下面這段引文可對左值、右值加深理解。
參考文獻：《ANSI/ISO C++ Professional Programmer's Handbook》
lvalues and rvalues
An object is a contiguous region of storage. An lvalue is an expression that refers to such an object. The original
definition of lvalue referred to an object that can appear on the left-hand side of an assignment. However, const
objects are lvalues that cannot be used in the left-hand side of an assignment. Similarly, an expression that can appear
in the right-hand side of an expression (but not in the left-hand side of an expression) is an rvalue. For example
#include?
<
string
>
using
?
namespace
?std;
int
&
?f();
void
?func()
{
????
int
?n;
????
char
?buf[
3
];
????n?
=
?
5
;?
//
?n?is?an?lvalue;?5?is?an?rvalue
????buf[
0
]?
=
?
'
a
'
;?
//
?buf[0]?is?an?lvalue,?'a'?is?an?rvalue
????
string
?s1?
=
?
"
a
"
,?s2?
=
?
"
b
"
,?s3?
=
??
"
c
"
;?
//
?"a",?"b",?"c"?are?rvalues
????s1?
=
?
//
?lvalue
????s2?
+
s3;?
//
s2?and?s3?are?lvalues?that?are??implicitly?converted?to?rvalues
????s1?
=
?
//
lvalue
????
string
(
"
z
"
);?
//
temporaries?are?rvalues
????
int
?
*
?p?
=
?
new
?
int
;?
//
p?is?an?lvalue;?'new?int'?is?an?rvalue
????f()?
=
?
0
;?
//
a?function?call?that?returns?a?reference?is?an?lvalue
?????s1.size();?
//
otherwise,?a?function?call?is?an?rvalue?expression
}
An lvalue can appear in a context that requires an rvalue; in this case, the lvalue is implicitly converted to an rvalue.
An rvalue cannot be converted to an lvalue. Therefore, it is possible to use every lvalue expression in the example as
an rvalue, but not vice versa.