Question

I have 4 files:

• shared.h
• main.cpp
• something.h
• something.cpp

shared.h:

#ifndef SHARED_H
#define SHARED_H

int* sth;

#endif

---

something.h:

#ifndef SOMETHING_H
#define SOMETHING_H

class foo
{
   public:
      void printVar();
};

#endif

---

something.cpp:

#include <iostream>
#include "something.h"
#include "shared.h"

using namespace std;

void foo::printVar()
{
    cout<<"Foo: "<<*sth<<endl;
};

---

main.cpp:

#include <cstdlib>
#include <iostream>
#include "shared.h"
#include "something.h"

using namespace std;

int main(int argc, char *argv[])
{
    sth=new int(32);

    foo x;
    cout<<"Main: "<<*sth<<endl;
    x.printVar();

    system("PAUSE");
    return EXIT_SUCCESS;
}

---

Compiler returns multipe definition of *sth;

I added static modifier to the *sth and it compiles, but crashes. I changed the printings to print the adresses of pointer and I had program returned:

Main: 0x3e0f20
Foo: 0

Why is the foo’s pointer not assigned? I want to assign it only once in main and then share in other files… How can I do this? Is it something with extern modifier?

Thanx for any replies.

Answer 1

Yes, use extern. Put extern int* sth; in the header, and then in one of the source files put int* sth;.

extern tells the compiler and linker that the actual variable/function definition is in another compilation unit (i.e. another source file).