<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
a	O
prefix	B-Language
header	I-Language
is	O
a	O
feature	O
found	O
in	O
some	O
C	B-Language
or	O
C++	B-Language
compilers	B-Language
used	O
to	O
ensure	O
that	O
a	O
certain	O
snippet	O
of	O
code	O
is	O
inserted	O
at	O
the	O
beginning	O
of	O
every	O
file	B-Operating_System
.	O
</s>
<s>
In	O
the	O
C	B-Language
and	O
C++	B-Language
programming	I-Language
languages	I-Language
,	O
a	O
header	B-Language
file	I-Language
is	O
a	O
file	B-Operating_System
whose	O
text	O
is	O
included	O
in	O
another	O
source	O
file	B-Operating_System
by	O
the	O
compiler	B-Language
,	O
usually	O
by	O
the	O
use	O
of	O
compiler	B-Language
directives	O
at	O
the	O
beginning	O
of	O
the	O
source	O
file	B-Operating_System
.	O
</s>
<s>
A	O
prefix	B-Language
header	I-Language
differs	O
from	O
a	O
normal	O
header	B-Language
file	I-Language
in	O
that	O
it	O
is	O
automatically	O
included	O
at	O
the	O
beginning	O
of	O
every	O
source	O
file	B-Operating_System
by	O
the	O
compiler	B-Language
,	O
without	O
the	O
use	O
of	O
any	O
compiler	B-Language
directives	O
.	O
</s>
<s>
Prefix	B-Language
headers	I-Language
are	O
usually	O
pre-compiled	B-Language
in	O
order	O
to	O
reduce	O
compilation	B-Language
times	O
.	O
</s>
<s>
Use	O
of	O
prefix	B-Language
headers	I-Language
outside	O
of	O
this	O
purpose	O
can	O
make	O
your	O
code	O
more	O
difficult	O
to	O
maintain	O
&	O
less	O
re-usable	O
.	O
</s>
<s>
Prefix	B-Language
headers	I-Language
can	O
also	O
be	O
used	O
for	O
cross-platform	O
support	O
.	O
</s>
<s>
On	O
*	B-Operating_System
NIX	I-Operating_System
systems	O
,	O
it	O
is	O
common	O
to	O
have	O
a	O
config.h	O
header	B-Language
file	I-Language
generated	O
at	O
build	O
time	O
(	O
via	O
something	O
like	O
autoconf	B-Application
)	O
that	O
describes	O
the	O
capabilities	O
of	O
the	O
system	O
.	O
</s>
<s>
However	O
,	O
when	O
using	O
certain	O
build	O
systems	O
such	O
as	O
Visual	O
Studio	O
or	O
Xcode	B-Operating_System
,	O
this	O
config.h	O
may	O
be	O
unavailable	O
.	O
</s>
<s>
An	O
alternative	O
,	O
would	O
be	O
for	O
the	O
build	O
system	O
to	O
add	O
config.h	O
as	O
a	O
prefix	B-Language
header	I-Language
instead	O
of	O
defining	O
HAVE_CONFIG_H	O
.	O
</s>
<s>
Of	O
course	O
the	O
downside	O
is	O
that	O
this	O
header	O
will	O
be	O
added	O
to	O
every	O
compilation	B-Language
unit	O
,	O
not	O
just	O
the	O
ones	O
that	O
include	O
it	O
explicitly	O
.	O
</s>
<s>
On	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
,	O
the	O
Xcode	B-Operating_System
build	O
system	O
generates	O
prefix	B-Language
headers	I-Language
automatically	O
for	O
new	O
projects	O
.	O
</s>
<s>
A	O
new	O
Cocoa	B-Operating_System
project	O
,	O
for	O
instance	O
,	O
gets	O
a	O
prefix	B-Language
header	I-Language
that	O
looks	O
like	O
this	O
:	O
</s>
<s>
As	O
a	O
result	O
,	O
explicit	O
includes	O
of	O
the	O
above	O
header	B-Language
files	I-Language
in	O
any	O
Objective-C	B-Language
code	O
file	B-Operating_System
do	O
not	O
imply	O
a	O
second	O
inclusion	O
because	O
of	O
the	O
#import	O
directive	O
of	O
Objective-C	B-Language
,	O
or	O
more	O
generally	O
with	O
#include	O
because	O
of	O
the	O
use	O
of	O
include	B-Language
guards	I-Language
;	O
hence	O
,	O
these	O
includes	O
can	O
be	O
forgotten	O
,	O
but	O
it	O
is	O
advocated	O
to	O
have	O
them	O
explicitly	O
written	O
in	O
order	O
to	O
keep	O
the	O
source	O
code	O
autonomous	O
and	O
reusable	O
,	O
and	O
make	O
the	O
library	O
dependencies	O
clear	O
.	O
</s>
<s>
Similar	O
prefix	B-Language
headers	I-Language
are	O
generated	O
for	O
other	O
types	O
of	O
project	O
.	O
</s>
