7 Expressions [expr]

7.2 Properties of expressions [expr.prop]

7.2.1 Value category [basic.lval]

Expressions are categorized according to the taxonomy in Figure 2 .

Figure 2: Expression category taxonomy [fig:basic.lval]

(1.1)
A glvalue is an expression whose evaluation determines the identity of an object or function.
(1.2)
A prvalue is an expression whose evaluation initializes an object or computes the value of an operand of an operator, as specified by the context in which it appears, or an expression that has type cv void.
(1.3)
An xvalue is a glvalue that denotes an object whose resources can be reused (usually because it is near the end of its lifetime).
(1.4)
An lvalue is a glvalue that is not an xvalue.
(1.5)
An rvalue is a prvalue or an xvalue.

Every expression belongs to exactly one of the fundamental categories in this taxonomy: lvalue, xvalue, or prvalue.

This property of an expression is called its value category.

[Note 1:

The discussion of each built-in operator in [expr.compound] 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]

[Note 2:

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 apply to expressions, not values.

— end note]

[Note 3:

An expression is an xvalue if it is:

(4.1)
a move-eligible id-expression ([expr.prim.id.unqual]),
(4.2)
the result of calling a function, whether implicitly or explicitly, whose return type is an rvalue reference to object type ([expr.call]),
(4.3)
a cast to an rvalue reference to object type ([expr.type.conv], [expr.dynamic.cast], [expr.static.cast], [expr.reinterpret.cast], [expr.const.cast], [expr.cast]),
(4.4)
a subscripting operation with an xvalue array operand ([expr.sub]),
(4.5)
a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue ([expr.ref]), or
(4.6)
a .* pointer-to-member expression in which the first operand is an xvalue and the second operand is a pointer to data member ([expr.mptr.oper]).

In general, the effect of this rule is that named rvalue references are treated as lvalues and unnamed rvalue references to objects are treated as xvalues; rvalue references to functions are treated as lvalues whether named or not.

— end note]

[Example 1: struct A { int m; }; A&& operator+(A, A); A&& f(); A a; A&& ar = static_cast<A&&>(a);

The expressions f(), f().m, static_cast<A&&>(a), and a + a are xvalues.

The expression ar is an lvalue.

— end example]

The result of a glvalue is the entity denoted by the expression.

The result of a prvalue is the value that the expression stores into its context; a prvalue that has type cv void has no result.

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 non-discarded prvalue that is used to compute the value of an operand of a built-in operator or a prvalue that has type cv void has no result object.

[Note 4:

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 that has type other than cv void, a temporary object is materialized; see [expr.context].

— end note]

Whenever a glvalue appears as an operand of an operator that requires a prvalue for that operand, the lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), or function-to-pointer ([conv.func]) standard conversions are applied to convert the expression to a prvalue.

[Note 5:

An attempt to bind an rvalue reference to an lvalue is not such a context; see [dcl.init.ref].

— end note]

[Note 6:

Because cv-qualifiers are removed from the type of an expression of non-class type when the expression is converted to a prvalue, an lvalue of type const int can, for example, be used where a prvalue of type int is required.

— end note]

[Note 7:

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]).

— end note]

Whenever a prvalue appears as an operand of an operator that expects a glvalue for that operand, the temporary materialization conversion is applied to convert the expression to an xvalue.

[Note 8:

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.

— end note]

Unless otherwise indicated ([dcl.type.decltype]), a prvalue shall always have complete type or the void type; if it has a class type or (possibly multidimensional) array of class type, that class shall not be an abstract class ([class.abstract]).

A glvalue shall not have type cv void.

[Note 9:

A glvalue can have complete or incomplete non-void type.

Class and array prvalues can have cv-qualified types; other prvalues always have cv-unqualified types.

See [expr.type].

— end note]

An lvalue is modifiable unless its type is const-qualified or is a function type.

[Note 10:

A program that attempts to modify an object through a nonmodifiable lvalue or through an rvalue is ill-formed ([expr.ass], [expr.post.incr], [expr.pre.incr]).

— end note]

An object of dynamic type

T_{obj}

is type-accessible through a glvalue of type

T_{ref}

T_{ref}

is similar ([conv.qual]) to:

(11.1)
$T_{obj}$ ,
(11.2)
a type that is the signed or unsigned type corresponding to $T_{obj}$ , or
(11.3)
a char, unsigned char, or std::byte type.

If a program attempts to access ([defns.access]) the stored value of an object through a glvalue through which it is not type-accessible, the behavior is undefined.43

If a program invokes a defaulted copy/move constructor or copy/move assignment operator for a union of type U with a glvalue argument that does not denote an object of type cv U within its lifetime, the behavior is undefined.

[Note 11:

In C, an entire object of structure type can be accessed, e.g., using assignment.

By contrast, C++ has no notion of accessing an object of class type through an lvalue of class type.

— end note]

43)43)

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