我无法构建一个派生类对象！谁能帮助我！ [英] I can't build a derive class object！who can help me!

问题描述

#include<iostream>
class CShape
{

private:
    char cType[20];
    float fArea;

public:

    CShape(){}
    CShape(char * cType_name , float fA) : fArea(fA) { std::strcpy(cType,cType_name); }
    virtual void print()
    {

        std::cout << "Type: " << cType << std::endl;
        std::cout << "Area: " << fArea << std::endl;

    }
    virtual float area() const = 0;

};

class CCircle : public CShape
{

private:
    float fRadius;

public:

    CCircle(){}
    CCircle (float fR=0 ) : CShape("CCircle",3.14*fR*fR) { fRadius=fR;}
    virtual void print ()
    {

        CShape::print();
        std::cout << "Radius: " << fRadius << std::endl;

    }

    float virtual area() { return 3.14*fRadius*fRadius; }


};


int main()
{

    CCircle cir(1);

}

以上是我的代码，系统提示CCircle是一个抽象类！ />
我不知道为什么！谁可以帮助我！

The above is my code , the system prompt that CCircle is a abstract class!
I don''t know why! who can help me!

推荐答案

当覆盖虚拟方法时，你必须确保方法签名完全匹配（请参阅布莱登的评论）。改变来自

When overriding virtual methods you have to make sure that the method signatures exactly match (please see Brydon''s remark). Change from

Quote：

float virtual area（）{return 3.14 * fRadius * fRadius; }

float virtual area() { return 3.14*fRadius*fRadius; }

to

to

virtual float area() const { return 3.14*fRadius*fRadius; }

从CCircle中的方法中取出 virtual 。请尝试覆盖。



任何包含虚拟函数是定义抽象的。

Take the virtual off the methods in CCircle. Try override instead.

Any class that contains a virtual function is abstract by definition.

编译器不允许声明对象d，因为CCircle是一个抽象类;它从AB继承了纯虚函数f（）。如果定义函数CCircle :: area（），编译器将允许声明对象d。



请注意，您可以从非抽象类派生抽象类，并且你可以使用纯虚函数覆盖非纯虚函数。



你可以做的是你可以创建一个指向该CCricle类的指针

ie，

CCricle * l_ptrcricle;

l_ptrcricle =新的CCricle; //你会给出错误

The compiler will not allow the declaration of object d because CCircle is an abstract class; it inherited the pure virtual function f()from AB. The compiler will allow the declaration of object d if you define function CCircle ::area().

Note that you can derive an abstract class from a nonabstract class, and you can override a non-pure virtual function with a pure virtual function.

what you can do is you can create a pointer to that CCricle class
i.e,
CCricle *l_ptrcricle;
l_ptrcricle= new CCricle;// will you give a error

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