error LNK2005 already defined

Question

I have 2 files  A cpp and B cpp  in a Win32 console application   Both 2 files contain only the following 2 lines of code    include  stdafx h  int k    When compiling it produces the error  Error   1   error LNK2005   int k    a  3HA  already defined in A obj   I don t understand what is happening   Can someone please explain this to me

User · Answer

Both files define variable k as an integer  int    As a result  the linker sees two variables with the same name  and is unsure which one it should use if you ever refer to k   To fix this  change one of the declarations to   extern int k    That means   k is an integer  declared here  but defined externally  ie  the other file     Now there is only one variable k  that can be properly referred to by two different files

User · Answer

If you want both to reference the same variable  one of them should have int k   and the other should have extern int k   For this situation  you typically put the definition  int k   in one  cpp file  and put the declaration  extern int k   in a header  to be included wherever you need access to that variable   If you want each k to be a separate variable that just happen to have the same name  you can either mark them as static  like  static int k   in all files  or at least all but one file   Alternatively  you can us an anonymous namespace   namespace       int k       Again  in all but at most one of the files   In C  the compiler generally isn t quite so picky about this  Specifically  C has a concept of a  tentative definition   so if you have something like int k  twice  in either the same or separate source files  each will be treated as a tentative definition  and there won t be a conflict between them  This can be a bit confusing  however  because you still can t have two definitions that both include initializers--a definition with an initializer is always a full definition  not a tentative definition  In other words  int k   1  appearing twice would be an error  but int k  in one place and int k   1  in another would not  In this case  the int k  would be treated as a tentative definition and the int k   1  as a definition  and both refer to the same variable

User · Answer

Why this error   You broke the one definition rule and hence the linking error      Suggested Solutions     If you need the same named variable in the two cpp files then You need to use Nameless namespace Anonymous Namespace  to avoid the error       namespace        int k        If you need to share the same variable across multiple files then you need to use extern   A h  extern int k    A cpp   include  A h  int k   0    B cpp   include  A h     Use  k  anywhere in the file

User · Answer

Assuming you want  k  to be a different value in different  cpp files  hence declaring it twice   try changing both files to   namespace       int k      This guarantees that the name  k  uniquely identifies  k  across translation units   The old version static int k  is deprecated   If you want them to point to the same value  change one to extern int k

User · Answer

The linker tells you that you have the variable k defined multiple times   Indeed  you have a definition in A cpp and another in B cpp   Both compilation units produce a corresponding object file that the linker uses to create your program   The problem is that in your case the linker does not know whic definition of k to use   In C   you can have only one defintion of the same construct  variable  type  function    To fix it  you will have to decide what your goal is   If you want to have two variables  both named k  you can use an anonymous namespace in both  cpp files  then refer to k as you are doing now       namespace     int k       You can rename one of the ks to something else  thus avoiding the duplicate defintion  If you want to have only once definition of k and use that in both  cpp files  you need to declare in one as extern int k   and leave it as it is in the other   This will tell the linker to use the one definition  the unchanged version  in both cases -- extern implies that the variable is defined in another compilation unit

User · Answer

Presence of int k  in the header file causes symbol k to be defined within each translation unit this header is included to while linker expects it to be defined only once  aka One Definition Rule Violation    While suggestion involving extern are not wrong  extern is a C-ism and should not be used   Pre C  17 solution that would allow variable in header file to be defined in multiple translation units without causing ODR violation would be conversion to template   template lt typename x Dummy   void gt  class t HeaderVariableHolder       public  static int s k      template lt typename x Dummy gt  int t HeaderVariableHolder lt x Dummy gt   s k        Getter is necessary to decouple variable storage implementation details from access to it  inline int  amp  Get K   noexcept       return t HeaderVariableHolder lt  gt   s k      With C  17 things become much simpler as it allows inline variables   inline int g k        Getter is necessary to decouple variable storage implementation details from access to it  inline int  amp  Get K   noexcept       return g k

User · Answer

In the Project   s Settings  add   FORCE MULTIPLE to the Linker   s Command Line options   From MSDN   Use  FORCE MULTIPLE to create an output file whether or not LINK finds more than one definition for a symbol

User · Answer

And if you want these translation units to share this variable  define int k  in A cpp and put extern int k  in B cpp

[c++] error LNK2005, already defined?

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