problem with linking errors in C++

[nabh4u]

i have 3 files namely acme.cpp, match.cpp and match.h
i am getting these errors :

Error1 error LNK2005: "int n" (?n@@3HA) already defined in acme.obj match.obj

Error2 error LNK2005: "int m" (?m@@3HA) already defined in acme.obj match.obj

following are my files:
/----------------------------------------------------------------------/
Header file match.h

#include <iostream>
#include<vector>

using namespace std;

/*------Declarations------*/

int n; // No. of companies.
int m; // No. of persons.
void read(); // Reads the input from user.

// class Data Structure for Company.
class company
{
public:
int openPos; // No. of open positions in a company.
int perAccept; // No. of persons acceptable by company.
vector <int> compPref; // Vector with preferences of companies.
vector <int> cId; // Vector containing Id's of companies.
};

// Class Data Structure for Person.
class person
{
public:
int comAccept; // No. of companies acceptable by person.
vector <int> pId; // Vector containing Id's of persons.
vector<int> perPref; // Vector with preferences of persons.
};
/----------------------------------------------------------------------------------------------------/
acme.cpp

# include"match.h"

using namespace std;

int main()
{
cout<<"xxxxx\n";
read(); // reads the values from the user.
return 0;
}
/----------------------------------------------------------------------------------------------------/
match.cpp

# include"match.h"

using namespace std;

void read()
{
cout<<"Enter the number of companies : ";
cin>>n;
cout<<"Enter the number of persons : ";
cin>>m;
}
/--------------------------------------------------------------------------------------------------/

[horace1]

you define n and m in match.h and include it in both your .cpp files.
Declare the variables extern in the header file and define them in one of the .cpp files, e.g.
match.h

#include <iostream>
#include<vector>
 
using namespace std;
 
/*------Declarations------*/
 
extern int n; // No. of companies.
extern int m; // No. of persons.
void read(); // Reads the input from user.
 
// class Data Structure for Company.
class company
{
public:
int openPos; // No. of open positions in a company.
int perAccept; // No. of persons acceptable by company.
vector <int> compPref; // Vector with preferences of companies.
vector <int> cId; // Vector containing Id's of companies.
};
 

and the .cpp files

# include"match.h"
 
using namespace std;
 
int main()
{
cout<<"xxxxx\n";
read(); // reads the values from the user.
return 0;
}
 

# include"match.h"
 
using namespace std;
int n; // No. of companies.
int m; // No. of persons.
 
void read()
{
cout<<"Enter the number of companies : ";
cin>>n;
cout<<"Enter the number of persons : ";
cin>>m;
}
 

then you won't get multiple definitions

[Motoma]

you define n and m in match.h and include it in both your .cpp files.
Declare the variables extern in the header file and define them in one of the .cpp files then you won't get multiple definitions

Correct me if I'm wrong, but this could also be solved by including a simple #ifndef directive:

#ifndef _MATCH_H_
#define _MATCH_H_
...
//rest of match.h
...
#endif
 

[nabh4u]

thank you...that works.

you define n and m in match.h and include it in both your .cpp files.
Declare the variables extern in the header file and define them in one of the .cpp files, e.g.
match.h

#include <iostream>
#include<vector>
 
using namespace std;
 
/*------Declarations------*/
 
extern int n; // No. of companies.
extern int m; // No. of persons.
void read(); // Reads the input from user.
 
// class Data Structure for Company.
class company
{
public:
int openPos; // No. of open positions in a company.
int perAccept; // No. of persons acceptable by company.
vector <int> compPref; // Vector with preferences of companies.
vector <int> cId; // Vector containing Id's of companies.
};
 

and the .cpp files

# include"match.h"
 
using namespace std;
 
int main()
{
cout<<"xxxxx\n";
read(); // reads the values from the user.
return 0;
}
 

# include"match.h"
 
using namespace std;
int n; // No. of companies.
int m; // No. of persons.
 
void read()
{
cout<<"Enter the number of companies : ";
cin>>n;
cout<<"Enter the number of persons : ";
cin>>m;
}
 

then you won't get multiple definitions

[horace1]

Correct me if I'm wrong, but this could also be solved by including a simple #ifndef directive:

#ifndef _MATCH_H_
#define _MATCH_H_
...
//rest of match.h
...
#endif
 

you use #ifndef when a file may include the same header file more than once, e.g. you have a header file that includes other header files etc etc.- a file may include b.h and c.h both of which include a.h and you can end up compiling the same header file multiple times (which if it only contains declarations would not cause errors but is very inefficent)

In this case two seperate C++ files included the same header file which defined n and m, hence memory was allocated for n and m in both object files leading to multiple definition of the variables at link time

have a look at
http://www.cs.odu.edu/~zeil/cs361/Le...headerdef.html