Unresolved External Symbol linker error (C++)

Question

I am trying to develop abstract design pattern code for one of my project as below.. But, I am not able to compile the code ..giving some compile errors(like "unresolved external symbol "public: virtual void __thiscall Xsecs::draw_lines(double,double)" (?draw_lines@Xsecs@@UAEXNN@Z)" ).. Can any one please help me out in this...

#include "stdafx.h"
#include <iostream>
#include <vector>
#include "Xsecs.h"
using namespace std;
//Product class

class Xsecs
{
public:
    virtual void draw_lines(double pt1, double pt2);
    virtual void draw_curves(double pt1, double rad);
};

class polyline: public Xsecs
{
public:
    virtual void draw_lines(double pt1,double pt2)
    {
        cout<<"draw_line in polygon"<<endl;
    }
     virtual void draw_curves(double pt1, double rad)
    {
        cout<<"Draw_curve in circle"<<endl;
    }
    /*void create_polygons()
    {
        cout<<"create_polygon_thru_draw_lines"<<endl;
    }*/
};

class circle: public Xsecs
{
 public:
     virtual void draw_lines(double pt1,double pt2)
    {
        cout<<"draw_line in polygon"<<endl;
    }
     virtual void draw_curves(double pt1, double rad)
    {
        cout<<"Draw_curve in circle"<<endl;
    }
    /*void create_circles()
    {
        cout<<"Create circle"<<endl;
    }*/
};

//Factory class
class Factory
{
public:
 virtual polyline* create_polyline()=0;
 virtual circle* create_circle()=0;
};

class Factory1: public Factory
{
public:
      polyline* create_polyline()
{
    return new polyline();
}
      circle* create_circle()
{
    return new circle();
}
};

class Factory2: public Factory
{
public:
      circle* create_circle()
{
    return new circle();
}
     polyline* create_polyline()
{
    return new polyline();
}
};

int _tmain(int argc, _TCHAR* argv[])
{
    Factory1 f1;
    Factory * fp=&f1;
    return 0;
}

Answer 1

I presume you were attempting to create a virtual base class. You need to add '= 0' to the end of the draw_lines and draw_curves methods in the class Xsecs

class Xsecs
{
public:
    virtual void draw_lines(double pt1, double pt2) = 0;
    virtual void draw_curves(double pt1, double rad) = 0;
};

the compiler is complaining as you haven't any implementation for the methods in question.

Answer 2

You should inherit publicly from A , like

class ProductA1 : public ProductA {
...

Without the public keyword, this relationship is private inheritance , which is not an is-a relationship, therefore you can't simply cast from ProductA1 to ProductA .

Scott Meyers explains this in Effective C++, Third Ed. , Item 39:

[...] compilers, when given a hierarchy in which a class Student publicly inherits from a class Person , implicitly convert Students to Persons when that is necessary for a function call to succeed.

[...] the first rule governing private inheritance you've just seen in action: in contrast to public inheritance, compilers will generally not convert a derived class object (such as Student ) into a base class object (such as Person ) if the inheritance relationship between the classes is private. [...] The second rule is that members inherited from a private base class become private members of the derived class, even if they were protected or public in the base class.

Private inheritance means is-implemented-in-terms-of . If you make a class D privately inherit from a class B , you do so because you are interested in taking advantage of some of the features available in class B , not because there is any conceptual relationship between objects of types B and D . As such, private inheritance is purely an implementation technique.

Update for the 2nd version of the post: if you want pure virtual functions, you should declare them so:

virtual void draw_lines(double pt1, double pt2) = 0;
virtual void draw_curves(double pt1, double rad) = 0;

Otherwise the linker will miss their definition.

Answer 3

In all your class definitions you forgot to use the public keyword :

class ProductA1 : ProductA

should be

class ProductA1 : public ProductA

and so on

Answer 4

You either need to add an implementation for Xsecs::draw_lines / Xsecs::draw_curves, or define them as pure virtual, by appending "= 0" to their definition.

class Xsecs
{
public:
    virtual void draw_lines(double pt1, double pt2)
    {
        // Do something
    }
   virtual void draw_curves(double pt1, double rad)
    {
        // Do something
    }
};

Or...

class Xsecs
{
public:
    virtual void draw_lines(double pt1, double pt2) = 0;
    virtual void draw_curves(double pt1, double rad) = 0;
};

Answer 5

将“class”改为“struct”这使得默认的公共继承而不是私有