c++ - Why does Declaring in header file and defining in file gives multiple definition error? -

i new programming in c++. had better knowledge on java. using hackerrank trying learn c++. keep track of each program sepearate entity started using header file , program file each program or challenge. trying hackerrank exercise input , output (https://www.hackerrank.com/challenges/cpp-input-and-output). tried implement program in manner;

inputandouput.h

 #ifndef inputandoutput_h_  #define inputandoutput_h_  int arr[3]; int m; int inputandoutput();  #endif 

inputandoutput.cpp

#include "inputandoutput.h" #include<iostream> #include<cmath> #include<cstdio>  int inputandoutput(){      int arr[3];      for(int i1 = 0; i1 < 3 ; i1++)         std::cin >> arr[i1];      for(int = 0; < 3 ; i++)        m = m + arr[i];      return m; } 

main.cpp

#include<iostream> //#include<day1datatypes> #include<cmath> #include<cstdio> //#include "day1datatypes.h" #include "inputandoutput.h"  int main() {     int k = inputandoutput();   \\the error persists whole block commented     std::cout << k << std::endl ; } 

this 1 giving following errors;

description resource    path    location    type first defined here  hackerrank      line 6  c/c++ problem first defined here  hackerrank      line 8  c/c++ problem make: *** [hackerrank] error 1  hackerrank          c/c++ problem multiple definition of `arr'    main.cpp    /hackerrank line 9  c/c++ problem multiple definition of `m'  main.cpp    /hackerrank line 12 c/c++ problem 

please explain me what's wrong notation.btw using eclipse , it's throwing error @ compile time.

to explain simplest problem first, let's take @ "int arr[3];"

for variable declaration, declared , implemented in inputandoutput.h header.

both main.cpp , inputandoutput.cpp include header file, implementing variable twice.

to declare variable, can used in other files, use:

inputandoutput.h

extern int arr[3]; extern int m; 

inputandoutput.cpp

int arr[3]; int m; 

this tells compiler there 2 variables, arr , m, being declared in .h file, implemented in external file, using extern keyword.

please note, code posted in question merely c within c++ files.

in c++, discouraged use global variables storing data.

so, if remove global variables , use c++ stl containers, have following:

inputandoutput.h

#include <array> int32_t inputandoutput(std::array<int32_t, 3>& arr, int32_t& m); 

inputandoutput.cpp

int32_t inputandoutput(std::array<int32_t, 3>& arr, int32_t& m) {          for(auto i1 = 0; i1 < 3 ; i1++)          std::cin >> arr[i1];      for(auto = 0; < 3 ; i++)          m = m + arr[i];      return m;  } 

main.cpp

int main() {     auto arr = std::array<int32_t, 3>{0,0,0};     auto m = 0;     const auto k = inputandoutput(arr, m);     std::cout << k << std::endl ; } 

now, should take care of of question, however, don't see in original code how getting input std::cin since don't prompt user input... , leads bug.

since learning c++, should learning modern c++ , not c++98.

i recommend read on https://github.com/isocpp/cppcoreguidelines

and, check out herb sutter's website, regard almost-always-auto @ https://herbsutter.com/2013/08/12/gotw-94-solution-aaa-style-almost-always-auto/