Question

I have declared the below namespace in sample.h

// namespace with identifier
namespace N1
{
    int b = 80;
}

sample1.cpp use the above namespace declaration

#include <iostream>
#include "sample.h"

using namespace std;
using namespace N1;

int main(void)
{
    cout << "b (in main) = " << b << endl;
      foo(); //written in sample2.cpp
      return 0;
}

sample2.cpp also use the namespace declared in sample.h

#include <iostream>
#include "sample.h"

using namespace std;
using namespace N1;

void foo(void)
{
    cout << "b = " << b << endl;
}

when I compiled, I got the below errors

$> g++ sample1.cpp sample2.cpp
/tmp/ccB25lEF.o:(.data+0x0): multiple definition of `N1::b'
/tmp/cchLecEj.o:(.data+0x0): first defined here

Let me know how to resolve and how “namespace std” implemented to avoid this problem ?

Answer 1

Include guards will work only during compile time, but the error is at link time. This is because the sample.h is included in both compilation units and a variable N1::b is created in both.
If you really want a variable (not a const) you have to declare it as extern in the header, and create a memory location for it in a separate compilation unit:

// sample.h
#ifndef N1
#define N1
namespace N1 {
    extern int b;
}
#endif

// sample.cpp
#include "sample.h"
namespace N1 {
    int b = 80;
}