C ++: LNK error: unresolved external character resulting from virtual functions

Question

Class overview, etc. my interface!

Animal.H:

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

Animals.CPP

= EMPTY

Cow.H:

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

Here you should know that the walk function is taken from the class in which it is displayed on the right? (e.g. Animal ..), when I did not define the walk function, it should say that I have to define it correctly ...?

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(); } } 

Should this work correctly? ... I mean the namespace, "class :: ..."? and how do I inherit and use the interface?

Because in this way I get with EVERY FUNCTION 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 really find is that it means that I did not declare the function on the right (somewhere in Cow.cpp ??)

Thanks in advance guys

Answer 1

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

You need to define this function or make it pure virtual.

Answer 2

If you have a virtual function that you do not 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 receive an error message if it is not defined, and with this you will receive an error message 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 ();

The myCow cow is NOT working explicitly!