Value categories

Each expression in C (an operator with its arguments, a function call, a constant, a variable name, etc) is characterized by two independent properties: a type and a value category

Every expression belongs to one of three value categories: lvalue, non-lvalue object (rvalue), and function designator.

Lvalue expressions

Lvalue expression is any expression with object type other than the type void, which potentially designates an object (the behavior is undefined if an lvalue does not actually designate an object when it is evaluated). In other words, lvalue expression evaluates to the object identity

Lvalue expressions can be used in the following lvalue contexts:

• as the operand of the address-of operator (except if the lvalue designates a bit-field or was declared register
• as the operand of the pre/post increment and decrement operators.
• as the left-hand operand of the member access (dot) operator.
• as the left-hand operand of the assignment and compound assignment operators.

If an lvalue expression is used in any context other than sizeof, _Alignof, or the operators listed above, non-array lvalues of any complete type undergo lvalue conversion, which models the memory load of the value of the object from its location. Similarly, array lvalues undergo array-to-pointer conversion when used in any context other than sizeof, _Alignof

The semantics of const/volatile/restrict-qualifiers and atomic

The following expressions are lvalues:

• identifiers, including function named parameters, provided they were declared as designating objects (not functions or enumeration constants)
• string literals
• (C99) compound literals
• parenthesized expression if the unparenthesized expression is an lvalue
• the result of a member access (dot) operator if its left-hand argument is lvalue
• the result of a member access through pointer -> operator
• the result of the indirection (unary *) operator applied to a pointer to object
• the result of the subscription operator ([])

Modifiable lvalue expressions

A modifiable lvalue is any lvalue expression of complete, non-array type which is not const-qualified, and, if it's a struct/union, has no members that are const

Only modifiable lvalue expressions may be used as arguments to increment/decrement, and as left-hand arguments of assignment and compound assignment operators.

Non-lvalue object expressions

Known as rvalues

The following expressions are non-lvalue object expressions:

• integer, character, and floating constants
• all operators not specified to return lvalues, including

• any function call expression
• any cast expression (note that compound literals, which look similar, are lvalues)
• member access operator (dot) applied to a non-lvalue structure/union, f().x, (x,s1).a, (s1=s2).m
• results of all arithmetic, relational, logical, and bitwise operators
• results of increment and decrement operators (note: pre-forms are lvalues in C++)
• results of assignment operators (note: also lvalues in C++)
• the conditional operator (note: is lvalue in C++ if both the second and third operands are lvalues of the same type)
• the comma operator (note: is lvalue in C++ if the second operand is)
• the address-of operator, even if neutralized by application to the result of unary * operator

As a special case, expressions of type void are assumed to be non-lvalue object expressions that yield a value which has no representation and requires no storage.

Note that a struct/union rvalue that has a member (possibly nested) of array type does in fact designate an object with temporary lifetime

Function designator expression

A function designator (the identifier introduced by a function declaration) is an expression of function type. When used in any context other than the address-of operator, sizeof, and _Alignof

References

See also

C++ documentation for Value categories