<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
a	O
default	B-Language
argument	I-Language
is	O
an	O
argument	O
to	O
a	O
function	O
that	O
a	O
programmer	O
is	O
not	O
required	O
to	O
specify	O
.	O
</s>
<s>
Usually	O
,	O
each	O
argument	O
must	O
be	O
specified	O
in	O
full	O
(	O
this	O
is	O
the	O
case	O
in	O
the	O
C	B-Language
programming	I-Language
language	I-Language
)	O
.	O
</s>
<s>
Later	O
languages	O
(	O
for	O
example	O
,	O
in	O
C++	B-Language
)	O
allow	O
the	O
programmer	O
to	O
specify	O
default	B-Language
arguments	I-Language
that	O
always	O
have	O
a	O
value	O
,	O
even	O
if	O
one	O
is	O
not	O
specified	O
when	O
calling	O
the	O
function	O
.	O
</s>
<s>
In	O
the	O
first	O
case	O
the	O
value	O
for	O
the	O
argument	O
called	O
c	B-Language
is	O
specified	O
as	O
normal	O
.	O
</s>
<s>
Because	O
default	B-Language
arguments	I-Language
 '	O
values	O
are	O
"	O
filled	O
in	O
"	O
at	O
the	O
call	O
site	O
rather	O
than	O
in	O
the	O
body	O
of	O
the	O
function	O
being	O
called	O
,	O
virtual	B-Application
functions	I-Application
take	O
their	O
default	B-Language
argument	I-Language
values	O
from	O
the	O
static	O
type	O
of	O
the	O
pointer	O
or	O
reference	O
through	O
which	O
the	O
call	O
is	O
made	O
,	O
rather	O
than	O
from	O
the	O
dynamic	O
type	O
of	O
the	O
object	O
supplying	O
the	O
virtual	B-Application
function	I-Application
's	O
body	O
.	O
</s>
<s>
Some	O
languages	O
,	O
such	O
as	O
Java	B-Language
,	O
do	O
not	O
have	O
default	B-Language
arguments	I-Language
.	O
</s>
<s>
However	O
,	O
the	O
same	O
behaviour	O
can	O
be	O
simulated	O
by	O
using	O
method	B-Language
overloading	I-Language
to	O
create	O
overloaded	O
methods	O
of	O
the	O
same	O
name	O
,	O
which	O
take	O
different	O
numbers	O
of	O
arguments	O
;	O
and	O
the	O
versions	O
with	O
fewer	O
arguments	O
simply	O
call	O
the	O
versions	O
with	O
more	O
arguments	O
,	O
with	O
the	O
default	B-Language
arguments	I-Language
as	O
the	O
missing	O
arguments	O
:	O
</s>
<s>
However	O
,	O
in	O
addition	O
to	O
,	O
since	O
the	O
default	B-Language
arguments	I-Language
are	O
not	O
modeled	O
in	O
the	O
type	O
system	O
,	O
the	O
type	O
of	O
a	O
callback	O
(	O
aka	O
higher-order	O
function	O
)	O
ca	O
n't	O
express	O
that	O
it	O
accepts	O
either	O
of	O
the	O
overloads	B-Language
nor	O
simulate	O
the	O
default	B-Language
arguments	I-Language
with	O
overloaded	B-Language
functions	I-Language
.	O
</s>
<s>
Whereas	O
,	O
the	O
non-overloaded	O
function	O
definition	O
can	O
substitute	O
the	O
default	O
when	O
the	O
input	O
value	O
is	O
undefined	O
(	O
regardless	O
if	O
it	O
was	O
implicitly	O
undefined	O
via	O
the	O
argument	O
's	O
absence	O
at	O
the	O
call	O
site	O
or	O
an	O
explicitly	O
passed	O
undefined	O
value	O
)	O
;	O
which	O
is	O
modeled	O
as	O
an	O
optional	B-Language
argument	I-Language
parameter	O
type	O
?	O
</s>
<s>
For	O
every	O
function	O
call	O
default	B-Language
argument	I-Language
values	O
must	O
be	O
passed	O
to	O
the	O
called	O
function	O
.	O
</s>
<s>
If	O
a	O
default	B-Language
argument	I-Language
value	O
contains	O
side-effects	O
,	O
it	O
is	O
significant	O
when	O
those	O
side	O
effects	O
are	O
evaluated	O
–	O
once	O
for	O
the	O
entire	O
program	O
(	O
at	O
parse	O
time	O
,	O
compile	O
time	O
,	O
or	O
load	O
time	O
)	O
,	O
or	O
once	O
per	O
function	O
call	O
,	O
at	O
call	O
time	O
.	O
</s>
<s>
Python	O
is	O
a	O
notable	O
language	O
that	O
evaluates	O
expressions	O
in	O
default	B-Language
arguments	I-Language
once	O
,	O
at	O
the	O
time	O
the	O
function	O
declaration	O
is	O
evaluated	O
.	O
</s>
<s>
If	O
evaluation	O
per	O
function	O
call	O
is	O
desired	O
,	O
it	O
can	O
be	O
replicated	O
by	O
having	O
the	O
default	B-Language
argument	I-Language
be	O
a	O
sentinel	B-Data_Structure
value	I-Data_Structure
,	O
such	O
as	O
None	O
,	O
and	O
then	O
having	O
the	O
body	O
of	O
the	O
function	O
evaluate	O
the	O
default	O
value	O
's	O
side	O
effects	O
only	O
if	O
the	O
sentinel	B-Data_Structure
value	I-Data_Structure
was	O
passed	O
in	O
.	O
</s>
<s>
Generally	O
a	O
default	B-Language
argument	I-Language
will	O
behave	O
identically	O
to	O
an	O
argument	O
passed	O
by	O
parameter	O
or	O
a	O
local	O
variable	O
declared	O
at	O
the	O
start	O
of	O
the	O
function	O
,	O
and	O
have	O
the	O
same	O
scope	B-Language
and	O
extent	O
(	O
lifetime	O
)	O
as	O
a	O
parameter	O
or	O
other	O
local	O
variable	O
,	O
namely	O
an	O
automatic	B-General_Concept
variable	I-General_Concept
which	O
is	O
deallocated	O
on	O
function	O
termination	O
.	O
</s>
<s>
In	O
other	O
cases	O
a	O
default	B-Language
argument	I-Language
may	O
instead	O
be	O
statically	O
allocated	O
.	O
</s>
<s>
If	O
the	O
variable	O
is	O
mutable	O
,	O
it	O
will	O
then	O
retain	O
its	O
value	O
across	O
function	O
calls	O
,	O
as	O
with	O
a	O
static	B-General_Concept
variable	I-General_Concept
.	O
</s>
<s>
As	O
with	O
evaluation	O
,	O
in	O
order	O
to	O
ensure	O
the	O
same	O
extent	O
as	O
a	O
local	O
variable	O
,	O
one	O
can	O
use	O
a	O
sentinel	B-Data_Structure
value	I-Data_Structure
:	O
</s>
