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Repeated Multiple Definition Errors from including same header in multiple cpps

Asked 2008-10-21T22:27:45.933
15

So, no matter what I seem to do, I cannot seem to avoid having Dev C++ spew out numerous Multiple Definition errors as a result of me including the same header file in multiple source code files in the same project. I'd strongly prefer to avoid having to dump all my source code into one file and only include the header once, as that's going to make my file very long and difficult to manage.

Essentially, this is what's going on:

#ifndef _myheader_h
#define _myheader_h

typedef struct MYSTRUCT{
int blah;
int blah2; } MYSTRUCT;

MYSTRUCT Job_Grunt;
MYSTRUCT *Grunt = &Job_Grunt;
MYSTRUCT Job_Uruk;
MYSTRUCT *Uruk = &Job_Grunt;

int Other_data[100];

void load_jobs();

#endif

Example Cpp File (They pretty much all look something like this):

#include "myheader.h"

void load_jobs(){

Grunt->blah = 1;
Grunt->blah2 = 14;

Uruk->blah = 2;
Uruk->blah2 = 15;

return; }

Bear in mind that I have about 5 cpp files that include this one header, each one dealing with a different type of struct found in the header file. In this example there was only the one struct containing a couple of members, when there are about 4-6 different structs with many more members in the actual header file. All the files I've included it in follow the same formula as you see in this example here.

Now I understand that the header guard only stops each individual cpp file from including the header file more than once. What would seem to be happening is that when the compiler reads the include at the start of each cpp, it defines the header file all over again, which is causing it to spit out lines and lines of:

Multiple Definition of Uruk, first defined here  
Multiple Definition of Job_Uruk, first defined here  
Multiple Definition of Grunt, first defined here  
Multiple Definition of Job_Grunt, first defined here  
Multiple Definition of Other_data, first defined here

                    
                    

                        
                        c++
                        
                        dev-c++
                        
                        multiple-definition-error
                        
                        multiple-inclusions
                        
                    

                    

                        
                        
                        
                        
                        
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2 Answers

5

You need to define your variables as extern in the header file, and then define them in a cpp file as well. i.e.:

extern MYSTRUCT Job_Grunt;

in your header file, and then in a cpp file in your project declare them normally.

The header file is only for definitions, when you instantiate a variable in the header file it will try to instantiate it every time the header is included in your project. Using the extern directive tells the compiler that it's just a definition and that the instantiation is done somewhere else.

answered 2008-10-21T22:32:03.350
2

I also received this error for a function defined in a .h file. The header file was not intended to make declarations of a class but definitions of some functions which are needed in various places in a project. (I may confuse the "definition" and "declaration" usages, but i hope i could give the main idea.) When I put an "inline" keyword just before the definition of the function which give the "multiple definitions" error, the error is avoided.

answered 2011-06-07T09:09:21.483

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