Re: #include within a namespace

From: (Carl Barron)
Sat, 2 Jun 2007 04:25:43 CST
<> wrote:


I have a couple questions regarding #including header files *within* a
namespace such as:

namespace ABC {
#include <iostream>

//code using iostream


This causes numerous compile errors with gcc, and from what I
understand goes explicitly against the c++ standard. My questions are:

1. Why is this prohibited by the standard?

   Your code puts the contents of <iostream> in namespace ABC
specifically all of iostream is in namespace std, which is now
nested insside namespace ABC. and not the unnested namespace std. That
is why it does not compile. The preprocessor which handles includes, is
not smart it does text substition, and file inclusion, but it does not
know much about C let alone C++.

2. Putting aside the fact that this goes against the standard, is
there a way to trick gcc into making this work?

  What is it 'supposed to do'?

3. (Related to question 1) I've been able to make this work when
including some of my own headers this way but not with certain headers
such as iostream, which makes me wonder: what are the attributes/
aspects of a header like iostream that prevent this from working?

   the standard io library is not a header only library ussually so even
if your code 'compiles' it probably would not like since the library is
expecting to find the functions in the namespace std, which is not
nested in another namespace.

Your code is eqiivalent to something like

// file_1.h

namespace foo
     struct A
         int operator () () const;

// file_1.cp
namespace foo
    int A::operator () () const {return 10;}

// your inclusion code
// file_2.cp
namespace bar
#include "file_1.h"
// rest of the file.

the prepocessing file2.cp produce the equivalent of this C++ code

namespace bar
   namespace foo
       struct A
         int operator ()()const;

and the linker will not find bar::foo::A's member functions.
This is one reason that including system /standara library headers
inside of a namespace nested in the global namespace is an error.

If your headers are complete, and nothing needs to be compiled with
them, such as nothing but templates then this places the templates
in another level of testing and it might work.

in general include all system and library headers in the global
namespace unless required otherwise by the library used.

if you really want to include namespace std in ABC as if it were in
the global namespace , cant think of a reason why, but something like

namespace ABC
    namespace std
        using namespace ::std;
         // ...

might work, not tested.

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