compilation - Multiple definitions error in C++ -

i'm writing c++ program each file has it's own set of global variable declarations. of these files make use of global variables defined in other files using extern.

here's example similar program:

main.cpp

#include "stdafx.h" #include <iostream> #include "other_file.cpp"  int var1; int var2;  int main() {     var1 = 1;     var2 = 2;     otherfunction();     var4 = 4; // other_file.cpp      std::cout << var1 << " " << var2 << " " << var3 << " " << var4 << std::endl;      return(0); } 

other_file.cpp

extern int var1; extern int var2;  int var3; int var4;  void otherfunction() {      var3 = var1 + var2;      var4 = 0; } 

when build code in visual studio (windows), runs fine , output correct. when attempt build using g++ on linux receive following error:

g++ -o testing testing.o other_file.o other_file.o:(.bss+0x0): multiple definition of var3' testing.o:(.bss+0x0): first defined here other_file.o:(.bss+0x4): multiple definition ofvar4' testing.o:(.bss+0x4): first defined here other_file.o: in function otherfunction()': other_file.cpp:(.text+0x0): multiple definition of otherfunction()' testing.o:testing.cpp:(.text+0x0): first defined here collect2: ld returned 1 exit status make: *** [testing] error 1

is because i'm "including" other file in main file?

if not what's issue code?

edit: content of makefile g++:

testing: testing.o other_file.o     g++ -o testing testing.o other_file.o  testing.o: testing.cpp     g++ -c -std=c++0x testing.cpp  other_file.o: other_file.cpp     g++ -c -std=c++0x other_file.cpp  clean:     rm *.o calculator 

don't #include source file source file. there times , places when okay, in less 0.001% of programs needed.

what should create header file contains declarations of things needed in both source files.

then code this:

1. main.cpp source file

   #include "stdafx.h" #include <iostream> #include "other_file.h"  // note inclusion of header file here  int var1; int var2;  int main() {     var1 = 1;     var2 = 2;     otherfunction();     var4 = 4; // other_file.cpp      std::cout << var1 << " " << var2 << " " << var3 << " " << var4 << std::endl; } 

2. other_file.cpp source file, have now

3. other_file.h header file, new file

   #pragma once  // declare variables, compiler knows exist somewhere extern int var3; extern int var4;  // forward declaration of function prototype void otherfunction(); 

both source files compiled separately , linked form final executable. linking step build fails. notice variables defined in other_source.cpp defined in object file created source file, since include main.cpp source file object file created source file well.

this why need learn translation units, compiler see. c++ source file goes through many phases of translation, each 1 doing own special part. translation unit single source file headers included.

this reason learn preprocessor #include directive does. inserts included file, is, source file being preprocessed. #include directive was, after preprocessing contents of included file, , compiler see.