8 Expressions [expr]

8.2 Properties of expressions [expr.prop]

8.2.1 Value category [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
  • A glvalue is an expression whose evaluation determines the identity of an object, bit-field, or function.
  • A prvalue is an expression whose evaluation initializes an object or a bit-field, or computes the value of the operand of an operator, as specified by the context in which it appears.
  • An xvalue is a glvalue that denotes an object or bit-field whose resources can be reused (usually because it is near the end of its lifetime).
  • An lvalue is a glvalue that is not an xvalue.
  • An rvalue is a prvalue or an xvalue.
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.
end note
]
[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.
end note
]
[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 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 [expr.prop].
end note
]
The result of a glvalue is the entity denoted by the expression.
Whenever a glvalue expression 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].
end note
]
[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 expression of type const int can, for example, be used where a prvalue expression of type int is required.
end note
]
[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]).
end note
]
Whenever a prvalue expression 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.
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:63
  • the dynamic type of the object,
  • a cv-qualified version of the dynamic type of the object,
  • a type similar (as defined in [conv.qual]) to the dynamic type of the object,
  • a type that is the signed or unsigned type corresponding to the dynamic type of the object,
  • a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,
  • an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),
  • a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
  • a char, unsigned char, or std​::​byte type.
The intent of this list is to specify those circumstances in which an object may or may not be aliased.

8.2.2 Type [expr.type]

If an expression initially has the type “reference to T” ([dcl.ref], [dcl.init.ref]), the type is adjusted to T prior to any further analysis.
The expression designates the object or function denoted by the reference, and the expression is an lvalue or an xvalue, depending on the expression.
[Note
:
Before the lifetime of the reference has started or after it has ended, the behavior is undefined (see [basic.life]).
end note
]
If a prvalue initially has the type “cv T”, where T is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to T prior to any further analysis.
The cv-combined type of two types T1 and T2 is a type T3 similar to T1 whose cv-qualification signature is:
  • for every , is the union of and ;
  • if the resulting is different from or , then const is added to every for .
[Note
:
Given similar types T1 and T2, this construction ensures that both can be converted to T3.
end note
]
The composite pointer type of two operands p1 and p2 having types T1 and T2, respectively, where at least one is a pointer or pointer-to-member type or std​::​nullptr_­t, is:
  • if both p1 and p2 are null pointer constants, std​::​nullptr_­t;
  • if either p1 or p2 is a null pointer constant, T2 or T1, respectively;
  • if T1 or T2 is “pointer to cv1 void” and the other type is “pointer to cv2 T”, where T is an object type or void, “pointer to cv12 void”, where cv12 is the union of cv1 and cv2;
  • if T1 or T2 is “pointer to noexcept function” and the other type is “pointer to function”, where the function types are otherwise the same, “pointer to function”;
  • if T1 is “pointer to cv1 C1” and T2 is “pointer to cv2 C2”, where C1 is reference-related to C2 or C2 is reference-related to C1, the cv-combined type of T1 and T2 or the cv-combined type of T2 and T1, respectively;
  • if T1 is “pointer to member of C1 of type cv1 U1” and T2 is “pointer to member of C2 of type cv2 U2” where C1 is reference-related to C2 or C2 is reference-related to C1, the cv-combined type of T2 and T1 or the cv-combined type of T1 and T2, respectively;
  • if T1 and T2 are similar types, the cv-combined type of T1 and T2;
  • otherwise, a program that necessitates the determination of a composite pointer type is ill-formed.
[Example
:
typedef void *p;
typedef const int *q;
typedef int **pi;
typedef const int **pci;
The composite pointer type of p and q is “pointer to const void”; the composite pointer type of pi and pci is “pointer to const pointer to const int.
end example
]

8.2.3 Context dependence [expr.context]

An unevaluated operand is not evaluated.
[Note
:
In an unevaluated operand, a non-static class member may be named ([expr.prim.id]) and naming of objects or functions does not, by itself, require that a definition be provided ([basic.def.odr]).
An unevaluated operand is considered a full-expression.
end note
]
In some contexts, an expression only appears for its side effects.
Such an expression is called a discarded-value expression.
The array-to-pointer and function-to-pointer standard conversions are not applied.
The lvalue-to-rvalue conversion is applied if and only if the expression is a glvalue of volatile-qualified type and it is one of the following:
[Note
:
Using an overloaded operator causes a function call; the above covers only operators with built-in meaning.
end note
]
If the (possibly converted) expression is a prvalue, the temporary materialization conversion is applied.
[Note
:
If the expression is an lvalue of class type, it must have a volatile copy constructor to initialize the temporary object that is the result object of the lvalue-to-rvalue conversion.
end note
]
The glvalue expression is evaluated and its value is discarded.