<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
an	O
opaque	B-General_Concept
data	I-General_Concept
type	I-General_Concept
is	O
a	O
data	O
type	O
whose	O
concrete	O
data	B-General_Concept
structure	I-General_Concept
is	O
not	O
defined	O
in	O
an	O
interface	B-Application
.	O
</s>
<s>
This	O
enforces	O
information	B-Application
hiding	I-Application
,	O
since	O
its	O
values	O
can	O
only	O
be	O
manipulated	O
by	O
calling	O
subroutines	O
that	O
have	O
access	O
to	O
the	O
missing	O
information	O
.	O
</s>
<s>
Opaque	B-General_Concept
data	I-General_Concept
types	I-General_Concept
are	O
frequently	O
used	O
to	O
implement	O
abstract	O
data	O
types	O
.	O
</s>
<s>
Typical	O
examples	O
of	O
opaque	B-General_Concept
data	I-General_Concept
types	I-General_Concept
include	O
handles	O
for	O
resources	B-General_Concept
provided	O
by	O
an	O
operating	B-General_Concept
system	I-General_Concept
to	O
application	B-Application
software	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
the	O
POSIX	B-Operating_System
standard	I-Operating_System
for	I-Operating_System
threads	I-Operating_System
defines	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
based	O
on	O
a	O
number	O
of	O
opaque	B-General_Concept
types	I-General_Concept
that	O
represent	O
threads	B-Operating_System
or	O
synchronization	O
primitives	O
like	O
mutexes	B-Operating_System
or	O
condition	O
variables	O
.	O
</s>
<s>
An	O
opaque	O
pointer	O
is	O
a	O
special	O
case	O
of	O
an	O
opaque	B-General_Concept
data	I-General_Concept
type	I-General_Concept
,	O
a	O
datatype	O
that	O
is	O
declared	O
to	O
be	O
a	O
pointer	O
to	O
a	O
record	O
or	O
data	B-General_Concept
structure	I-General_Concept
of	O
some	O
unspecified	O
data	O
type	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
standard	B-Library
library	I-Library
that	O
forms	O
part	O
of	O
the	O
specification	O
of	O
the	O
C	B-Language
programming	I-Language
language	I-Language
provides	O
functions	O
for	O
file	B-Operating_System
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
that	O
return	O
or	O
take	O
values	O
of	O
type	O
"	O
pointer	O
to	O
FILE	B-Operating_System
"	O
that	O
represent	O
file	B-Operating_System
streams	O
(	O
see	O
C	B-Language
file	I-Language
input/output	I-Language
)	O
,	O
but	O
the	O
concrete	O
implementation	O
of	O
the	O
type	O
FILE	B-Operating_System
is	O
not	O
specified	O
.	O
</s>
<s>
Some	O
languages	O
,	O
such	O
as	O
C	B-Language
,	O
allow	O
the	O
declaration	O
of	O
opaque	O
records	O
(	O
structs	O
)	O
,	O
whose	O
size	O
and	O
fields	B-Application
are	O
hidden	O
from	O
the	O
client	O
.	O
</s>
<s>
The	O
only	O
thing	O
that	O
the	O
client	O
can	O
do	O
with	O
an	O
object	O
of	O
such	O
a	O
type	O
is	O
to	O
take	O
its	O
memory	B-General_Concept
address	I-General_Concept
,	O
to	O
produce	O
an	O
opaque	O
pointer	O
.	O
</s>
<s>
If	O
the	O
information	O
provided	O
by	O
the	O
interface	B-Application
is	O
sufficient	O
to	O
determine	O
the	O
type	O
's	O
size	O
,	O
then	O
clients	O
can	O
declare	O
variables	O
,	O
fields	B-Application
,	O
and	O
arrays	B-Data_Structure
of	O
that	O
type	O
,	O
assign	O
their	O
values	O
,	O
and	O
possibly	O
compare	O
them	O
for	O
equality	O
.	O
</s>
<s>
In	O
some	O
languages	O
,	O
such	O
as	O
Java	B-Language
,	O
the	O
only	O
kind	O
of	O
opaque	B-General_Concept
type	I-General_Concept
provided	O
is	O
the	O
opaque	O
pointer	O
.	O
</s>
<s>
Indeed	O
,	O
in	O
Java	B-Language
(	O
and	O
several	O
other	O
languages	O
)	O
records	O
are	O
always	O
handled	O
through	O
pointers	O
.	O
</s>
<s>
Some	O
languages	O
allow	O
partially	O
opaque	B-General_Concept
types	I-General_Concept
,	O
e.g.	O
</s>
<s>
a	O
record	O
which	O
has	O
some	O
public	O
fields	B-Application
,	O
known	O
and	O
accessible	O
to	O
all	O
clients	O
,	O
and	O
some	O
hidden	O
fields	B-Application
which	O
are	O
not	O
revealed	O
in	O
the	O
interface	B-Application
.	O
</s>
<s>
Such	O
types	O
play	O
a	O
fundamental	O
role	O
in	O
object-oriented	B-Language
programming	I-Language
.	O
</s>
<s>
The	O
information	O
which	O
is	O
missing	O
in	O
the	O
interface	B-Application
may	O
be	O
declared	O
in	O
its	O
implementation	O
,	O
or	O
in	O
another	O
"	O
friends-only	O
"	O
interface	B-Application
.	O
</s>
