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 1.
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  [fig:basic.lval]
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 [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.
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 ]
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
 ]
Note
:
An expression is an xvalue if it is:
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.
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 ]
Example
:
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
:
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].
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 ]
Whenever a glvalue appears as an operand of an operator that expects a prvalue for that operand, the lvalue-to-rvalue, array-to-pointer, or function-to-pointer standard conversions are applied to convert the expression to a prvalue.
Note
:
An attempt to bind an rvalue reference to an lvalue is not such a context; see [dcl.init.ref].
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 ]
Note
:
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.
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 ]
Note
:
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]).
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 ]
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.
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 ([dcl.type.simple]), a prvalue shall always have complete type or the void type; if it has a class type or (possibly multi-dimensional) array of class type, that class shall not be an abstract class ([class.abstract]).
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.
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 ]
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 or through an rvalue is ill-formed ([expr.ass], [expr.post.incr], [expr.pre.incr]).
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 ]
If a program attempts to access the stored value of an object through a glvalue whose type is not similar ([conv.qual]) to one of the following types the behavior is undefined:52
  • the dynamic type of the object,
  • a type that is the signed or unsigned type corresponding to the dynamic type of the object, or
  • a char, unsigned char, or std​::​byte type.
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
:
Unlike in C, C++ has no accesses of class type.
— end note
 ]
The intent of this list is to specify those circumstances in which an object may or may not be aliased.