Help with header file

[yepMe]

Hi All,

I am never comfortable writing a header file, but I am trying to write from the net and try to understand

 #ifndef Sample_h
#define Sample_h
namespace
{
         int x;
         int y;
         void add(int , int);
}
#endif 

#include "Sample_h.h"
#include<iostream>
using namespace std;

void add(int a, int b)
{
     x = a+b;
     y = a-b;
     cout << "x" << x << endl ;
     cout << "y" << y << endl;
}

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

using namespace std;

int main()
{
    add(4,5);
    system("pause");
    return 0;
}

But it's giving me error
[Linking error] undefined reference to '(anonymous namespace) :: add(int,int)'
And my another doubt is can we write the header file without the #macros i.r #ifndef or #define ?

[Peter87]

You probably don't want to have the add function inside an anonymous namespace.

If you want the function add to be inside a namespace give the namespace a name by writing the name at the end of line 3 in Sample_h.h.

namespace foo

Also add the namespace name before the function name, separated with :: when you define the function on line 5 in Sample_h.cpp.

void foo::add(int a, int b)

And do the same when you call the function in main.

foo::add(4,5);

#ifndef/#define/#endif prevents the content of the header from being included more than once. They are called include guards. http://en.wikipedia.org/wiki/Include_guard

[Stewbond]

You can do one of two things:

First, name and use the namespace:

#ifndef Sample_h
#define Sample_h
namespace ns
{
         int x;
         int y;
         void add(int , int);
}
#endif 

then

int main()
{
    ns::add(4,5);
    ...
}

The other suggestion is to just remove the namespace:

#ifndef Sample_h
#define Sample_h

void add(int , int);

#endif  

By the way, avoid defining globals, especially in headers. The reason is that you'll come into issues with "duplicate names" which is a whole other barrel of worms. Instead use locals, especially in this situation. Drop the int x, and int y definitions in the headers and define it like so:

void add(int a, int b)
{
     int x = a+b;
     int y = a-b;
     cout << "x" << x << endl ;
     cout << "y" << y << endl;
}

To answer your last question: You don't need the #ifdef around a header, but the advantage is that it avoids errors when you include the headerfile recursively by accident, or include the same header twice.

[yepMe]

Thanks for the reply but the problem that I was trying to solve is "Create a header file with an unnamed namespace . Include the header file in two separate cpp files and show that an unnamed space is unique for each translation unit.

How do I solve this then if I do not use the unnamed namespace?

[Peter87]

Well, in that case I think you should just move the declaration of add outside the namespace and leave x and y where they are.