避免定义基类(base class)虚函数的缺省参数, 如果基类虚函数包含缺省参数, 派生类不要重新定义, 会产生异常;
虚函数(virtual)是动态绑定(dynamically bound),缺省参数是静态绑定(statically bound).
如果派生类(derived class)重新定义缺省值, 实例化, 执行函数正常;
但在动态绑定时, 会出现,执行基类的缺省参数, 却是派生类的行为, 这样的异常举动.
talk is cheap, show you the code.
注意观察动态绑定时, pr的输出, 表现为基类的缺省参数, 派生类的动作.
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, 静态绑定无法继承参数
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, 使用基类的默认参数, 完成派生类的动作
pc->draw();
return 0;
}
更多精彩内容：http://www.bianceng.cn/Programming/cplus/
输出:
Rectangle : color = Green Rectangle : color = Blue Circle : color = Blue Rectangle : color = Red Circle : color = Red
如果想使用基类虚函数的缺省参数, 派生类不提供参数, 则需要使用NVI(non-virtual interface)的方法, 可以直接使用默认参数;
但是, 绝对不要重新定义一个继承而来的缺省参数值.
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
