Unresolved External Symbol linker error (C++)

Question

Hi,

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

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 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

change "class" to "struct" this make the default public inheritance rather than private

Answer 4

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 5

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;
};