c++ compiler error multiple definition of a variable

Question

I am trying to learn the C++ language (so kindly bear with my noobness). On following the tutorial in several books I decided to experiment with headers

I have a file named Untitled2.cpp which contains

#include <iostream>
#include "findaverage.h"
#include <string>
using namespace std;

int main(){
cout<<find_average();
}

A header file named findaverage.h which contain

#ifndef FINDAVERAGE_H
#define FINDAVERAGE_H
int find_average();
double first_no;
double second_no;
#endif

and a findaverage.cpp file which contain

#include "findaverage.h"
#include <iostream>
using namespace std;
int find_average(){
std::cout << "Enter a number"<<endl;
std::cin>>first_no;
std::cout<<"Enter another number"<<endl;
std::cin>>second_no;
return (first_no+second_no)/2;
};

when i compile the program in Code::Blocks, I get the following error

||=== Build: Debug in test (compiler: GNU GCC Compiler) ===|
obj/Debug/Untitled2.o||In function `main':|
/root/Untitled2.cpp|7|multiple definition of `second_no'|
obj/Debug/findaverage.o:/root/findaverage.cpp|5|first defined here|
||error: ld returned 1 exit status|
||=== Build failed: 3 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

I am trying to follow the an approach similar to this picture

Question: how can I successfully resolve the multiple definition issue in my experimental code?

1. Not a good idea to use [using namespace std;](http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-in-c-considered-bad-practice) 2. Please do not post images unless yo have to — Ed Heal, Jun 06 '16 at 00:01
Why is `double first_no; double second_no;` in a header file — Ed Heal, Jun 06 '16 at 00:02
I guess you are trying to use global variables - this is also a bad idea — Ed Heal, Jun 06 '16 at 00:03
@Ed Heal..I am of the opinion that we can put it in a header? I am looking at the snap from the picture and experiment...I can't say where I am going wrong though. — repzero, Jun 06 '16 at 00:04
Do not use global variables. Put them into the .cpp file and inside the function — Ed Heal, Jun 06 '16 at 00:05

score 4 · Answer 1 · answered Jun 06 '16 at 00:04

4

The problem is that you include a header with definition of your global variables. Headers should include only declarations, not definitions:

#ifndef FINDAVERAGE_H
#define FINDAVERAGE_H
int find_average();
extern double first_no;
extern double second_no;
#endif

This would fail to build with a different error, however, because you need to provide definitions for your variables in one of your cpp files - for example, in Untitled2.cpp:

#include <iostream>
#include "findaverage.h"
#include <string>
using namespace std;

int main(){
    cout<<find_average();
}

double first_no;
double second_no;

Note that if you do not need these variables to be global, you could make them local to your function, or local to your cpp file.

answered Jun 06 '16 at 00:04

Sergey Kalinichenko

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This appears to solve the issue but I am getting another error "undefined reference to first_no" in findaverage.cpp – repzero Jun 06 '16 at 00:07
@repzero Did you add definitions to your `Untitled2.cpp` as shown above? – Sergey Kalinichenko Jun 06 '16 at 00:09
Excellent!!!!.....I think I get the picture now...I'll need to read up some more on extern...I noticed it but never grasp it mentally... – repzero Jun 06 '16 at 00:19
1

@repzero The deal with extern is pretty simple. I think you already see what's going on with functions vs prototypes - i.e. the header has the declaration without a body, which makes it a declaration, while the cpp has the header again plus the body, which makes it a definition. Variables don't have "body" either way, so `extern` says "this is a variable with no body", i.e. a promise to have a variable by the same name elsewhere. The same variable declared in cpp file provides the "body", so that he linker can find it. – Sergey Kalinichenko Jun 06 '16 at 00:23
..thanks for breaking this down to the ground for me..:D – repzero Jun 06 '16 at 00:26

score 3 · Accepted Answer · answered Jun 06 '16 at 00:09

Change the header file to

#ifndef FINDAVERAGE_H
#define FINDAVERAGE_H
int find_average();
// No - wrong place double first_no;
// No - wrong place double second_no;
#endif

as the variables will be included where ever the header file is in a .cpp file

and change the function to this

int find_average(){
  double first_no;
  double second_no;

   std::cout << "Enter a number"<<endl;
   std::cin>>first_no;
   std::cout<<"Enter another number"<<endl;
   std::cin>>second_no;
   return (first_no+second_no)/2;
};

Should add error checking in case the user does not enter a number - but I leave that to the reader

Gr8...Seems simplified and on the spot – repzero Jun 06 '16 at 00:19 — repzero, Jun 06 '16 at 00:19

c++ compiler error multiple definition of a variable

2 Answers2