C++: error LNK: unresolved external symbol, resulting from virtual functions

Question

Overview of classes etc of my interface!

Animal.H:

class Animal
{
public:
   virtual void walk();
}

Animals.CPP

=EMPTY

Cow.H:

class Cow : public Animal
{
public:
   virtual void walk();
}

Here it should outomatically know the function walk is taken from the class where it's derived from right? (e.a. Animal..) when i wouldnt define the function walk, it should say i should define it right...?

Cow.CPP:

void Cow::walk()
{
   //do something specific for cow
}

SomeOtherClass.H

namespace SomeNamespace
{
   void LetAnimalWalk();
}

SomeOtherClass.CPP

Cow myCow;
namespace SomeNamespace
{
   void LetAnimalWalk()
   {
      myCow.walk();
   }
}

This should work right?... i mean, the namespace, the "Class::..." things? and the way i inherit and use the interface?

Because this way i get with EVERY FUNCTION i made from the interface, so every virtual function gives me the following error:

SomeOtherClass.obj : error LNK2019: unresolved external symbol "public: virtual void __thiscall Cow::Walk (...etc etc...) referenced in function "void __cdecl SomeNamespace::LetAnimalWalk() (...etc etc...)

Does anyone know what i'm doing wrong, what i do find mostly is that it means i've not declared a function right (somewhere in Cow.cpp??)

Thanks in advance guys

Answer 1

class Animal
{
public:
   virtual void walk();
}

You need to define that function or make it pure virtual like

class Animal
{
public:
   virtual void walk() = 0;
}

Answer 2

When you have a virtual function that you don't want to define in the base class, you need to make it abstract, this is done using the following syntax:

class Animal
{
public:
   virtual void walk() = 0;
}

Without this you will get an error if it is not defined, and with this you will get an error if any derived class does not define it.

Answer 3

either make the function in the base class pure virtual:

class Animal
{
    public:
        virtual void walk() = 0;
}

or define a trivial implementation:

void Animal::walk()
{
   return;    // could also throw an exception here
}

Answer 4

Animal* animal = new Cow(); animal->walk();

Cow myCow does NOT work obviously!