简单程序实现中的 C++ 继承和虚函数错误

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： 请解释为什么我在编译后会收到此错误？请用简单的语言解释我不是术语专业人士，但仍在学习 OOPS 概念。提前致谢（代码和错误如下）

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 draw 不带参数：

virtual void Draw(void)=0;

而子类 circle 和 square 有 draw 期望双：

void Draw(double radius)

在circle 中，我认为radius（可能连同中心之类的东西）应该传递给构造函数并且draw 不应该接受任何内容。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;
    }
};

然后你把它当作

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班级也有同样的问题。

Answer 2

Draw 方法的参数不匹配。

形状：

virtual void Draw(void)=0;

在圆形和方形中：

void Draw(double radius)

如果你想要虚拟方法，参数必须匹配。

Answer 3

Shape中的这个方法应该是（不需要void）

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

当您声明 Circle 时，您需要一个 Draw 方法（即没有半径位的方法）