6 Basic concepts [basic]

6.10 Lvalues and rvalues [basic.lval]

Expressions are categorized according to the taxonomy in Figure [fig:categories].

categories expression expression glvalue glvalue expression->glvalue rvalue rvalue expression->rvalue lvalue lvalue glvalue->lvalue xvalue xvalue glvalue->xvalue rvalue->xvalue prvalue prvalue rvalue->prvalue
Figure 1 — Expression category taxonomy

[Note: Historically, lvalues and rvalues were so-called because they could appear on the left- and right-hand side of an assignment (although this is no longer generally true); glvalues are “generalized” lvalues, prvalues are “pure” rvalues, and xvalues are “eXpiring” lvalues. Despite their names, these terms classify expressions, not values. end note] Every expression belongs to exactly one of the fundamental classifications in this taxonomy: lvalue, xvalue, or prvalue. This property of an expression is called its value category. [Note: The discussion of each built-in operator in Clause [expr] indicates the category of the value it yields and the value categories of the operands it expects. For example, the built-in assignment operators expect that the left operand is an lvalue and that the right operand is a prvalue and yield an lvalue as the result. User-defined operators are functions, and the categories of values they expect and yield are determined by their parameter and return types. end note]

The result of a prvalue is the value that the expression stores into its context. A prvalue whose result is the value V is sometimes said to have or name the value V. The result object of a prvalue is the object initialized by the prvalue; a prvalue that is used to compute the value of an operand of an operator or that has type cv void has no result object. [Note: Except when the prvalue is the operand of a decltype-specifier, a prvalue of class or array type always has a result object. For a discarded prvalue, a temporary object is materialized; see Clause [expr]. end note] The result of a glvalue is the entity denoted by the expression.

[Note: Whenever a glvalue appears in a context where a prvalue is expected, the glvalue is converted to a prvalue; see [conv.lval], [conv.array], and [conv.func]. An attempt to bind an rvalue reference to an lvalue is not such a context; see [dcl.init.ref]. end note] [Note: There are no prvalue bit-fields; if a bit-field is converted to a prvalue, a prvalue of the type of the bit-field is created, which might then be promoted. end note]

[Note: Whenever a prvalue appears in a context where a glvalue is expected, the prvalue is converted to an xvalue; see [conv.rval]. end note]

The discussion of reference initialization in [dcl.init.ref] and of temporaries in [class.temporary] indicates the behavior of lvalues and rvalues in other significant contexts.

Unless otherwise indicated ([expr.call]), a prvalue shall always have complete type or the void type. A glvalue shall not have type cv void. [Note: A glvalue may have complete or incomplete non-void type. Class and array prvalues can have cv-qualified types; other prvalues always have cv-unqualified types. See Clause [expr]. end note]

An lvalue is modifiable unless its type is const-qualified or is a function type. [Note: A program that attempts to modify an object through a nonmodifiable lvalue expression or through an rvalue expression is ill-formed ([expr.ass], [expr.post.incr], [expr.pre.incr]). end note]

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:56

The intent of this list is to specify those circumstances in which an object may or may not be aliased.