Re: what will be the value of #define

On 2 Mar, 13:22, Eric Sosman <esos...@xxxxxxxxxxxxxxxxxxx> wrote:
raghu wrote:
Hello
I have a doubt plz clarify that
#ifndef SYSTEM_H
#define SYSTEM_H
what will be the value of SYATEM_H after this #define statement and
before that statement.

(Just in case it's a trick question): Impossible to tell,
based on the information provided.

(Assuming it's just a typo): Before the #define, SYSTEM_H
is not defined and has no value at all. After it, SYSTEM_H is
defined as an object-like macro whose replacement list is empty.

To explain a little more for "raghu" - the approach is a standard
convention for ensuring that multiple inclusions of header files
(usually due to being nested in other header files) are "harmless".

If my header called "fred.h" has the form:-

#ifndef FRED_H
#define FRED_H
....
#endif /*FRED_H*/

then the first inclusion will have the macro "FRED_H" undefined, will
define the macro (as an empty macro) and do the other declarations,
definitions etc which the header defines.

Any subsequent inclusions will have "FRED_H" defined, so will do
nothing.

.

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