我无法理解析构函数有什么问题？

Question

I have a class called polygon which is my base class in which I have area and perimeter and I need to derive a rectangle class from it. Right now the program below doesn't work work and it gives me the following error:

GS_Inheritance_Program.obj : error LNK2019: unresolved external symbol "public: virtual
 __thiscall rectangle::~rectangle(void)" (??1rectangle@@UAE@XZ) referenced in function 
"public: virtual void * __thiscall rectangle::`scalar deleting destructor'(unsigned int)" 
(??_Grectangle@@UAEPAXI@Z)

---

It is due to destructors that I added to the program but when I remove them both it works. I did some research and found out that it might be due to me not compiling the program .cpp file correctly. Is that my problem? If not, what is my problem?

#include <iostream>
using namespace std;

class polygon
{
protected:
    double area;
    double perimeter;

public:
    polygon(){}
    ~polygon();
    double printperimeter();
    double printarea();
};

double polygon::printperimeter()
{
    return perimeter;
}

double polygon::printarea()
{
    return area;
}

class rectangle:public polygon
{
protected:
    double length;
    double width;
public:
    rectangle(double = 1.0, double = 1.0);
    ~rectangle();
    double calcarea();
    double calcperimeter();
};

rectangle::rectangle(double l, double w)
{
    length = l;
    width = w;
}

double rectangle::calcarea()
{
    area = length*width;
    return printarea();
}

double rectangle::calcperimeter()
{
    perimeter = 2*(length+width);
    return printperimeter();
}

void main()
{
    rectangle rect_1 (9.0, 5.0);

    cout<<"The Area of Rect_1 is " <<rect_1.calcarea() <<endl;

    system("pause");
}

Answer 1

You declared destructors in your classes. But you never defined them. Why would you declare functions and then fail to define them? You declared polygon::~polygon() and rectangle::~rectangle() . Neither is defined though.

You are basically lying to the compiler. You make a promise by declaring a function, and then you break that promise by failing to define it. Hence the error.

PS And that's int main() , not void main() .

Answer 2

You didn't add destructors. You said you added them, but you didn't actually add them. So the linker is looking for them and not finding them.

You could just change:

            ~rectangle();

to

            ~rectangle() { ; }