使用从抽象类继承的 C++ 默认构造函数

Question

您好，我是 C++ 新手，在使用以下代码时遇到问题:

#include "stdafx.h"
#include <iostream>

using namespace std;

class Shape
{
public:
    virtual void area() = 0;
    virtual void perimeter() = 0;
    virtual void volume() = 0;
};


class Circle : public Shape
{
protected:
    int r;


public:
    Circle()
    {
        r = 0;
    }


    Circle(int num)
    {
        r = num;
    }


    void area()
    {
        double area = (r * r) * (3.14);
        cout << "The area of the circle = " << area << endl;
    }


    void perimeter()
    {
        double perimeter = 2 * (3.14) * r;
        cout << "The circumference of the circle = " << perimeter << endl;

    }


    void volume()
    {
        cout << "You are tapping into another dimension. AHHH Error!" << endl;
    }

};




class Sphere : public Circle
{

protected:
    int r;

public:
    Sphere() :Circle()
    {
        r = 0;
    }

    Sphere(int num) : Circle(num)
    {
        r = num;
    }

    void area()
    {
        double area = 4 * (3.14) * (r * r);
        cout << "The surface area of the sphere = " << area << endl;
    }

    void perimeter()
    {
        double perimeter = 2 * (3.14) * r;
        cout << "The circumference of the sphere = " << perimeter << endl;
    }

    void volume()
    {
        double volume = ((4 / 3) * (3.14) * (r * r * r));
        cout << "The volume of the sphere = " << volume << endl;
    }

};


int _tmain(int argc, _TCHAR* argv[])
{
    Shape *ptr;

    Circle c1(5);
    Circle c2();

    Sphere sp1(6);
    Sphere sp2();

    ptr = &c1;
    ptr->perimeter();
    ptr->area();
    ptr->volume();

    ptr = &c2;
    ptr->perimeter();
    ptr->area();
    ptr->volume();

    ptr = &sp1;
    ptr->perimeter();
    ptr->area();
    ptr->volume();

    ptr = &sp2;
    ptr->perimeter();
    ptr->area();
    ptr->volume();

    return 0;
}

控制台告诉我这个:

error C2440: '=' : cannot convert from 'Circle (__cdecl *)(void)' to 'Shape *'
error C2440: '=' : cannot convert from 'Sphere (__cdecl *)(void)' to 'Shape *'

---

注意: 我在包含以下内容的行中收到这些错误:“ptr = &c2;”和“ptr = &sp2;”

这仅适用于使用不带参数的构造函数的实例。 带参数的构造函数出于某种原因工作??
你能帮忙的话，我会很高兴。谢谢!!!

Answer 1

使用无参数构造函数构建对象的正确方法是

 Circle c2;

不是

 Circle c2();

后一行是函数声明。