X does not name a type error in C

Question

I have two classes declared as below   class User   public    MyMessageBox dataMsgBox      class MyMessageBox   public    void sendMessage Message  msg  User  recvr     Message receiveMessage      vector lt Message gt   dataMessageList       When I try to compile it using gcc  it gives the following error      MyMessageBox does not name a type

User · Answer

On a related note  if you had       class User     let the compiler know such a class will be defined      class MyMessageBox           public          User  myUser              class User           public             also ok  since it s now defined         MyMessageBox dataMsgBox           Then that would also work  because the User is defined in MyMessageBox as a pointer

User · Answer

It is always encouraged in C   that you have one class per header file  see this discussion in SO  1   GManNickG answer s tells why this happen  But the best way to solve this is to put User class in one header file  User h  and MyMessageBox class in another header file  MyMessageBox h   Then in your User h you include MyMessageBox h and in MyMessageBox h you include User h  Do not forget  include gaurds   2  so that your code compiles successfully

User · Answer

Forward declare User Put the declaration of MyMessageBox before User

User · Answer

When the compiler compiles the class User and gets to the MyMessageBox line  MyMessageBox has not yet been defined  The compiler has no idea MyMessageBox exists  so cannot understand the meaning of your class member   You need to make sure MyMessageBox is defined before you use it as a member  This is solved by reversing the definition order  However  you have a cyclic dependency  if you move MyMessageBox above User  then in the definition of MyMessageBox the name User won t be defined   What you can do is forward declare User  that is  declare it but don t define it  During compilation  a type that is declared but not defined is called an incomplete type  Consider the simpler example   struct foo     foo is  declared  to be a struct  but that struct is not yet defined  struct bar          this is okay  it s just a pointer         we can point to something without knowing how that something is defined     foo  fp           likewise  we can form a reference to it     void some func foo amp  fr           but this would be an error  as before  because it requires a definition        foo fooMember         struct foo    okay  now define foo        int fooInt      double fooDouble      void bar  some func foo amp  fr           now that foo is defined  we can read that reference      fr fooInt   111605      fr foDouble   123 456      By forward declaring User  MyMessageBox can still form a pointer or reference to it   class User     let the compiler know such a class will be defined  class MyMessageBox   public         this is ok  no definitions needed yet for User  or Message      void sendMessage Message  msg  User  recvr         Message receiveMessage        vector lt Message gt   dataMessageList      class User   public         also ok  since it s now defined     MyMessageBox dataMsgBox       You cannot do this the other way around  as mentioned  a class member needs to have a definition   The reason is that the compiler needs to know how much memory User takes up  and to know that it needs to know the size of its members   If you were to say   class MyMessageBox   class User   public         size not available  it s an incomplete type     MyMessageBox dataMsgBox       It wouldn t work  since it doesn t know the size yet     On a side note  this function    void sendMessage Message  msg  User  recvr     Probably shouldn t take either of those by pointer  You can t send a message without a message  nor can you send a message without a user to send it to  And both of those situations are expressible by passing null as an argument to either parameter  null is a perfectly valid pointer value    Rather  use a reference  possibly const     void sendMessage const Message amp  msg  User amp  recvr

User · Answer

C   compilers process their input once  Each class you use must have been defined first  You use MyMessageBox before you define it  In this case  you can simply swap the two class definitions

User · Answer

You must declare the prototype it before using it   class User   class MyMessageBox   public   void sendMessage Message  msg  User  recvr    Message receiveMessage     vector lt Message gt   dataMessageList      class User   public   MyMessageBox dataMsgBox       edit  Swapped the types

User · Answer

You need to define MyMessageBox before User -- because User include object of MyMessageBox by value  and so compiler should know its size    Also you ll need to forward declare User befor MyMessageBox -- because MyMessageBox include member of User  type

[c++] "X does not name a type" error in C++

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