C++ - 虚函数(virtual function)缺省参数的错误表现 及 正确使用方法

C++ - 虚函数(virtual function)缺省参数的异常表现 及 正确使用方法

虚函数(virtual function)缺省参数的异常表现 及 正确使用方法

本文地址: http://blog.****.net/caroline_wendy/article/details/23679691

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;
}

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