我的 Design Pattern 之旅 [1]：Strategy Pattern (2) - C++ Interface 經典實現

Strategy Pattern ISO C++ 經典介面實現：虛函數多態與 OCP 原則完美結合

核心代碼

class IDrawStrategy { public: virtual void draw() const = 0; };

class Grapher {
public: Grapher(IDrawStrategy* drawStrategy = 0) : _drawStrategy(drawStrategy) {}
void drawShape() const { if (_drawStrategy) _drawStrategy->draw(); }
void setShape(IDrawStrategy* drawStrategy) { _drawStrategy = drawStrategy; }
protected: IDrawStrategy* _drawStrategy;
};

class Triangle : public IDrawStrategy { public: void draw() const { cout << "Draw Triangle" << endl; } };
class Circle : public IDrawStrategy { public: void draw() const { cout << "Draw Circle" << endl; } };
class Square : public IDrawStrategy { public: void draw() const { cout << "Draw Square" << endl; } };

int main() {
    Grapher grapher(&Square()); grapher.drawShape();
    grapher.setShape(&Circle()); grapher.drawShape();
}

執行結果

Draw Square
Draw Circle

動態換演算法如 Plugin 般靈活。擴充新 Shape 僅需實作 IDrawStrategy，既符合 OCP Closed for Modification 又符合 Open for Extension。