<s>
FLUID	O
(	O
Fast	O
Light	O
User	O
Interface	O
Designer	O
)	O
is	O
a	O
graphical	O
editor	O
and	O
GUI	O
builder	O
that	O
is	O
used	O
to	O
produce	O
FLTK	B-Language
source	O
code	O
.	O
</s>
<s>
After	O
designing	O
the	O
application	O
,	O
FLUID	O
compiles	O
the	O
.fl	O
file	O
into	O
a	O
.cxx	B-Language
file	O
,	O
which	O
defines	O
all	O
the	O
objects	O
from	O
the	O
.fl	O
file	O
,	O
and	O
an	O
.h	B-Language
file	I-Language
,	O
which	O
declares	O
all	O
the	O
global	O
ones	O
.	O
</s>
<s>
FLUID	O
also	O
supports	O
localization	B-General_Concept
of	O
label	O
strings	O
using	O
message	O
files	O
and	O
the	O
GNU	B-Application
gettext	I-Application
or	O
POSIX	O
catgets	O
interfaces	O
.	O
</s>
<s>
A	O
simple	O
program	O
can	O
be	O
made	O
by	O
putting	O
all	O
non-interface	O
code	O
(	O
including	O
a	O
main	O
function	O
)	O
into	O
the	O
.fl	O
file	O
,	O
thus	O
making	O
the	O
.cxx	B-Language
file	O
a	O
single	O
source	O
file	O
to	O
compile	O
.	O
</s>
<s>
Most	O
programs	O
are	O
more	O
complex	O
than	O
this	O
,	O
so	O
other	O
.cxx	B-Language
files	O
can	O
be	O
written	O
that	O
call	O
the	O
FLUID	O
functions	O
.	O
</s>
<s>
These	O
.cxx	B-Language
files	O
must	O
#include	O
the	O
.h	B-Language
file	I-Language
,	O
or	O
they	O
can	O
#include	O
the	O
.cxx	B-Language
file	O
so	O
it	O
still	O
appears	O
to	O
be	O
a	O
single	O
source	O
file	O
.	O
</s>
<s>
Normally	O
the	O
FLUID	O
file	O
defines	O
one	O
or	O
more	O
functions	O
or	O
classes	O
,	O
which	O
output	O
C++	B-Language
code	I-Language
.	O
</s>
<s>
Each	O
function	O
defines	O
one	O
or	O
more	O
FLTK	B-Language
windows	O
,	O
and	O
all	O
the	O
widgets	O
that	O
go	O
inside	O
those	O
windows	O
.	O
</s>
<s>
A	O
named	O
widget	O
has	O
a	O
legal	O
C++	B-Language
variable	O
identifier	O
as	O
its	O
name	O
(	O
i.e.	O
</s>
<s>
Widgets	O
may	O
either	O
call	O
a	O
named	O
callback	O
function	O
that	O
one	O
writes	O
in	O
another	O
source	O
file	O
,	O
or	O
one	O
can	O
supply	O
a	O
small	O
piece	O
of	O
C++	B-Language
source	O
and	O
FLUID	O
will	O
write	O
a	O
private	O
callback	O
function	O
into	O
the	O
.cxx	B-Language
file	O
.	O
</s>
<s>
While	O
editing	O
the	O
program	O
FLUID	O
the	O
user	O
can	O
look	O
at	O
the	O
Code	O
View	O
window	O
which	O
shows	O
exactly	O
how	O
the	O
C++	B-Language
output	O
of	O
the	O
program	O
would	O
look	O
.	O
</s>
