Each C++
(an operator with its operands, a literal, a variable name, etc.) is characterized by two independent properties: a
and a value category. Each expression has some non-reference type, and each expression belongs to exactly one of the three primary value categories:
,
, and
.
a
(“generalized” lvalue) is an expression whose evaluation determines the identity of an object or function;
a
(“pure” rvalue) is an expression whose evaluation
computes the value of an operand of a built-in operator (such prvalue has no result object), or
initializes an object (such prvalue is said to have a result object).
The result object may be a variable, an object created by
, a temporary created by
, or a member thereof. Note that non-void
expressions have a result object (the materialized temporary). Also, every class and array prvalue has a result object except when it is the operand of
;an
(an “eXpiring” value) is a glvalue that denotes an object whose resources can be reused;
an
is a glvalue that is not an xvalue;
Extended contentSo-called, historically, because lvalues could appear on the left-hand side of an assignment expression. In general, it's not always the case:
voidfoo();voidbaz(){inta;// Expression `a` is lvaluea=4;// OK, could appear on the left-hand side of an assignment expressionint&b{a};// Expression `b` is lvalueb=5;// OK, could appear on the left-hand side of an assignment expressionconstint&c{a};// Expression `c` is lvaluec=6;// ill-formed, assignment of read-only reference// Expression `foo` is lvalue// address may be taken by built-in address-of operatorvoid(*p)()=&foo;foo=baz;// ill-formed, assignment of function}an
is a prvalue or an xvalue;
Extended contentSo-called, historically, because rvalues could appear on the right-hand side of an assignment expression. In general, it's not always the case:
Run this code
#include<iostream>structS{S():m{42}{}S(inta):m{a}{}intm;};intmain(){Ss;// Expression `S{}` is prvalue// May appear on the right-hand side of an assignment expressions=S{};std::cout<<s.m<<'\n';// Expression `S{}` is prvalue// Can be used on the left-hand side toostd::cout<<(S{}=S{7}).m<<'\n';}Output:
42 7 Note: this taxonomy went through significant changes with past C++ standard revisions, see
below for details.
Extended contentDespite their names, these terms classify expressions, not values.
Run this code
#include<type_traits>#include<utility>template<classT>structis_prvalue:std::true_type{};template<classT>structis_prvalue<T&>:std::false_type{};template<classT>structis_prvalue<T&&>:std::false_type{};template<classT>structis_lvalue:std::false_type{};template<classT>structis_lvalue<T&>:std::true_type{};template<classT>structis_lvalue<T&&>:std::false_type{};template<classT>structis_xvalue:std::false_type{};template<classT>structis_xvalue<T&>:std::false_type{};template<classT>structis_xvalue<T&&>:std::true_type{};intmain(){inta{42};int&b{a};int&&r{std::move(a)};// Expression `42` is prvaluestatic_assert(is_prvalue<decltype((42))>::value);// Expression `a` is lvaluestatic_assert(is_lvalue<decltype((a))>::value);// Expression `b` is lvaluestatic_assert(is_lvalue<decltype((b))>::value);// Expression `std::move(a)` is xvaluestatic_assert(is_xvalue<decltype((std::move(a)))>::value);// Type of variable `r` is rvalue referencestatic_assert(std::is_rvalue_reference<decltype(r)>::value);// Type of variable `b` is lvalue referencestatic_assert(std::is_lvalue_reference<decltype(b)>::value);// Expression `r` is lvaluestatic_assert(is_lvalue<decltype((r))>::value);}Primary categories
lvalue
The following expressions are lvalue expressions:
the name of a variable, a function, a
(since C++20), or a data member, regardless of type, such as std::cin or std::endl. Even if the variable's type is rvalue reference, the expression consisting of its name is an lvalue expression (but see
);
Extended contentvoidfoo(){}voidbaz(){// `foo` is lvalue// address may be taken by built-in address-of operatorvoid(*p)()=&foo;}structfoo{};template<fooa>voidbaz(){constfoo*obj=&a;// `a` is an lvalue, template parameter object}a function call or an overloaded operator expression, whose return type is lvalue reference, such as std::getline(std::cin,str), std::cout<<1, str1=str2, or ++it;
Extended contentint&a_ref(){staticinta{3};returna;}voidfoo(){a_ref()=5;// `a_ref()` is lvalue, function call whose return type is lvalue reference}a=b, a+=b, a%=b, and all other built-in
assignment and compound assignment
expressions;
++a and --a, the built-in
pre-increment and pre-decrement
expressions;
*p, the built-in
expression;
a[n] and p[n], the built-in
expressions, where one operand in a[n] is an array lvalue(since C++11);
a.m, the
expression, except where m is a member enumerator or a non-static member function, or where a is an rvalue and m is a non-static data member of object type;
Extended contentstructfoo{enumbar{m// member enumerator};};voidbaz(){fooa;a.m=42;// ill-formed, lvalue required as left operand of assignment}structfoo{voidm(){}// non-static member function};voidbaz(){fooa;// `a.m` is a prvalue, hence the address cannot be taken by built-in// address-of operatorvoid(foo::*p1)()=&a.m;// ill-formedvoid(foo::*p2)()=&foo::m;// OK: pointer to member function}structfoo{staticvoidm(){}// static member function};voidbaz(){fooa;void(*p1)()=&a.m;// `a.m` is an lvaluevoid(*p2)()=&foo::m;// the same}p->m, the built-in
expression, except where m is a member enumerator or a non-static member function;
a.*mp, the
expression, where a is an lvalue and mp is a pointer to data member;
p->*mp, the built-in
expression, where mp is a pointer to data member;
a,b, the built-in
expression, where b is an lvalue;
a?b:c, the
expression for certain b and c (e.g., when both are lvalues of the same type, but see
for detail);
a
, such as "Hello, world!";
a cast expression to lvalue reference type, such as static_cast<int&>(x) or static_cast<void(&)(int)>(x);
a non-type
of an lvalue reference type;
template<int&v>voidset(){v=5;// template parameter is lvalue}inta{3};// static variable, fixed address is known at compile-timevoidfoo(){set<a>();}a function call or an overloaded operator expression, whose return type is rvalue reference to function;
a cast expression to rvalue reference to function type, such as static_cast<void(&&)(int)>(x).
(since C++11)Properties:
Same as
(below).
Address of an lvalue may be taken by built-in address-of operator: &++i
and &std::endl are valid expressions.
A modifiable lvalue may be used as the left-hand operand of the built-in assignment and compound assignment operators.
An lvalue may be used to
initialize an lvalue reference
; this associates a new name with the object identified by the expression.
prvalue
The following expressions are prvalue expressions:
a
(except for
), such as 42, true or nullptr;
a function call or an overloaded operator expression, whose return type is non-reference, such as str.substr(1,2), str1+str2, or it++;
a++ and a--, the built-in
post-increment and post-decrement
expressions;
a+b, a%b, a&b, a<<b, and all other built-in
expressions;
a&&b, a||b, !a, the built-in
expressions;
a<b, a==b, a>=b, and all other built-in
expressions;
&a, the built-in
expression;
a.m, the
expression, where m is a member enumerator or a non-static member function
;
p->m, the built-in
expression, where m is a member enumerator or a non-static member function
;
a.*mp, the
expression, where mp is a pointer to member function
;
p->*mp, the built-in
expression, where mp is a pointer to member function
;
a,b, the built-in
expression, where b is an prvalue;
a?b:c, the
expression for certain b and c (see
for detail);
a cast expression to non-reference type, such as static_cast<double>(x), std::string{}, or (int)42;
the
pointer;
an
;
a non-type
of a scalar type;
template<intv>voidfoo(){// not an lvalue, `v` is a template parameter of scalar type intconstint*a=&v;// ill-formedv=3;// ill-formed: lvalue required as left operand of assignment}a
, such as [](intx){returnx*x;};
(since C++11)a
, such as requires(Ti){typenameT::type;};
a specialization of a
, such as std::equality_comparable<int>.
(since C++20)Properties:
Same as
(below).
A prvalue cannot be
: the
of the object it denotes is always the type of the expression.
A non-class non-array prvalue cannot be
. (Note: a function call or cast expression may result in a prvalue of non-class cv-qualified type, but the cv-qualifier is
immediately stripped out.)
A prvalue cannot have
(except for type void, see below, or when used in
specifier).
A prvalue cannot have
or an array thereof.
xvalue
The following expressions are xvalue expressions:
a.m, the
expression, where a is an rvalue and m is a non-static data member of an object type;
a.*mp, the
expression, where a is an rvalue and mp is a pointer to data member;
a,b, the built-in
expression, where b is an xvalue;
a?b:c, the
expression for certain b and c (see
for detail);
a function call or an overloaded operator expression, whose return type is rvalue reference to object, such as std::move(x);
a[n], the built-in
expression, where one operand is an array rvalue;
a cast expression to rvalue reference to object type, such as static_cast<char&&>(x);
(since C++11)any expression that designates a temporary object, after
;
(since C++17)a
.
(since C++23)Properties:
Same as rvalue (below).
Same as glvalue (below).
In particular, like all rvalues, xvalues bind to rvalue references, and like all glvalues, xvalues may be
, and non-class xvalues may be
.
Extended contentRun this code
#include<type_traits>template<classT>structis_prvalue:std::true_type{};template<classT>structis_prvalue<T&>:std::false_type{};template<classT>structis_prvalue<T&&>:std::false_type{};template<classT>structis_lvalue:std::false_type{};template<classT>structis_lvalue<T&>:std::true_type{};template<classT>structis_lvalue<T&&>:std::false_type{};template<classT>structis_xvalue:std::false_type{};template<classT>structis_xvalue<T&>:std::false_type{};template<classT>structis_xvalue<T&&>:std::true_type{};// Example from C++23 standard: 7.2.1 Value category [basic.lval]structA{intm;};A&&operator+(A,A);A&&f();intmain(){Aa;A&&ar=static_cast<A&&>(a);// Function call with return type rvalue reference is xvaluestatic_assert(is_xvalue<decltype((f()))>::value);// Member of object expression, object is xvalue, `m` is a non-static data memberstatic_assert(is_xvalue<decltype((f().m))>::value);// A cast expression to rvalue referencestatic_assert(is_xvalue<decltype((static_cast<A&&>(a)))>::value);// Operator expression, whose return type is rvalue reference to objectstatic_assert(is_xvalue<decltype((a+a))>::value);// Expression `ar` is lvalue, `&ar` is validstatic_assert(is_lvalue<decltype((ar))>::value);[[maybe_unused]]A*ap=&ar;}Mixed categories
glvalue
A glvalue expression is either lvalue or xvalue.
Properties:
A glvalue may be implicitly converted to a prvalue with lvalue-to-rvalue, array-to-pointer, or function-to-pointer
.
A glvalue may be
: the
of the object it identifies is not necessarily the static type of the expression.
A glvalue can have
, where permitted by the expression.
rvalue
An rvalue expression is either prvalue or xvalue.
Properties:
Address of an rvalue cannot be taken by built-in address-of operator: &int(), &i++
, &42, and &std::move(x) are invalid.
An rvalue can't be used as the left-hand operand of the built-in assignment or compound assignment operators.
An rvalue may be used to
initialize a const lvalue reference
, in which case the lifetime of the temporary object identified by the rvalue is
until the scope of the reference ends.
An rvalue may be used to
initialize an rvalue reference
, in which case the lifetime of the temporary object identified by the rvalue is
until the scope of the reference ends.
When used as a function argument and when
of the function are available, one taking rvalue reference parameter and the other taking lvalue reference to const parameter, an rvalue binds to the rvalue reference overload (thus, if both copy and move constructors are available, an rvalue argument invokes the
, and likewise with copy and move assignment operators).
(since C++11)Special categories
Pending member function call
The expressions a.mf and p->mf, where mf is a
, and the expressions a.*pmf and p->*pmf, where pmf is a
, are classified as prvalue expressions, but they cannot be used to initialize references, as function arguments, or for any purpose at all, except as the left-hand argument of the function call operator, e.g. (p->*pmf)(args).
Void expressions
Function call expressions returning void, cast expressions to void, and
are classified as prvalue expressions, but they cannot be used to initialize references or as function arguments. They can be used in discarded-value contexts (e.g. on a line of its own, as the left-hand operand of the comma operator, etc.) and in the return statement in a function returning void. In addition, throw-expressions may be used as the second and the third operands of the
.
Void expressions have no result object.
(since C++17)Bit-fields
An expression that designates a
(e.g. a.m, where a is an lvalue of type struct A { int m: 3; }) is a glvalue expression: it may be used as the left-hand operand of the assignment operator, but its address cannot be taken and a non-const lvalue reference cannot be bound to it. A const lvalue reference or rvalue reference can be initialized from a bit-field glvalue, but a temporary copy of the bit-field will be made: it won't bind to the bit-field directly.
Move-eligible expressions
Although an expression consisting of the name of any variable is an lvalue expression, such expression may be move-eligible if it appears as the operand of
a
statement
a
statement (since C++20)
a
expression (since C++17)
If an expression is move-eligible, it is treated either as an rvalue or as an lvalue(until C++23)as an rvalue(since C++23) for the purpose of
(thus it may select the
). See
Automatic move from local variables and parameters
for details.
(since C++11)History
CPL
The programming language
was first to introduce value categories for expressions: all CPL expressions can be evaluated in "right-hand mode", but only certain kinds of expression are meaningful in "left-hand mode". When evaluated in right-hand mode, an expression is regarded as being a rule for the computation of a value (the right-hand value, or rvalue). When evaluated in left-hand mode an expression effectively gives an address (the left-hand value, or lvalue). "Left" and "Right" here stood for "left of assignment" and "right of assignment".
C
The C programming language followed a similar taxonomy, except that the role of assignment was no longer significant: C expressions are categorized between "lvalue expressions" and others (functions and non-object values), where "lvalue" means an expression that identifies an object, a "locator value"
.
C++98
Pre-2011 C++ followed the C model, but restored the name "rvalue" to non-lvalue expressions, made functions into lvalues, and added the rule that references can bind to lvalues, but only references to const can bind to rvalues. Several non-lvalue C expressions became lvalue expressions in C++.
C++11
With the introduction of move semantics in C++11, value categories were redefined to characterize two independent properties of expressions
:
has identity: it's possible to determine whether the expression refers to the same entity as another expression, such as by comparing addresses of the objects or the functions they identify (obtained directly or indirectly);
can be moved from:
,
, or another function overload that implements move semantics can bind to the expression.
In C++11, expressions that:
have identity and cannot be moved from are called lvalue expressions;
have identity and can be moved from are called xvalue expressions;
do not have identity and can be moved from are called prvalue ("pure rvalue") expressions;
do not have identity and cannot be moved from are not used
.
The expressions that have identity are called "glvalue expressions" (glvalue stands for "generalized lvalue"). Both lvalues and xvalues are glvalue expressions.
The expressions that can be moved from are called "rvalue expressions". Both prvalues and xvalues are rvalue expressions.
C++17
In C++17,
was made mandatory in some situations, and that required separation of prvalue expressions from the temporary objects initialized by them, resulting in the system we have today. Note that, in contrast with the C++11 scheme, prvalues are no longer moved from.
Footnotes
Assuming i has built-in type or the pre-increment operator is
to return by lvalue reference.
↑
Special rvalue category, see
.
Assuming i has built-in type or the post-increment operator is not
to return by lvalue reference.
"A difference of opinion within the C community centered around the meaning of lvalue, one group considering an lvalue to be any kind of object locator, another group holding that an lvalue is meaningful on the left side of an assigning operator. The C89 Committee adopted the definition of lvalue as an object locator." -- ANSI C Rationale, 6.3.2.1/10.
by Bjarne Stroustrup, 2010.
const prvalues (only allowed for class types) and const xvalues do not bind to T&& overloads, but they bind to the const T&& overloads, which are also classified as "move constructor" and "move assignment operator" by the standard, satisfying the definition of "can be moved from" for the purpose of this classification. However, such overloads cannot modify their arguments and are not used in practice; in their absence const prvalues and const xvalues bind to const T& overloads.
References
C++23 standard (ISO/IEC 14882:2024):
7.2.1 Value category [basic.lval]
C++20 standard (ISO/IEC 14882:2020):
7.2.1 Value category [basic.lval]
C++17 standard (ISO/IEC 14882:2017):
6.10 Lvalues and rvalues [basic.lval]
C++14 standard (ISO/IEC 14882:2014):
3.10 Lvalues and rvalues [basic.lval]
C++11 standard (ISO/IEC 14882:2011):
3.10 Lvalues and rvalues [basic.lval]
C++98 standard (ISO/IEC 14882:1998):
3.10 Lvalues and rvalues [basic.lval]
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++11 member access and member access through
pointer to member of an rvalue resulted in prvalue reclassified as xvalue
C++11 array prvalues could not be cv-qualified allowed
C++11 subscripting an array rvalue resulted in lvalue reclassified as xvalue See also
External links