  
	cout<<"\n--------------start of the main()---------------"<<endl;  
    PieceBoard pieceBoard("A","B");  
    pieceBoard.SetPiece(BLACK, PiecePos(4, 4));  
    pieceBoard.SetPiece(WHITE, PiecePos(4, 16));  
    pieceBoard.SetPiece(BLACK, PiecePos(16, 4));  
    pieceBoard.SetPiece(WHITE, PiecePos(16, 16));  

	cout<<"\n--------------end of the main()---------------"<<endl;  
	return 0;
}