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What are the header defines for?

I am re-learning c++. And I see lots of code. All the time I stumble
over the following pattern (in header files):

#ifndef some_filename_h
#define some_filename_h
..
..
..
#endif

What is this "filename" define good for? I found this pattern in many
examples and in many books, but it is nowhere explained what it is
good for? Can anyone explain this to me?

Thanks a lot,
Marco
Jul 22 '05 #1
3 1044
On 29 Dec 2003 02:53:19 -0800,
Marco Aschwanden <PP**********@spammotel.com> wrote:
I am re-learning c++. And I see lots of code. All the time I stumble
over the following pattern (in header files):

#ifndef some_filename_h
#define some_filename_h
.
.
.
#endif

What is this "filename" define good for? I found this pattern in many
examples and in many books, but it is nowhere explained what it is
good for? Can anyone explain this to me?


They are include guards and are used so that a header being included
multiple times doesn't cause compilation errors (by defining something
twice, for example).

--
Sam Holden
Jul 22 '05 #2
"Marco Aschwanden" <PP**********@spammotel.com> wrote in message
news:15*************************@posting.google.co m
I am re-learning c++. And I see lots of code. All the time I stumble
over the following pattern (in header files):

#ifndef some_filename_h
#define some_filename_h
.
.
.
#endif

What is this "filename" define good for? I found this pattern in many
examples and in many books, but it is nowhere explained what it is
good for? Can anyone explain this to me?

Thanks a lot,
Marco


Suppose that you #include two files, "a.h" and "b.h" into the file
"filename.cpp". Suppose that "b.h" itself #includes "a.h". That means that
"a.h" is being #included into "filename.cpp" twice: once directly and once
indirectly via "b.h". The scheme you have described stops this from
happening.

The first time "a.h" is #included, some_filename_h is not yet defined.
Accordingly

#ifndef some_filename_h

returns true, some_filename_h is then defined, and the code that follows is
read.

The second time "a.h" is #included, some_filename_h has already been
defined,

#ifndef some_filename_h

returns false and execution skips to the #endif, so none of the header code
is read.
--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

Jul 22 '05 #3
The following article explains duplicate inclusion and other
rules related to header file inclusion in C++.

http://www.eventhelix.com/RealtimeMa...dePatterns.htm

Sandeep
--
http://www.EventHelix.com/EventStudio
EventStudio 2.0 - Go Beyond UML Use Case and Sequence Diagrams
Jul 22 '05 #4

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