Should I include header file within a namespace?

Question

Currently I am working on a c framework and I want to embed a c++ package into it. However, there are lots of naming conflicts occurs. So I decided to to add a namespace to the C++ source. Now the question is should I move the #include "header.h" within namespace { } block? I just spent some time to figure out a bug which was resulted from the following codes.

Original C++ source

In a.h

#include <unistd.h>
struct File 
{
     void func(int fd);
};

In a.cpp

#include "a.h"
void File::func(int fd)
{
    ::close( fd );
}

And I added the namespace like this

New a.h

namespace MyAddedNameSpace 
{ 
    #include <unistd.h>
    struct File 
    {
        void func(int fd);
    }; 
}

New a.cpp

#include "a.h"
namespace MyAddedNameSpace 
{
    void File::func(int fd)
    {
        ::close( fd );
    }
}

And compiler complains that ::close() has not been declared.

The reason Why I put the #include directive inside namespace block is because the c++ package I imported also use #ifndef flag to include header files as follows. So I think the easy way is to put all codes within namespace block {}

#ifndef 
    #include <header1.h> 
    #include <header2.h>
    ...
#else 
    #include <header3.h>
    #include <header4.h>
    ...
#endif

Now I solved this problem by adding the extra line in cpp file

#include <unistd.h>  //new added line 
#include "a.h"
namespace MyNameSpace 
{
    void File::func(int fd)
    {
        ::close( fd );
    }
}

But I am not satisfied with this solution since unistd.h header has already been included in a.h, but inside the namespace MyAddedNameSpace, or should I added the prefix MyNameSpace to all the function calls where the compiler complain no such function declared ?

Thanks for the reply.

There is no harm in #include <unistd.h> for your case. It's clear that, <unistd.h> is not visible to a.cpp. — iammilind
– iammilind, Commented Mar 19, 2012 at 3:27

aldo · Accepted Answer · 2012-03-19 06:02:25Z

You SHOULD NOT place #include directives inside your own namespace. What you have done is to place all of the contents of unistd.h inside your namespace as well; thus what used to be (and should remain!) ::close() is now declared as MyAddedNameSpace::close(). This is NOT what you want.

Your "solution" of adding the line #include <unistd.h> at the top of your .cpp file fixes the problem, but only for this one .cpp file. What you've done is included the library header the correct way (without your namespace), and then when your header (a.h) is included, it does #include <unistd.h> again (this time within your namespace), but this time the include guards in that file prevent it from being processed again. So for this .cpp file you're OK, but any other file that does #include <a.h> will have the same problem that you originally had.

There may be some rare occasion where you have a good reason for using #include within your own namespace, but you will most likely be including one of your own headers (or some other file) - NOT a library header! - and even then, it's probably not the ideal solution.

In a.h

#include <unistd.h>

namespace MyAddedNameSpace {

struct File
{
  void func(int fd);
};

}

In a.cpp

#include "a.h"

namespace MyAddedNameSpace {

void File::func(int fd)
{
  ::close( fd );
}

}

Dmitriy Kachko · Accepted Answer · 2012-03-19 03:13:04Z

-1

Usually it is enough to place the directive using namespace in the .cpp file. Like that:

using namespace MyAddedNameSpace;

    void File::func(int fd)
    {
        close( fd );
    }

Hope it helps..

edited Mar 19, 2012 at 3:13

answered Mar 19, 2012 at 3:06

Dmitriy Kachko

2,9241 gold badge21 silver badges23 bronze badges

1 Comment

Spacen Jasset Over a year ago

aldo privides a good reason why you should not do this. Because header guards mean that unistd.h can only be included once per translation unit it means that all users of your header file must then place unistd.h before your include for compilation to work. It is not a good idea to have a header ordering dependancy, and there is no need to do this either.

Collectives™ on Stack Overflow

Should I include header file within a namespace?

2 Answers 2

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related