Sharing an object between separate modules

I have compiled (with g++) and run the code "combined.cpp". It consists of the class LIBS, main and funct_correlation. A LIBS object is created as "L". Then main invokes L.getHABlib. funct_correlation then does the same.

Now I want to separate the code into the 2 pieces shown.(separate1.cpp,separate2.cpp) This does not compile-the function "correlation" cannot "see" the "L" object. Functionally I would think this code is the same as the above but I clearly am missing something to tie them together (the gnu linker does not).

Any help appreciated
Fritz

You need a .h file that you include in both files, containing the following.

Call it separate.h

Code:

---

#ifdef SEPARATE_H_INCLUDED
#define SEPARATE_H_INCLUDED

//////////////////////////////////////////////////////////////////////////
class LIBS
{
private:
  char  HABlib[100];
public:
    void setLIBS(int argc, char  *argv[])
    {
        strcpy(HABlib,"test");
    }
    char* getHABlib(void)  {return HABlib;} //return the ADDRESS of HABlib
};

//////////////////////////////////////////////////////////////////////////
extern LIBS L;
//////////////////////////////////////////////////////////////////////////
#endif

---

Then the two sources are

Code:

---

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <time.h>
#include <errno.h>
#include <iostream>

#include "separate.h"  // the file created above

using namespace std;
void funct_correlation(void);

//////////////////////////////////////////////////////////////////////////

// Create an instance of your object
LIBS L;

//////////////////////////////////////////////////////////////////////////
int main( int argc, char *argv[] )
{
  L.setLIBS(argc,argv);
  printf("HAB=%s\n", L.getHABlib()  ); //print string at location of address
  funct_correlation();

  return 0;
} //end main

---

Code:

---

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <time.h>
#include <errno.h>
#include <iostream>

#include "separate.h"  // the file created above

using namespace std;

//////////////////////////////////////////////////////////////////////////
void funct_correlation(void)
{
    char HL[100];
    strcpy( HL, L.getHABlib() );
    printf("HAB from funct=%s\n", HL  ); //print string at location of address
   
    return;
}

---

Each .cpp file (along with it's included text) forms a "compilation unit" that must be able to be compiled into an object file separately from the other .cpp files. As it stands, separate2.cpp has no knowledge of LIBS or L. Usually you would put class LIBS in a .h file and include it in separate1.cpp and separate2.cpp so they each know about that class.

As for L, if you really want it global like you have it, then you need to let separate2.cpp know about it by adding the following at global scope in separate2.cpp:

Code:

---

extern LIBS L;  // indicate the existence of L without defining it here

---

Alternatively you could define the object locally in main and pass it to the other function:

Code:

---

// libs.h ////////////////////////////////////////////////////////
 
#ifndef LIBS_H
#define LIBS_H
 
#include <cstring>
 
class LIBS
{
private:
    char  HABlib[100];
public:
    void setLIBS( int argc, char  *argv[] )
    {
        strcpy(HABlib,"test");
    }
    char* getHABlib(void) { return HABlib; }
};
 
#endif
 
// separate1.cpp /////////////////////////////////////////////////
 
#include <iostream>
#include "libs.h"
void funct_correlation( LIBS &libs );
using namespace std;
 
int main( int argc, char *argv[] )
{
  LIBS L;
  L.setLIBS(argc, argv);
  cout << "HAB=" << L.getHABlib() << "\n";
  funct_correlation(L);
 
  return 0;
}
 
// separate2.cpp /////////////////////////////////////////////////
 
#include <iostream>
#include <cstring>
#include "libs.h"
using namespace std;
 
void funct_correlation( LIBS &L )
{
    char HL[100];
    strcpy( HL, L.getHABlib() );
    cout << "HAB from funct=" << HL << "\n";
}

---

You can compile each unit separately (the -c file says to compile only, do not link yet) :

Code:

---

g++ -Wall -W -c separate1.cpp      # this will create separate1.o
g++ -Wall -W -c separate2.cpp      # this will create separate2.o
g++ separate1.o sepatate2.o -o separate  # this links the .o files
 
# Or you could compile and link everything at once:
g++ -Wall -W separate1.cpp separate2.cpp -o separate

---

BTW, we don't generally mix C-style I/O and C++-style I/O, so if you're including iostream and using cout, etc., then there's no reason to include stdio.h.
And <stdio.h> is usually spelled <cstdio> in C++; and <string.h> is <cstring>).
We also don't put (void) for an empty parameter list, but instead just write ().

Ok and thanks for this useful input. I guess I was expecting too much of the linker.
I added the modifications using "extern" however I preferred to move this extern to the called module. This way my class in the .h file remains untouched and I can put the external logic for each called subroutine as I use it. Works fine.
The added comment about passing the &L as a variable makes sense. However I am never sure exactly which procedure is preferred by C++.
Thanks for the comments on the use of void and cout. I don't use C++ enough and can't always keep track of all these nuances along with the other plethora of languages these days!

cheers
Fritz