Error with multiple definitions of function

Question

I am trying to relearn C   after taking an intro course a few years ago and I   m having some basic problems  My current problem occurs when trying to use a friend function  Here is my code in 2 files   First      fun cpp   include  lt iostream gt  using namespace std   class classA       friend void funct    public      classA int a 1 int b 2  propa a  propb b  cout lt  lt  constructor n    private      int propa      int propb      void outfun            cout lt  lt  propa   lt  lt propa lt  lt endl lt  lt  propb   lt  lt propb lt  lt endl           void funct                           ERROR HERE     cout lt  lt  enter funct  lt  lt endl      classA tmp 1 2       tmp outfun        cout lt  lt  exit funct  lt  lt endl      Second      mainfile cpp  include  lt iostream gt   include  fun cpp  using namespace std   int main int nargin char  varargin          cout lt  lt  call funct  lt  lt endl      funct        cout lt  lt  exit main  lt  lt endl      return 0      The error I am getting is  multiple definition of  funct      Am I using the wrong syntax when declaring it as a friend function

User · Answer

Here is a highly simplified but hopefully relevant view of what happens when you build your code in C++.

C++ splits the load of generating machine executable code in following different phases -

Preprocessing - This is where any macros - #defines etc you might be using get expanded.
Compiling - Each cpp file along with all the #included files in that file directly or indirectly (together called a compilation unit) is converted into machine readable object code.

This is where C++ also checks that all functions defined (i.e. containing a body in { } e.g. void Foo( int x){ return Boo(x); }) are referring to other functions in a valid manner.

The way it does that is by insisting that you provide at least a declaration of these other functions (e.g. void Boo(int); ) before you call it so it can check that you are calling it properly among other things. This can be done either directly in the cpp file where it is called or usually in an included header file.

Note that only the machine code that corresponds to functions defined in this cpp and included files gets built as the object (binary) version of this compilation unit (e.g. Foo) and not the ones that are merely declared (e.g. Boo).
Linking - This is the stage where C++ goes hunting for stuff declared and called in each compilation unit and links it to the places where it is getting called. Now if there was no definition found of this function the linker gives up and errors out. Similarly if it finds multiple definitions of the same function signature (essentially the name and parameter types it takes) it also errors out as it considers it ambiguous and doesn't want to pick one arbitrarily.

The latter is what is happening in your case. By doing a #include of the fun.cpp file, both fun.cpp and mainfile.cpp have a definition of funct() and the linker doesn't know which one to use in your program and is complaining about it.

The fix as Vaughn mentioned above is to not include the cpp file with the definition of funct() in mainfile.cpp and instead move the declaration of funct() in a separate header file and include that in mainline.cpp. This way the compiler will get the declaration of funct() to work with and the linker would get just one definition of funct() from fun.cpp and will use it with confidence.

User · Answer

The problem is that if you include fun cpp in two places in your program  you will end up defining it twice  which isn t valid   You don t want to include cpp files   You want to include header files   The header file should just have the class definition   The corresponding cpp file  which you will compile separately  will have the function definition   fun hpp    include  lt iostream gt   class classA       friend void funct    public      classA int a 1 int b 2  propa a  propb b  std  cout lt  lt  constructor n    private      int propa      int propb      void outfun            std  cout lt  lt  propa   lt  lt propa lt  lt endl lt  lt  propb   lt  lt propb lt  lt  std  endl             fun cpp    include  fun hpp   using namespace std   void funct        cout lt  lt  enter funct  lt  lt endl      classA tmp 1 2       tmp outfun        cout lt  lt  exit funct  lt  lt endl      mainfile cpp    include  lt iostream gt   include  fun hpp  using namespace std   int main int nargin char  varargin          cout lt  lt  call funct  lt  lt endl      funct        cout lt  lt  exit main  lt  lt endl      return 0      Note that it is generally recommended to avoid using namespace std in header files

User · Answer

You have  include  quot fun cpp quot  in mainfile cpp so compiling with  g   -o hw1 mainfile cpp  will work  however if you compile by linking these together like g   -g -std c  11 -Wall -pedantic   -c -o fun o fun cpp g   -g -std c  11 -Wall -pedantic   -c -o mainfile o mainfile cpp  As they mention above  adding  include  quot fun hpp quot  will need to be done or it won t work  However  your case with the funct   function is slightly different than my problem  I had this issue when doing a HW assignment and the autograder compiled by the  lower bash recipe  yet locally it worked using the upper bash

User · Answer

This problem happens because you are calling fun cpp instead of fun hpp  So c   compiler finds func cpp definition twice and throws this error  Change line 3 of your main cpp file  from  include  quot fun cpp quot  to  include  quot fun hpp quot

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