C++：虚函数(virtual function)缺省参数的异常表示及正确利用

制止界说基类(base class)虚函数的缺省参数, 假如基类虚函数包括缺省参数, 派生类不要从头界说, 会发生异常;

虚函数(virtual)是动态绑定(dynamically bound),缺省参数是静态绑定(statically bound).

假如派生类(derived class)从头界说缺省值, 实例化, 执行函数正常;

但在动态绑按时, 会呈现,执行基类的缺省参数, 却是派生类的行为, 这样的异常流动.

talk is cheap, show you the code.

留意调查动态绑按时, pr的输出, 表示为基类的缺省参数, 派生类的行动.

代码:

/* 
 * 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(); //报错, 静态绑定无法担任参数  
      
    Shape* pr = new Rectangle; //动态绑定  
    Shape* pc = new Circle;  
      
    pr->draw(Shape::Blue);  
    pc->draw(Shape::Blue);  
    pr->draw(); //静态绑定, 利用基类的默认参数, 完成派生类的行动  
    pc->draw();  
      
      
    return 0;  
}

输出:

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

假如想利用基类虚函数的缺省参数, 派生类不提供参数, 则需要利用NVI(non-virtual interface)的要领, 可以直接利用默认参数;

可是, 绝对不要从头界说一个担任而来的缺省参数值.

如, 以下代码中, Rectangle的私有虚函数, doDraw()不要提供默认参数.

代码:

/* 
 * 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; //动态绑定  
      
    pr->draw(Shape::Blue);  
    pr->draw();  
      
      
    return 0;  
}

输出:

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

作者：csdn博客 Spike_King