c Incomplete type not allowed error accessing class reference information Circular dependency with forward declaration

Question

Had some issues in my code recently surrounding what I now know of as a Circular dependency  In short there are two classes  Player and Ball  which both need to use information from the other  Both at some point in the code will be passed a reference of the other  from another class that will include both  h files    After reading up on it  I removed the  include h files from each one and went with forward declaration  This solved the issue of being able to declare the classes in eachother  but I m now left with an  Incomplete type error  when trying to access a passed reference to the object  There seem to be a few similar examples around  though often mixed with more complex code and hard to narrow down to the basics   I ve rewritten the code in it s simplest form  a skeleton essentially    Ball h   class Player   class Ball   public      Player amp  PlayerB      float ballPosX   800  private        Player h   class Ball   class Player   public      void doSomething Ball amp  ball   private       Player cpp    include  Player h   void Player  doSomething Ball amp  ball        ball ballPosX    10                       incomplete type error occurs here      Any help understanding why this is the case would be greatly appreciated

User · Answer

Here is what I had and what caused my  incomplete type error     include  X h     another already declared class class Big          full declaration of class A  class Small   Big       Small          Small X      line 6          all other stuff   What I did in the file  Big cpp   where I declared the A2 s constructor with X as a parameter is    Big cpp  Small  Big X my x       line 9  lt --- LOOK at this       I wrote  Small  Big  instead of  Small  Small   what a dumb mistake   I received the error  incomplete type is now allowed  for the class X all the time  in lines 6 and 9   which made a total confusion    Anyways  that is where a mistake can happen  and the main reason is that I was tired when I wrote it and I needed 2 hours of exploring and rewriting the code to reveal it

User · Answer

Player cpp require the definition of Ball class  So simply add  include  Ball h   Player cpp    include  Player h   include  Ball h   void Player  doSomething Ball amp  ball        ball ballPosX    10                       incomplete type error occurs here

User · Answer

If you will place your definitions in this order then the code will be compiled  class Ball   class Player   public      void doSomething Ball amp  ball   private      class Ball   public      Player amp  PlayerB      float ballPosX   800  private       void Player  doSomething Ball amp  ball        ball ballPosX    10                       incomplete type error occurs here     int main         The definition of function doSomething requires the complete definition of class Ball because it access its data member   In your code example module Player cpp has no access to the definition of class Ball so the compiler issues an error

[c++] c++ "Incomplete type not allowed" error accessing class reference information (Circular dependency with forward declaration)

Big.cpp

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