C++：虛函數(virtual function)預設參數的異常表現及正確使用

避免定義基類(base class)虛函數的預設參數, 如果基類虛函數包含預設參數, 衍生類別不要重新定義, 會產生異常;

虛函數(virtual)是動態綁定(dynamically bound),預設參數是靜態繫結(statically bound).

如果衍生類別(derived class)重新定義預設值, 執行個體化, 執行函數正常;

但在動態綁定時, 會出現,執行基類的預設參數, 卻是衍生類別的行為, 這樣的異常舉動.

talk is cheap, show you the code.

注意觀察動態綁定時, pr的輸出, 表現為基類的預設參數, 衍生類別的動作.

代碼:

/*  * test.cpp  *  *  Created on: 2014.04.14  *      Author: Spike  */      /*eclipse cdt, gcc 4.8.1*/      #include <iostream>  #include <string>        using namespace std;        class Shape {  public:      enum ShapeColor {Red, Green, Blue};      virtual void draw(ShapeColor color = Red) const = 0;  };        class Rectangle : public Shape {  public:      virtual void draw(ShapeColor color = Green) const {          std::cout << "Rectangle : ";          switch (color) {          case Red :              std::cout << "color = " << "Red" << std::endl;              break;          case Green :              std::cout << "color = " << "Green" << std::endl;              break;          case Blue :              std::cout << "color = " << "Blue" << std::endl;              break;          default:              break;          }      }  };        class Circle : public Shape {  public:      virtual void draw(ShapeColor color) const {          std::cout << "Circle : ";          switch (color) {          case Red :              std::cout << "color = " << "Red" << std::endl;              break;          case Green :              std::cout << "color = " << "Green" << std::endl;              break;          case Blue :              std::cout << "color = " << "Blue" << std::endl;              break;          default:              break;          }      }  };        int main() {      Rectangle R;      Circle C;      R.draw();      //C.draw(); //報錯, 靜態繫結無法繼承參數            Shape* pr = new Rectangle; //動態綁定      Shape* pc = new Circle;            pr->draw(Shape::Blue);      pc->draw(Shape::Blue);      pr->draw(); //靜態繫結, 使用基類的預設參數, 完成衍生類別的動作      pc->draw();                  return 0;  }

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輸出:

Rectangle : color = Green  Rectangle : color = Blue  Circle : color = Blue  Rectangle : color = Red  Circle : color = Red

如果想使用基類虛函數的預設參數, 衍生類別不提供參數, 則需要使用NVI(non-virtual interface)的方法, 可以直接使用預設參數;

但是, 絕對不要重新定義一個繼承而來的預設參數值.

如, 以下代碼中, Rectangle的私人虛函數, doDraw()不要提供預設參數.

代碼:

/*  * test.cpp  *  *  Created on: 2014.04.14  *      Author: Spike  */      /*eclipse cdt, gcc 4.8.1*/      #include <iostream>  #include <string>        using namespace std;        class Shape {  public:      enum ShapeColor {Red, Green, Blue};      void draw(ShapeColor color = Red) const {          doDraw(color);      }  private:      virtual void doDraw(ShapeColor color) const = 0;  };        class Rectangle : public Shape {  private:      virtual void doDraw(ShapeColor color) const {          std::cout << "Rectangle : ";          switch (color) {          case Red :              std::cout << "color = " << "Red" << std::endl;              break;          case Green :              std::cout << "color = " << "Green" << std::endl;              break;          case Blue :              std::cout << "color = " << "Blue" << std::endl;              break;          default:              break;          }      }  };        int main() {      Rectangle R;      R.draw();            Shape* pr = new Rectangle; //動態綁定            pr->draw(Shape::Blue);      pr->draw();                  return 0;  }

輸出:

Rectangle : color = Red  Rectangle : color = Blue  Rectangle : color = Red

作者：csdn部落格 Spike_King