简单程序实现中的C++继承与虚函数错误

Question

我正在学习C++，我正在尝试实现一个简单的c++程序来创建一个抽象类，如下所示:请解释为什么编译后会出现这个错误？请用简单的语言来解释，我不是术语专家，而且还在学习OOPS的概念。预先谢谢(代码和错误)

代码：

/*
 * shape.cpp
 *
 *  Created on: Mar 9, 2014
 *      Author: Drix
*/


#include <iostream>
using namespace std;

class shape{
public:
virtual void Draw(void)=0;
};


class circle:public shape{
public:
void Draw(double radius){
    radii = radius;
    cout << "The radius of the circle is "  << radii<<endl;
}
private:
double radii;
};

class square:public shape{
public:
void Draw(double side){
    s = side;
    cout << "The length of side of sqare is "  << s<<endl;
}
private:
double s;
};


int main(){

cout <<"Welcome to shape drwaing program"<<endl;
cout <<"Enter 1 to draw a sqare or 2 to draw a circle"<<endl;
int input;

cin>>input;
if(input == 1)
{
    cout << "Please enter the radius of the circle: ";
    double radius;
    cin >> radius;
    circle *p = new circle;
    p->Draw(radius);
}
if(input == 2)
    {
        cout << "Please enter the length of the side of a square: ";
        double side;
        cin >> side;
        square *t = new square;
        t->Draw(side);
    }
 }

错误：

10:58:15 **** Incremental Build of configuration Debug for project Shape ****
Info: Internal Builder is used for build
g++ -O0 -g3 -Wall -c -fmessage-length=0 -o shape.o "..\\shape.cpp" 
..\shape.cpp: In function 'int main()':
..\shape.cpp:51:19: error: cannot allocate an object of abstract type 'circle'
..\shape.cpp:18:7: note:   because the following virtual functions are pure within 'circle':
..\shape.cpp:14:15: note:   virtual void shape::Draw()
..\shape.cpp:59:20: error: cannot allocate an object of abstract type 'square'
..\shape.cpp:28:7: note:   because the following virtual functions are pure within 'square':
..\shape.cpp:14:15: note:   virtual void shape::Draw()

10:58:16 Build Finished (took 884ms)

Answer 1

问题如下:在shape类中，您声明了不接受参数的绘制：

virtual void Draw(void)=0;

而子类circle和square的draw需要一个double：

void Draw(double radius)

在circle中，我认为radius应该传递给构造函数(可能还有类似中心的东西)，而绘图不应该接受任何东西。例如：

class circle:public shape{
private:
    double radii;
public:
    circle(double radius) radii(radius) {};
    void Draw(){
        cout << "The radius of the circle is "  << radii<<endl;
    }
};

然后你用它作为

circle *p = new circle(radius);
p->Draw();

或者如果您不需要动态分配：

circle c(radius);
c.Draw()

当然，square类也有同样的问题。

Answer 2

绘制方法的参数不匹配。

形状：

virtual void Draw(void)=0;

在圆圈和正方形中：

void Draw(double radius)

如果您想拥有虚拟方法，则参数必须匹配。

Answer 3

这种方法的形状应该是(不需要空的)。

class shape{ 
public: 
    virtual void Draw()=0;
};

当您声明循环时，您需要一个Draw方法(即没有半径位的方法)。