Lvalues and rvalues
- Lvalue
- An expression can appear on the left side of an assignment expression if the expression is not const qualified.
Xvalue
An rvalue reference that is to expire.
- (Prvalue) rvalue
- A non-xvalue expression that appears only
on the right side of an assignment expression.
Rvalues include
both xvalues and prvalues. Lvalues and xvalues can be referred as
glvalues.- Class (prvalue) rvalues can be cv-qualified,
but non-class (prvalue) rvalues cannot be cv-qualified.
- Lvalues and xvalues can be of incomplete types, but (prvalue) rvalues must be of complete
types or void types.
or xvalue is an expression that
refers to such an object. An lvalue does not necessarily permit modification
of the object it designates. For example, a const object
is an lvalue that cannot be modified. The term modifiable lvalue is
used to emphasize that the lvalue allows the designated object to
be changed as well as examined. Lvalues of the following
object types are not modifiable lvalues:- An array type
- An incomplete type
- A const-qualified type
- A structure or union type with one of its members qualified as a const type
except where a suitable assignment operator exists
.
C defines a function designator as
an expression that has function type. A function designator is distinct
from an object type or an lvalue. It can be the name of a function
or the result of dereferencing a function pointer. The C language
also differentiates between its treatment of a function pointer and
an object pointer.
A function
call that returns an lvalue reference is an lvalue.
Expressions can produce an lvalue, an xvalue,
a (prvalue) rvalue, or no value.
built-in operators require lvalues
for some of their operands. The following table
lists these operators and additional constraints on their usage. | Operator | Requirement |
|---|---|
| & (unary) | Operand must be an lvalue. |
| ++ -- | Operand must be a modifiable lvalue. This applies to both prefix and postfix forms. |
| = += -= *= %= <<= >>= &= ‸= |= | Left operand must be a modifiable lvalue. |
For example, all assignment operators evaluate their right operand and assign that value to their left operand. The left operand must be a modifiable lvalue.
| Expression | Lvalue |
|---|---|
| x = 42 | x |
| *ptr = newvalue | *ptr |
| a++ | a |
| f() |
The function call to f() |
- The result of calling a function whose return type is of an rvalue reference type
- A cast to an rvalue reference
- A nonstatic data member of a non-reference type accessed through an xvalue expression
- A pointer to member access expression in which the first operand is an xvalue expression and the second operand is of a pointer to member type
int a;
int&& b= static_cast<int&&>(a);
struct str{
int c;
};
int&& f(){
int&& var =1;
return var;
}
str&& g();
int&& rc = g().c;
When GNU C language extensions are enabled, compound expressions,
conditional expressions, and casts are allowed as lvalues, if their
operands are lvalues.
(x + 1, y) *= 42;
x + 1, (y *=42);&(x + 1, y);
x + 1, &y;A conditional expression can be a
valid lvalue if its type is not void and both of its branches for
true and false are valid lvalues. Casts are valid lvalues if the operand
is an lvalue. The primary restriction is that you cannot take the
address of an lvalue cast. 
