C Calling a function from another class

Question

Very new to c   having trouble calling a function from another class    Class B inherites from Class A   and i want class A to be able to call a function created in class B    using namespace std   class B   class A     public       void CallFunction                  B b          b bFunction               class B  public A   public      virtual void bFunction                           stuff done here                 It all looks fine on screen  no obvious errors  but when I try to compile it i get an error C2079  b  uses undefined class B   I ve tried making them pointers  friends but i m getting the same error

User · Answer

Forward declare class B and swap order of A and B definitions  1st B and 2nd A    You can not call methods of forward declared B class

User · Answer

in A you have used a definition of B which is not given until then   that s why the compiler is giving error

User · Answer

What you should do  is put CallFunction into   cpp file  where you include B h  After edit  files will look like  B h   pragma once   or other specific to compiler    using namespace std   class A    public      void CallFunction         class B  public A   public      virtual void bFunction                           stuff done here               B cpp  include  quot B h quot  void A  CallFunction      use B object here       Referencing to your explanation  that you have tried to change B b  into pointer- it would be okay  if you wouldn t use it in that same place  You can use pointer of undefined class but declared   because ALL pointers have fixed byte size 4   so compiler doesn t have problems with that  But it knows nothing about the object they are pointing to simply  knows the size boundary  not the content   So as long as you are using the knowledge  that all pointers are same size  you can use them anywhere  But if you want to use the object  they are pointing to  the class of this object must be already defined and known by compiler  And last clarification  objects may differ in size  unlike pointers  Pointer is a number index  which indicates the place in RAM  where something is stored for example index  0xf6a7b1

User · Answer

void CallFunction                lt ----- At this point the compiler knows                   nothing about the members of B          B b          b bFunction            This happens for the same reason that functions in C cannot call each other without at least one of them being declared as a function prototype   To fix this issue we need to make sure both classes are declared before they are used  We separate the declaration from the definition  This MSDN article explains in more detail about the declarations and definitions   class A   public      void CallFunction         class B  public A   public      virtual void bFunction                   void A  CallFunction          B b      b bFunction

User · Answer

class B is only declared but not defined at the beginning  which is what the compiler complains about  The root cause is that in class A s Call Function  you are referencing instance b of type B  which is incomplete and undefined  You can modify source like this without introducing new file just for sake of simplicity  not recommended in practice    using namespace std   class A    public       void CallFunction         class B  public A   public      virtual void bFunction                   stuff done here                postpone definition of CallFunction here   void A  CallFunction            B b       b bFunction

User · Answer

Here s my solution to the issue  Tried to keep it straight and simple   include  lt iostream gt  using namespace std   class Game  public    void init        cout  lt  lt   quot Hi quot   lt  lt  endl       g   class b   Game     class b uses imports class Game public    void h        init      Use function from class Game      A   int main       A h      return 0

[c++] C++ Calling a function from another class

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