<s>
The	O
C	B-Language
and	O
C++	B-Language
programming	I-Language
languages	I-Language
are	O
closely	O
related	O
but	O
have	O
many	O
significant	O
differences	O
.	O
</s>
<s>
C++	B-Language
began	O
as	O
a	O
fork	O
of	O
an	O
early	O
,	O
pre-standardized	O
C	B-Language
,	O
and	O
was	O
designed	O
to	O
be	O
mostly	O
source-and-link	O
compatible	O
with	O
C	B-Language
compilers	B-Language
of	O
the	O
time	O
.	O
</s>
<s>
Due	O
to	O
this	O
,	O
development	O
tools	O
for	O
the	O
two	O
languages	O
(	O
such	O
as	O
IDEs	B-Application
and	O
compilers	B-Language
)	O
are	O
often	O
integrated	O
into	O
a	O
single	O
product	O
,	O
with	O
the	O
programmer	O
able	O
to	O
specify	O
C	B-Language
or	O
C++	B-Language
as	O
their	O
source	O
language	O
.	O
</s>
<s>
However	O
,	O
C	B-Language
is	O
not	O
a	O
subset	O
of	O
C++	B-Language
,	O
and	O
nontrivial	O
C	B-Language
programs	I-Language
will	O
not	O
compile	B-Language
as	O
C++	B-Language
code	I-Language
without	O
modification	O
.	O
</s>
<s>
Likewise	O
,	O
C++	B-Language
introduces	O
many	O
features	O
that	O
are	O
not	O
available	O
in	O
C	B-Language
and	O
in	O
practice	O
almost	O
all	O
code	O
written	O
in	O
C++	B-Language
is	O
not	O
conforming	O
C	B-Language
code	O
.	O
</s>
<s>
This	O
article	O
,	O
however	O
,	O
focuses	O
on	O
differences	O
that	O
cause	O
conforming	O
C	B-Language
code	O
to	O
be	O
ill-formed	O
C++	B-Language
code	I-Language
,	O
or	O
to	O
be	O
conforming/well	O
-formed	O
in	O
both	O
languages	O
but	O
to	O
behave	O
differently	O
in	O
C	B-Language
and	O
C++	B-Language
.	O
</s>
<s>
Bjarne	O
Stroustrup	O
,	O
the	O
creator	O
of	O
C++	B-Language
,	O
has	O
suggested	O
that	O
the	O
incompatibilities	O
between	O
C	B-Language
and	O
C++	B-Language
should	O
be	O
reduced	O
as	O
much	O
as	O
possible	O
in	O
order	O
to	O
maximize	O
interoperability	O
between	O
the	O
two	O
languages	O
.	O
</s>
<s>
Others	O
have	O
argued	O
that	O
since	O
C	B-Language
and	O
C++	B-Language
are	O
two	O
different	O
languages	O
,	O
compatibility	O
between	O
them	O
is	O
useful	O
but	O
not	O
vital	O
;	O
according	O
to	O
this	O
camp	O
,	O
efforts	O
to	O
reduce	O
incompatibility	O
should	O
not	O
hinder	O
attempts	O
to	O
improve	O
each	O
language	O
in	O
isolation	O
.	O
</s>
<s>
The	O
official	O
rationale	O
for	O
the	O
1999	O
C	B-Language
standard	O
(	O
C99	B-Language
)	O
"endorse[d]	O
the	O
principle	O
of	O
maintaining	O
the	O
largest	O
common	O
subset	O
"	O
between	O
C	B-Language
and	O
C++	B-Language
"	O
while	O
maintaining	O
a	O
distinction	O
between	O
them	O
and	O
allowing	O
them	O
to	O
evolve	O
separately	O
"	O
,	O
and	O
stated	O
that	O
the	O
authors	O
were	O
"	O
content	O
to	O
let	O
C++	B-Language
be	O
the	O
big	O
and	O
ambitious	O
language.	O
"	O
</s>
<s>
Several	O
additions	O
of	O
C99	B-Language
are	O
not	O
supported	O
in	O
the	O
current	O
C++	B-Language
standard	O
or	O
conflicted	O
with	O
C++	B-Language
features	O
,	O
such	O
as	O
variable-length	B-Data_Structure
arrays	I-Data_Structure
,	O
native	O
complex	O
number	O
types	O
and	O
the	O
restrict	B-Language
type	B-Language
qualifier	I-Language
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
C99	B-Language
reduced	O
some	O
other	O
incompatibilities	O
compared	O
with	O
C89	O
by	O
incorporating	O
C++	B-Language
features	O
such	O
as	O
//	O
comments	O
and	O
mixed	O
declarations	O
and	O
code	O
.	O
</s>
<s>
C++	B-Language
enforces	O
stricter	O
typing	O
rules	O
(	O
no	O
implicit	O
violations	O
of	O
the	O
static	O
type	O
system	O
)	O
,	O
and	O
initialization	O
requirements	O
(	O
compile-time	O
enforcement	O
that	O
in-scope	O
variables	O
do	O
not	O
have	O
initialization	O
subverted	O
)	O
than	O
C	B-Language
,	O
and	O
so	O
some	O
valid	O
C	B-Language
code	O
is	O
invalid	O
in	O
C++	B-Language
.	O
</s>
<s>
A	O
rationale	O
for	O
these	O
is	O
provided	O
in	O
Annex	O
C.1	O
of	O
the	O
ISO	O
C++	B-Language
standard	O
.	O
</s>
<s>
C99	B-Language
and	O
C11	O
added	O
several	O
additional	O
features	O
to	O
C	B-Language
that	O
have	O
not	O
been	O
incorporated	O
into	O
standard	O
C++	B-Language
as	O
of	O
C++20	B-Language
,	O
such	O
as	O
complex	O
numbers	O
,	O
variable	B-Data_Structure
length	I-Data_Structure
arrays	I-Data_Structure
(	O
note	O
that	O
complex	O
numbers	O
and	O
variable	B-Data_Structure
length	I-Data_Structure
arrays	I-Data_Structure
are	O
designated	O
as	O
optional	O
extensions	O
in	O
C11	O
)	O
,	O
flexible	B-Language
array	I-Language
members	I-Language
,	O
the	O
restrict	B-Language
keyword	I-Language
,	O
array	O
parameter	O
qualifiers	O
,	O
and	O
compound	O
literals	O
.	O
</s>
<s>
C++	B-Language
adds	O
numerous	O
additional	O
keywords	O
to	O
support	O
its	O
new	O
features	O
.	O
</s>
<s>
This	O
renders	O
C	B-Language
code	O
using	O
those	O
keywords	O
for	O
identifiers	O
invalid	O
in	O
C++	B-Language
.	O
</s>
<s>
is	O
valid	O
C	B-Language
code	O
,	O
but	O
is	O
rejected	O
by	O
a	O
C++	B-Language
compiler	B-Language
,	O
since	O
the	O
keywords	O
template	O
,	O
new	O
and	O
class	O
are	O
reserved	O
.	O
</s>
<s>
There	O
are	O
a	O
few	O
syntactic	O
constructs	O
that	O
are	O
valid	O
in	O
both	O
C	B-Language
and	O
C++	B-Language
but	O
produce	O
different	O
results	O
in	O
the	O
two	O
languages	O
.	O
</s>
<s>
Character	O
literals	O
such	O
as	O
'	O
a	O
 '	O
are	O
of	O
type	O
int	O
in	O
C	B-Language
and	O
of	O
type	O
char	O
in	O
C++	B-Language
,	O
which	O
means	O
that	O
sizeof	B-Language
'	O
a	O
 '	O
will	O
generally	O
give	O
different	O
results	O
in	O
the	O
two	O
languages	O
:	O
in	O
C++	B-Language
,	O
it	O
will	O
be	O
1	O
,	O
while	O
in	O
C	B-Language
it	O
will	O
be	O
sizeof(int )	O
.	O
</s>
<s>
As	O
another	O
consequence	O
of	O
this	O
type	O
difference	O
,	O
in	O
C	B-Language
,	O
'	O
a	O
 '	O
will	O
always	O
be	O
a	O
signed	O
expression	O
,	O
regardless	O
of	O
whether	O
or	O
not	O
char	O
is	O
a	O
signed	O
or	O
unsigned	O
type	O
,	O
whereas	O
for	O
C++	B-Language
this	O
is	O
compiler	B-Language
implementation	O
specific	O
.	O
</s>
<s>
C++	B-Language
assigns	O
internal	B-Language
linkage	I-Language
to	O
namespace-scoped	O
const	O
variables	O
unless	O
they	O
are	O
explicitly	O
declared	O
extern	O
,	O
unlike	O
C	B-Language
in	O
which	O
extern	O
is	O
the	O
default	O
for	O
all	O
file-scoped	O
entities	O
.	O
</s>
<s>
Note	O
that	O
in	O
practice	O
this	O
does	O
not	O
lead	O
to	O
silent	O
semantic	O
changes	O
between	O
identical	O
C	B-Language
and	O
C++	B-Language
code	I-Language
but	O
instead	O
will	O
lead	O
to	O
a	O
compile-time	O
or	O
linkage	B-Language
error	O
.	O
</s>
<s>
In	O
C	B-Language
use	O
of	O
inline	O
functions	O
requires	O
manually	O
adding	O
a	O
prototype	O
declaration	O
of	O
the	O
function	O
using	O
the	O
extern	B-Language
keyword	I-Language
in	O
exactly	O
one	O
translation	B-Language
unit	I-Language
to	O
ensure	O
a	O
non-inlined	O
version	O
is	O
linked	O
in	O
,	O
whereas	O
C++	B-Language
handles	O
this	O
automatically	O
.	O
</s>
<s>
In	O
more	O
detail	O
,	O
C	B-Language
distinguishes	O
two	O
kinds	O
of	O
definitions	O
of	O
inline	O
functions	O
:	O
ordinary	O
external	O
definitions	O
(	O
where	O
extern	O
is	O
explicitly	O
used	O
)	O
and	O
inline	O
definitions	O
.	O
</s>
<s>
C++	B-Language
,	O
on	O
the	O
other	O
hand	O
,	O
provides	O
only	O
inline	O
definitions	O
for	O
inline	O
functions	O
.	O
</s>
<s>
In	O
C	B-Language
,	O
an	O
inline	O
definition	O
is	O
similar	O
to	O
an	O
internal	O
(	O
i.e.	O
</s>
<s>
static	O
)	O
one	O
,	O
in	O
that	O
it	O
can	O
coexist	O
in	O
the	O
same	O
program	O
with	O
one	O
external	O
definition	O
and	O
any	O
number	O
of	O
internal	O
and	O
inline	O
definitions	O
of	O
the	O
same	O
function	O
in	O
other	O
translation	B-Language
units	I-Language
,	O
all	O
of	O
which	O
can	O
differ	O
.	O
</s>
<s>
This	O
is	O
a	O
separate	O
consideration	O
from	O
the	O
linkage	B-Language
of	O
the	O
function	O
,	O
but	O
not	O
an	O
independent	O
one	O
.	O
</s>
<s>
C	B-Language
compilers	B-Language
are	O
afforded	O
the	O
discretion	O
to	O
choose	O
between	O
using	O
inline	O
and	O
external	O
definitions	O
of	O
the	O
same	O
function	O
when	O
both	O
are	O
visible	O
.	O
</s>
<s>
C++	B-Language
,	O
however	O
,	O
requires	O
that	O
if	O
a	O
function	O
with	O
external	B-Language
linkage	I-Language
is	O
declared	O
inline	O
in	O
any	O
translation	B-Language
unit	I-Language
then	O
it	O
must	O
be	O
so	O
declared	O
(	O
and	O
therefore	O
also	O
defined	O
)	O
in	O
every	O
translation	B-Language
unit	I-Language
where	O
it	O
is	O
used	O
,	O
and	O
that	O
all	O
the	O
definitions	O
of	O
that	O
function	O
be	O
identical	O
,	O
following	O
the	O
ODR	B-Language
.	O
</s>
<s>
Note	O
that	O
static	O
inline	O
functions	O
behave	O
identically	O
in	O
C	B-Language
and	O
C++	B-Language
.	O
</s>
<s>
Both	O
C99	B-Language
and	O
C++	B-Language
have	O
a	O
boolean	O
type	O
bool	O
with	O
constants	O
true	O
and	O
false	O
,	O
but	O
they	O
are	O
defined	O
differently	O
.	O
</s>
<s>
In	O
C++	B-Language
,	O
bool	O
is	O
a	O
built-in	O
type	O
and	O
a	O
reserved	O
keyword	O
.	O
</s>
<s>
In	O
C99	B-Language
,	O
a	O
new	O
keyword	O
,	O
_Bool	O
,	O
is	O
introduced	O
as	O
the	O
new	O
boolean	O
type	O
.	O
</s>
<s>
Therefore	O
,	O
true	O
and	O
false	O
have	O
type	O
int	O
in	O
C	B-Language
.	O
This	O
is	O
likely	O
to	O
change	O
in	O
C2x	B-Language
however	O
,	O
whose	O
draft	O
includes	O
changing	O
bool	O
,	O
true	O
,	O
and	O
false	O
to	O
become	O
keywords	O
,	O
and	O
giving	O
true	O
and	O
false	O
the	O
type	O
bool	O
.	O
</s>
<s>
Several	O
of	O
the	O
other	O
differences	O
from	O
the	O
previous	O
section	O
can	O
also	O
be	O
exploited	O
to	O
create	O
code	O
that	O
compiles	B-Language
in	O
both	O
languages	O
but	O
behaves	O
differently	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
following	O
function	O
will	O
return	O
different	O
values	O
in	O
C	B-Language
and	O
C++	B-Language
:	O
</s>
<s>
This	O
is	O
due	O
to	O
C	B-Language
requiring	O
struct	B-Language
in	O
front	O
of	O
structure	O
tags	O
(	O
and	O
so	O
sizeof(T )	O
refers	O
to	O
the	O
variable	O
)	O
,	O
but	O
C++	B-Language
allowing	O
it	O
to	O
be	O
omitted	O
(	O
and	O
so	O
sizeof(T )	O
refers	O
to	O
the	O
implicit	O
typedef	B-Language
)	O
.	O
</s>
<s>
Beware	O
that	O
the	O
outcome	O
is	O
different	O
when	O
the	O
extern	O
declaration	O
is	O
placed	O
inside	O
the	O
function	O
:	O
then	O
the	O
presence	O
of	O
an	O
identifier	O
with	O
same	O
name	O
in	O
the	O
function	O
scope	O
inhibits	O
the	O
implicit	O
typedef	B-Language
to	O
take	O
effect	O
for	O
C++	B-Language
,	O
and	O
the	O
outcome	O
for	O
C	B-Language
and	O
C++	B-Language
would	O
be	O
the	O
same	O
.	O
</s>
<s>
Observe	O
also	O
that	O
the	O
ambiguity	O
in	O
the	O
example	O
above	O
is	O
due	O
to	O
the	O
use	O
of	O
the	O
parenthesis	O
with	O
the	O
sizeof	B-Language
operator	O
.	O
</s>
<s>
Using	O
sizeof	B-Language
T	O
would	O
expect	O
T	O
to	O
be	O
an	O
expression	O
and	O
not	O
a	O
type	O
,	O
and	O
thus	O
the	O
example	O
would	O
not	O
compile	B-Language
with	O
C++	B-Language
.	O
</s>
<s>
While	O
C	B-Language
and	O
C++	B-Language
maintain	O
a	O
large	O
degree	O
of	O
source	O
compatibility	O
,	O
the	O
object	O
files	O
their	O
respective	O
compilers	B-Language
produce	O
can	O
have	O
important	O
differences	O
that	O
manifest	O
themselves	O
when	O
intermixing	O
C	B-Language
and	O
C++	B-Language
code	I-Language
.	O
</s>
<s>
C	B-Language
compilers	B-Language
do	O
not	O
name	B-Language
mangle	I-Language
symbols	O
in	O
the	O
way	O
that	O
C++	B-Language
compilers	B-Language
do	O
.	O
</s>
<s>
Depending	O
on	O
the	O
compiler	B-Language
and	O
architecture	O
,	O
it	O
also	O
may	O
be	O
the	O
case	O
that	O
calling	O
conventions	O
differ	O
between	O
the	O
two	O
languages	O
.	O
</s>
<s>
For	O
these	O
reasons	O
,	O
for	O
C++	B-Language
code	I-Language
to	O
call	O
a	O
C	B-Language
function	O
foo( )	O
,	O
the	O
C++	B-Language
code	I-Language
must	O
prototype	O
foo( )	O
with	O
extern	O
"	O
C	B-Language
"	O
.	O
</s>
<s>
Likewise	O
,	O
for	O
C	B-Language
code	O
to	O
call	O
a	O
C++	B-Language
function	O
bar( )	O
,	O
the	O
C++	B-Language
code	I-Language
for	O
bar( )	O
must	O
be	O
declared	O
with	O
extern	O
"	O
C	B-Language
"	O
.	O
</s>
<s>
A	O
common	O
practice	O
for	O
header	B-Language
files	I-Language
to	O
maintain	O
both	O
C	B-Language
and	O
C++	B-Language
compatibility	O
is	O
to	O
make	O
its	O
declaration	O
be	O
extern	O
"	O
C	B-Language
"	O
for	O
the	O
scope	O
of	O
the	O
header	O
:	O
</s>
<s>
Differences	O
between	O
C	B-Language
and	O
C++	B-Language
linkage	B-Language
and	O
calling	O
conventions	O
can	O
also	O
have	O
subtle	O
implications	O
for	O
code	O
that	O
uses	O
function	B-Language
pointers	I-Language
.	O
</s>
<s>
Some	O
compilers	B-Language
will	O
produce	O
non-working	O
code	O
if	O
a	O
function	B-Language
pointer	I-Language
declared	O
extern	O
"	O
C	B-Language
"	O
points	O
to	O
a	O
C++	B-Language
function	O
that	O
is	O
not	O
declared	O
extern	O
"	O
C	B-Language
"	O
.	O
</s>
<s>
Using	O
Sun	O
Microsystems	O
 '	O
C++	B-Language
compiler	B-Language
,	O
this	O
produces	O
the	O
following	O
warning	O
:	O
</s>
<s>
This	O
is	O
because	O
my_function( )	O
is	O
not	O
declared	O
with	O
C	B-Language
linkage	I-Language
and	O
calling	O
conventions	O
,	O
but	O
is	O
being	O
passed	O
to	O
the	O
C	B-Language
function	O
foo( )	O
.	O
</s>
