<s>
In	O
the	O
C++	B-Language
programming	I-Language
language	I-Language
,	O
a	O
reference	O
is	O
a	O
simple	O
reference	O
datatype	O
that	O
is	O
less	O
powerful	O
but	O
safer	O
than	O
the	O
pointer	O
type	O
inherited	O
from	O
C	B-Language
.	O
The	O
name	O
C++	B-Language
reference	I-Language
may	O
cause	O
confusion	O
,	O
as	O
in	O
computer	O
science	O
a	O
reference	O
is	O
a	O
general	O
concept	O
datatype	O
,	O
with	O
pointers	O
and	O
C++	B-Language
references	I-Language
being	O
specific	O
reference	O
datatype	O
implementations	O
.	O
</s>
<s>
The	O
definition	O
of	O
a	O
reference	O
in	O
C++	B-Language
is	O
such	O
that	O
it	O
does	O
not	O
need	O
to	O
exist	O
.	O
</s>
<s>
const	O
int&	O
ref	O
is	O
an	O
lvalue	B-Language
reference	I-Language
to	O
const	O
int	O
pointing	O
to	O
a	O
piece	O
of	O
storage	O
having	O
value	O
65	O
.	O
</s>
<s>
Since	O
the	O
name	O
of	O
an	O
rvalue	O
reference	O
is	O
itself	O
an	O
lvalue	O
,	O
std::move	O
must	O
be	O
used	O
to	O
pass	O
an	O
rvalue	O
reference	O
to	O
a	O
function	O
overload	B-Language
accepting	O
an	O
rvalue	O
reference	O
parameter	O
.	O
</s>
<s>
Types	O
which	O
are	O
of	O
kind	O
"	O
reference	O
to	O
<Type>	O
"	O
are	O
sometimes	O
called	O
reference	B-Language
types	I-Language
.	O
</s>
<s>
Identifiers	O
which	O
are	O
of	O
reference	B-Language
type	I-Language
are	O
called	O
reference	O
variables	O
.	O
</s>
<s>
Declaring	O
references	O
as	O
const	O
or	O
volatile( )	O
also	O
fails	O
unless	O
a	O
typedef/decltype	O
is	O
used	O
in	O
which	O
case	O
the	O
const/volatile	O
is	O
ignored	O
.	O
</s>
<s>
However	O
if	O
template	O
argument	O
deduction	O
takes	O
place	O
and	O
a	O
reference	B-Language
type	I-Language
is	O
deduced	O
(	O
which	O
happens	O
when	O
forwarding	O
references	O
are	O
used	O
and	O
an	O
lvalue	O
is	O
passed	O
to	O
the	O
function	O
)	O
or	O
if	O
typedef	B-Language
,	O
using	O
or	O
decltype	B-Language
denote	O
a	O
reference	B-Language
type	I-Language
it	O
is	O
possible	O
to	O
take	O
a	O
reference	O
to	O
that	O
type	O
.	O
</s>
<s>
In	O
that	O
case	O
the	O
rule	O
that	O
is	O
used	O
to	O
determine	O
the	O
type	O
of	O
reference	O
is	O
called	O
reference	O
collapsing	O
and	O
works	O
like	O
this	O
:	O
Assuming	O
a	O
type	O
T	O
and	O
a	O
reference	B-Language
type	I-Language
to	O
T	O
TR	O
,	O
attempting	O
to	O
create	O
an	O
rvalue	O
reference	O
to	O
TR	O
creates	O
a	O
TR	O
while	O
an	O
lvalue	B-Language
reference	I-Language
to	O
TR	O
creates	O
an	O
lvalue	B-Language
reference	I-Language
to	O
T	O
.	O
In	O
other	O
words	O
,	O
lvalue	B-Language
references	I-Language
override	O
rvalue	O
references	O
and	O
rvalue	O
references	O
of	O
rvalue	O
references	O
stay	O
unchanged	O
.	O
</s>
<s>
This	O
qualifier	O
participates	O
in	O
overload	B-Language
resolution	I-Language
and	O
applies	O
to	O
the	O
implicit	O
object	O
parameter	O
like	O
const	O
and	O
volatile	O
but	O
unlike	O
those	O
two	O
,	O
it	O
does	O
not	O
change	O
the	O
properties	O
of	O
this	O
.	O
</s>
<s>
C++	B-Language
references	I-Language
differ	O
from	O
pointers	O
in	O
several	O
essential	O
ways	O
:	O
</s>
<s>
Though	O
that	O
is	O
usually	O
the	O
case	O
,	O
the	O
C++	B-Language
Standard	O
does	O
not	O
force	O
compilers	O
to	O
implement	O
references	O
using	O
pointers	O
.	O
</s>
<s>
However	O
,	O
the	O
following	O
call	O
would	O
give	O
a	O
compiler	O
error	O
,	O
since	O
lvalue	B-Language
reference	I-Language
parameters	O
not	O
qualified	O
with	O
const	O
can	O
only	O
be	O
bound	O
to	O
addressable	O
values	O
:	O
</s>
<s>
Returning	O
an	O
lvalue	B-Language
reference	I-Language
allows	O
function	O
calls	O
to	O
be	O
assigned	O
to	O
:	O
</s>
<s>
Continuing	O
the	O
relationship	O
between	O
references	O
and	O
pointers	O
(	O
in	O
C++	B-Language
context	O
)	O
,	O
the	O
former	O
exhibit	O
polymorphic	O
capabilities	O
,	O
as	O
one	O
might	O
expect	O
:	O
</s>
<s>
The	O
source	O
above	O
is	O
valid	O
C++	B-Language
and	O
generates	O
the	O
following	O
output	O
:	O
</s>
