Including cpp files

Question

I have read in places like here that you have to include  h files and not  cpp files  because otherwise then you get an error  So for example  main cpp   include  lt iostream gt   include  foop h   int main int argc  char  argv      int x 42  std  cout  lt  lt  x  lt  lt std  endl  std  cout  lt  lt  foo x   lt  lt  std  endl  return 0      foop h   ifndef FOOP H  define FOOP H int foo int a    endif   foop cpp  int foo int a       return   a      works  but if I replace  include  foop h  with  include  foop cpp  I get an error  Using Dev C   4 9 9 2  Windows    multiple definition of foo int  first defined here   Why is this

User · Answer

Because your program now contains two copies of the foo function, once inside foo.cpp and once inside main.cpp

Think of #include as an instruction to the compiler to copy/paste the contents of that file into your code, so you'll end up with a processed main.cpp that looks like this

#include <iostream> // actually you'll get the contents of the iostream header here, but I'm not going to include it!
int foo(int a){
    return ++a;
}

int main(int argc, char *argv[])
{
int x=42;
std::cout << x <<std::endl;
std::cout << foo(x) << std::endl;
return 0;
}

and foo.cpp

int foo(int a){
    return ++a;
}

hence the multiple definition error

User · Answer

So I found that if you are compiling from Visual Studios you just have to exclude the included  cpp file from the final build  that which you are extending from       Visual Studios   cpp file   right click    properties   configuration properties     general   excluded from build   yes   I believe you can also exclude the file when compiling from the command line

User · Answer

Because of the One Definition Rule  probably1    In C    each non-inline object and function must have exactly one definition within the program   By  includeing the file in which foo int  is defined  the CPP file   it is defined both in every file where foop cpp is  included  and in foop cpp itself  assuming foop cpp is compiled    You can make a function inline to override this behavior  but I m not recommending that here   I have never seen a situation where it is necessary or even desirable to  include a CPP file   There are situations where it is desireable to include a definition of something   This is specially true when you try to seperate the definition of a template from the declaration of it   In those cases  I name the file HPP rather than CPP to denote the difference     1    probably    I say probably here because the actual code you ve posted should compile without errors  but given the compiler error it seems likely that the code you posted isn t exactly the same as the code you re compiling

User · Answer

This boils down to a difference between definitions and declarations    You can declare functions and variables multiple times  in different translation units  or in the same translation unit  Once you declare a function or a variable  you can use it from that point on  You can define a non-static function or a variable only once in all of your translation units  Defining non-static items more than once causes linker errors    Headers generally contain declarations  cpp files contain definitions  When you include a file with definitions more than once  you get duplicates during linking   In your situation one defintion comes from foo cpp  and the other definition comes from main cpp  which includes foo cpp   Note  if you change foo to be static  you would have no linking errors  Despite the lack of errors  this is not a good thing to do

User · Answer

When you say  include  foop cpp   it is as if you had copied the entire contents of foop cpp and pasted it into main cpp   So when you compile main cpp  the compiler emits a main obj that contains the executable code for two functions  main and foo   When you compile foop cpp itself  the compiler emits a foop obj that contains the executable code for function foo   When you link them together  the compiler sees two definitions for function foo  one from main obj and the other from foop obj  and complains that you have multiple definitions

User · Answer

Using   h  method is better But if you really want to include the  cpp file then make foo int  static in foo cpp

User · Answer

There are many reasons to discourage including a  cpp file  but it isn t strictly disallowed  Your example should compile fine   The problem is probably that you re compiling both main cpp and foop cpp  which means two copies of foop cpp are being linked together  The linker is complaining about the duplication

User · Answer

What include does is copying all the contents from the file  which is the argument inside the  lt  gt  or the       so when the preproccesor finishes its work main cpp will look like       iostream stuff  int foo int a       return   a     int main int argc  char  argv         int x 42     std  cout  lt  lt  x  lt  lt std  endl     std  cout  lt  lt  foo x   lt  lt  std  endl     return 0      So foo will be defined in main cpp  but a definition also exists in foop cpp  so the compiler  gets confused  because of the function duplication

User · Answer

You should just include header file s    If you include header file  header file automatically finds  cpp file   --  This process is done by LINKER

[c++] Including .cpp files

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