An object is a region of storage that can be examined and stored into. An lvalue 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. The following object types are lvalues, but not modifiable lvalues:
Because these lvalues are not modifiable, they cannot appear on the left side of an assignment statement.
The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it. Both a literal constant and a variable can serve as an rvalue. When an lvalue appears in a context that requires an rvalue, the lvalue is implicitly converted to an rvalue. The reverse, however, is not true: an rvalue cannot be converted to an lvalue. Rvalues always have complete types or the void type.
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.
On the other hand, in C++, a function
call that returns a reference is an lvalue. Otherwise, a function
call is an rvalue expression. In C++, every expression produces an
lvalue, an rvalue, or no value.
In both C and C++, certain operators require lvalues for some of their operands. The table below lists these operators and additional constraints on their usage.
| Operator | Requirement |
|---|---|
| & (unary) | Operand must be an lvalue. |
| ++ -- | Operand must be an lvalue. This applies to both prefix and postfix forms. |
| = += -= *= %= <<= >>= &= ^= |= | Left operand must be an 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 or a reference to a modifiable object.
The address operator (&) requires an lvalue as an operand while the increment (++) and the decrement (--) operators require a modifiable lvalue as an operand. The following example shows expressions and their corresponding lvalues.
| Expression | Lvalue |
|---|---|
| x = 42 | x |
| *ptr = newvalue | *ptr |
| a++ | a |
|
int& f() |
The function call to f() |
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