6 Basic concepts [basic]

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.

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.

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].

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].

There are no prvalue bit-fields; if a bit-field is converted to a prvalue ([conv.lval]), a prvalue of the type of the bit-field is created, which might then be promoted ([conv.prom]).

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

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].

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]).