Constructs a
(an unnamed function object capable of capturing variables in scope).
Syntax
Lambda expressions without an explicit template parameter list (possibly non-generic)[captures]front-attr(optional)(params)specs(optional)except(optional)
back-attr(optional)trailing(optional)requires(optional)contract-specs(optional){body}(1) [captures] {body}(2) (until C++23)[captures]front-attr(optional)trailing(optional)contract-specs(optional){body}(2) (since C++23)[captures]front-attr(optional)except
back-attr(optional)trailing(optional)contract-specs(optional){body}(3) (since C++23)[captures]front-attr(optional)specsexcept(optional)
back-attr(optional)trailing(optional)contract-specs(optional){body}(4) (since C++23)Lambda expressions with an explicit template parameter list (always generic) (since C++20)[captures] <tparams>t-requires(optional)
front-attr(optional)(params)specs(optional)except(optional)
back-attr(optional)trailing(optional)requires(optional)contract-specs(optional){body}(1) [captures] <tparams>t-requires(optional){body}(2) (until C++23)[captures] <tparams>t-requires(optional)
front-attr(optional)trailing(optional)contract-specs(optional){body}(2) (since C++23)[captures] <tparams>t-requires(optional)front-attr(optional)except
back-attr(optional)trailing(optional)contract-specs(optional){body}(3) (since C++23)[captures] <tparams>t-requires(optional)front-attr(optional)specsexcept(optional)
back-attr(optional)trailing(optional)contract-specs(optional){body}(4) (since C++23)1) The lambda expression with a parameter list.
2-4) The lambda expression without a parameter list.
2) The simplest syntax. back-attr cannot be applied.
3,4)back-attr can only be applied if any of specs and except is present.
Explanation
captures- Specifies the entities to be
. tparams- A non-empty comma-separated list of
, used to provide names to the template parameters of a generic lambda (see ClosureType::operator() below). t-requires- Adds
to tparams. If t-requires ends with an attribute specifier sequence, the attributes in the sequence are treated as attributes in front-attr.
(since C++23)front-attr- (since C++23) An
applies to operator() of the closure type (and thus the [[
]] attribute can be used). params- The
of operator() of the closure type. It can have an
.
(since C++23)specs- A list of the following specifiers, each specifier is allowed at most once in each sequence. Specifier Effect mutableAllows body to modify the objects captured by copy, and to call their non-const member functions. Cannot be used if an explicit object parameter is present.
(since C++23)constexpr
(since C++17)Explicitly specifies that operator() is a
. If operator() satisfy all constexpr function requirements, operator() will be constexpr even if constexpr is not present.
consteval
(since C++20)Specifies that operator() is an
. consteval and constexpr cannot be specified at the same time.
static
(since C++23)Specifies that operator() is a
. static and mutable cannot be specified at the same time.
Cannot be used if captures is not empty, or an explicit object parameter is present.
except- Provides the
dynamic exception specification
or(until C++17) the
for operator() of the closure type. back-attr- An
applies to the type of operator() of the closure type (and thus the [[
]] attribute cannot be used). trailing- ->ret, where ret specifies the return type. requires- (since C++20) Adds
to operator() of the closure type. contract-specs- (since C++26) A list of
for operator() of the closure type. body- The function body. If
is used as a type of a parameter or an explicit template parameter list is provided(since C++20), the lambda is a generic lambda.
(since C++14)A variable __func__ is implicitly defined at the beginning of body, with semantics as described
.
Closure type
The lambda expression is a prvalue expression of unique unnamed non-
non-
class type, known as closure type, which is declared (for the purposes of
) in the smallest block scope, class scope, or namespace scope that contains the lambda expression.
The closure type is a
type if and only if captures is empty.
(since C++20)The closure type has the following members, they cannot be
,
, or(since C++14) named in a
:
ClosureType::operator()(params)
retoperator()(params){body}(static and const may be present, see below)template<template-params>retoperator()(params){body}(since C++14)
(generic lambda, static and const may be present, see below)Executes the body of the lambda expression, when invoked. When accessing a variable, accesses its captured copy (for the entities captured by copy), or the original object (for the entities captured by reference).
The parameter list of operator() is params if it is provided, otherwise the parameter list is empty.
The return type of operator() is the type specified in trailing .
If trailing is not provided, the return type of operator() is automatically
.
Unless the keyword mutable was used in the lambda specifiers, or an explicit object parameter is present(since C++23), the cv-qualifier of operator() is const and the objects that were captured by copy are non-modifiable from inside this operator(). Explicit const qualifier is not allowed. operator() is never virtual and cannot have the volatile qualifier.
operator() is always constexpr if it satisfies the requirements of a
. It is also constexpr if the keyword constexpr was used in the lambda specifiers.
(since C++17)operator() is an
if the keyword consteval was used in the lambda specifiers.
(since C++20)operator() is a
if the keyword static was used in the lambda specifiers.
operator() is an
explicit object member function
if params contains an explicit object parameter.
(since C++23)For each parameter in params whose type is specified as auto, an invented template parameter is added to template-params, in order of appearance. The invented template parameter may be a
if the corresponding function member of params is a function parameter pack.
// generic lambda, operator() is a template with two parametersautoglambda=[](autoa,auto&&b){returna<b;};boolb=glambda(3,3.14);// OK// generic lambda, operator() is a template with one parameterautovglambda=[](autoprinter){return[=](auto&&...ts)// generic lambda, ts is a parameter pack{printer(std::forward<decltype(ts)>(ts)...);// nullary lambda (takes no parameters):return[=]{printer(ts...);};};};autop=vglambda([](autov1,autov2,autov3){std::cout<<v1<<v2<<v3;});autoq=p(1,'a',3.14);// outputs 1a3.14q();// outputs 1a3.14(since C++14)If the lambda definition uses an explicit template parameter list, that template parameter list is used with operator(). For each parameter in params whose type is specified as auto, an additional invented template parameter is appended to the end of that template parameter list:
// generic lambda, operator() is a template with two parametersautoglambda=[]<classT>(Ta,auto&&b){returna<b;};// generic lambda, operator() is a template with one parameter packautof=[]<typename...Ts>(Ts&&...ts){returnfoo(std::forward<Ts>(ts)...);};(since C++20)The exception specification except on the lambda expression applies to operator().
For the purpose of
, determining the type and value of the
and for accessing non-static class members, the body of the closure type's operator() is considered in the context of the lambda expression.
structX{intx,y;intoperator()(int);voidf(){// the context of the following lambda is the member function X::f[=]()->int{returnoperator()(this->x+y);// X::operator()(this->x + (*this).y)// this has type X*};}};Dangling references
If a non-reference entity is captured by reference, implicitly or explicitly, and operator() of the closure object is invoked after the entity's lifetime has ended, undefined behavior occurs. The C++ closures do not extend the lifetimes of objects captured by reference.
Same applies to the lifetime of the current *this object captured via this.
Although function return type deduction is introduced in C++14, its rule is available for lambda return type deduction in C++11.
ClosureType::operator ret(*)(params)()
capture-less non-generic lambda
usingF=ret(*)(params);operatorF()constnoexcept;(until C++17)usingF=ret(*)(params);constexproperatorF()constnoexcept;(since C++17)capture-less generic lambda
template<template-params>usingfptr_t=/* see below */;template<template-params>operatorfptr_t<template-params>()constnoexcept;(since C++14)
(until C++17)template<template-params>usingfptr_t=/* see below */;template<template-params>constexproperatorfptr_t<template-params>()constnoexcept;(since C++17)This
user-defined conversion function
is only defined if the lambda expression has no captures and has no explicit object parameter(since C++23). It is a public, constexpr,(since C++17) non-virtual, non-explicit, const noexcept member function of the closure object.
This function is an
if the function call operator (or specialization, for generic lambdas) is an immediate function.
(since C++20)A generic capture-less lambda has a user-defined conversion function template with the same invented template parameter list as operator().
voidf1(int(*)(int)){}voidf2(char(*)(int)){}voidh(int(*)(int)){}// #1voidh(char(*)(int)){}// #2autoglambda=[](autoa){returna;};f1(glambda);// OKf2(glambda);// error: not convertibleh(glambda);// OK: calls #1 since #2 is not convertibleint&(*fpi)(int*)=[](auto*a)->auto&{return*a;};// OK(since C++14)The value returned by the conversion function is a pointer to a function with C++
that, when invoked, has the same effect as invoking the closure type's function call operator on a default-constructed instance of the closure type.
(until C++14)The value returned by the conversion function (template) is a pointer to a function with C++
that, when invoked, has the same effect as:
for non-generic lambdas, invoking the closure type's operator() on a default-constructed instance of the closure type.
for generic lambdas, invoking the generic lambda's corresponding operator() specialization on a default-constructed instance of the closure type.
(since C++14)
(until C++23)The value returned by the conversion function (template) is
if operator() is static, a pointer to that operator() with C++
,
otherwise, a pointer to a function with C++
that, when invoked, has the same effect as: for non-generic lambdas, invoking the closure type's operator() on a default-constructed instance of the closure type.
for generic lambdas, invoking the generic lambda's corresponding operator() specialization on a default-constructed instance of the closure type.
(since C++23)This function is constexpr if the function call operator (or specialization, for generic lambdas) is constexpr.
autoFwd=[](int(*fp)(int),autoa){returnfp(a);};autoC=[](autoa){returna;};static_assert(Fwd(C,3)==3);// OKautoNC=[](autoa){staticints;returna;};static_assert(Fwd(NC,3)==3);// error: no specialization can be// constexpr because of static sIf the closure object's operator() has a non-throwing exception specification, then the pointer returned by this function has the type pointer to noexcept function.
(since C++17)ClosureType::ClosureType()
ClosureType()=default;(since C++20)
(only if no captures are specified)ClosureType(constClosureType&)=default;ClosureType(ClosureType&&)=default;Closure types are not
. Closure types have no default constructor.
(until C++20)If no captures are specified, the closure type has a defaulted default constructor. Otherwise, it has no default constructor (this includes the case when there is a capture-default , even if it does not actually capture anything).
(since C++20)The copy constructor and the move constructor are declared as defaulted and may be implicitly-defined according to the usual rules for
and
.
ClosureType::operator=(const ClosureType&)
ClosureType&operator=(constClosureType&)=delete;(until C++20)ClosureType&operator=(constClosureType&)=default;ClosureType&operator=(ClosureType&&)=default;(since C++20)
(only if no captures are specified)ClosureType&operator=(constClosureType&)=delete;(since C++20)
(otherwise)The copy assignment operator is defined as deleted (and the move assignment operator is not declared). Closure types are not
.
(until C++20)If no captures are specified, the closure type has a defaulted copy assignment operator and a defaulted move assignment operator. Otherwise, it has a deleted copy assignment operator (this includes the case when there is a capture-default , even if it does not actually capture anything).
(since C++20)ClosureType::~ClosureType()
~ClosureType()=default;The destructor is implicitly-declared.
ClosureType::Captures
T1a;T2b;...If the lambda expression captures anything by copy (either implicitly with capture clause [=] or explicitly with a capture that does not include the character &, e.g. [a, b, c]), the closure type includes unnamed non-static data members, declared in unspecified order, that hold copies of all entities that were so captured.
Those data members that correspond to captures without initializers are
when the lambda expression is evaluated. Those that correspond to captures with initializers are initialized as the initializer requires (could be copy- or direct-initialization). If an array is captured, array elements are direct-initialized in increasing index order. The order in which the data members are initialized is the order in which they are declared (which is unspecified).
The type of each data member is the type of the corresponding captured entity, except if the entity has reference type (in that case, references to functions are captured as lvalue references to the referenced functions, and references to objects are captured as copies of the referenced objects).
For the entities that are captured by reference (with the capture-default[&] or when using the character &, e.g. [&a, &b, &c]), it is unspecified if additional data members are declared in the closure type, but any such additional members must satisfy
(since C++17).
Lambda capture
The captures defines the outside variables that are accessible from within the lambda function body. Its syntax is defined as follows:
capture-default(1) capture-list(2) capture-default,capture-list(3) capture-default- one of & and =capture-list- a comma-separated list of capture s
The syntax of capture is defined as follows:
identifier(1) identifier...(2) identifierinitializer(3) (since C++14)&identifier(4) &identifier...(5) &identifierinitializer(6) (since C++14)this(7) *this(8) (since C++17)...identifierinitializer(9) (since C++20)&...identifierinitializer(10) (since C++20)1) simple by-copy capture
4) simple by-reference capture
6) by-reference capture with an initializer
7) simple by-reference capture of the current object
8) simple by-copy capture of the current object
9) by-copy capture with an initializer that is a pack expansion
10) by-reference capture with an initializer that is a pack expansion
If the capture-default is &, subsequent simple captures must not begin with &.
structS2{voidf(inti);};voidS2::f(inti){[&]{};// OK: by-reference capture default[&,i]{};// OK: by-reference capture, except i is captured by copy[&,&i]{};// Error: by-reference capture when by-reference is the default[&,this]{};// OK, equivalent to [&][&,this,i]{};// OK, equivalent to [&, i]}If the capture-default is =, subsequent simple captures must begin with & or be *this(since C++17)or this(since C++20).
structS2{voidf(inti);};voidS2::f(inti){[=]{};// OK: by-copy capture default[=,&i]{};// OK: by-copy capture, except i is captured by reference[=,*this]{};// until C++17: Error: invalid syntax// since C++17: OK: captures the enclosing S2 by copy[=,this]{};// until C++20: Error: this when = is the default// since C++20: OK, same as [=]}Any capture may appear only once, and its name must be different from any parameter name:
structS2{voidf(inti);};voidS2::f(inti){[i,i]{};// Error: i repeated[this,*this]{};// Error: "this" repeated (C++17)[i](inti){};// Error: parameter and capture have the same name}A lambda expression can use a variable without capturing it if the variable
is a non-local variable or has static or thread local
(in which case the variable cannot be captured), or
is a reference that has been initialized with a
.
A lambda expression can read the value of a variable without capturing it if the variable
has const non-volatile integral or enumeration type and has been initialized with a
, or
is constexpr and has no mutable members.
The current object (*this) can be implicitly captured if either capture default is present. If implicitly captured, it is always captured by reference, even if the capture default is =. The implicit capture of *this when the capture default is = is deprecated.(since C++20)
Only lambda expressions satisfying any of the following conditions may have a capture-default or capture without initializers:
Its innermost
is a
.
It appears within a
, and its innermost enclosing scope is the corresponding
.
For such lambda expression, the reaching scope is defined as the set of enclosing scopes up to and including the innermost enclosing function (and its parameters). This includes nested block scopes and the scopes of enclosing lambdas if this lambda is nested.
The identifier in any capture without an initializer (other than the this-capture) is looked up using usual
in the reaching scope of the lambda. The result of the lookup must be a
with automatic storage duration declared in the reaching scope, or a
whose corresponding variable satisfies such requirements(since C++20). The entity is explicitly captured.
A capture with an initializer, called init-capture, acts as if it declares and explicitly captures a variable declared with type specifier
and the same initializer, whose declarative region is the body of the lambda expression (that is, it is not in scope within its initializer), except that:
if the capture is by-copy, the introduced non-static data member of the closure object is another way to refer to that variable; in other words, the source variable does not actually exist, and the type deduction via auto and the initialization are applied to the non-static data member;
if the capture is by-reference, the reference variable's lifetime ends when the lifetime of the closure object ends.
This is used to capture move-only types with a capture such as x=std::move(x).
This also makes it possible to capture by const reference, with &cr=std::as_const(x) or similar.
intx=4;autoy=[&r=x,x=x+1]()->int{r+=2;returnx*x;}();// updates ::x to 6 and initializes y to 25.(since C++14)If captures has a capture-default and does not explicitly capture the enclosing object (as this or *this), or an automatic variable that is
in the lambda body, or a
whose corresponding variable has atomic storage duration(since C++20), it captures the entity implicitly if the entity is named in a
expression within an expression (including when the implicit this-> is added before a use of non-static class member).
For the purpose of determining implicit captures,
is never considered to make its operands unevaluated.
Entities might be implicitly captured even if they are only named within a
after instantiation of the lambda body.
(since C++17)voidf(int,constint(&)[2]={}){}// #1voidf(constint&,constint(&)[1]){}// #2structNoncopyableLiteralType{constexprexplicitNoncopyableLiteralType(intn):n_(n){}NoncopyableLiteralType(constNoncopyableLiteralType&)=delete;intn_;};voidtest(){constintx=17;autol0=[]{f(x);};// OK: calls #1, does not capture xautog0=[](autoa){f(x);};// same as aboveautol1=[=]{f(x);};// OK: captures x (since P0588R1) and calls #1// the capture can be optimized awayautog1=[=](autoa){f(x);};// same as aboveautoltid=[=]{typeid(x);};// OK: captures x (since P0588R1)// even though x is unevaluated// the capture can be optimized awayautog2=[=](autoa){intselector[sizeof(a)==1?1:2]={};f(x,selector);// OK: is a dependent expression, so captures x};autog3=[=](autoa){typeid(a+x);// captures x regardless of// whether a + x is an unevaluated operand};constexprNoncopyableLiteralTypew{42};autol4=[]{returnw.n_;};// OK: w is not odr-used, capture is unnecessary// auto l5 = [=]{ return w.n_; }; // error: w needs to be captured by copy}If the body of a lambda
an entity captured by copy, the member of the closure type is accessed. If it is not odr-using the entity, the access is to the original object:
voidf(constint*);voidg(){constintN=10;[=]{intarr[N];// not an odr-use: refers to g's const int Nf(&N);// odr-use: causes N to be captured (by copy)// &N is the address of the closure object's member N, not g's N}();}If a lambda odr-uses a reference that is captured by reference, it is using the object referred-to by the original reference, not the captured reference itself:
Run this code
#include<iostream>automake_function(int&x){return[&]{std::cout<<x<<'\n';};}intmain(){inti=3;autof=make_function(i);// the use of x in f binds directly to ii=5;f();// OK: prints 5}Within the body of a lambda with capture default =, the type of any capturable entity is as if it were captured (and thus const-qualification is often added if the lambda is not mutable), even though the entity is in an unevaluated operand and not captured (e.g. in
):
voidf3(){floatx,&r=x;[=]{// x and r are not captured (appearance in a decltype operand is not an odr-use)decltype(x)y1;// y1 has type floatdecltype((x))y2=y1;// y2 has type float const& because this lambda// is not mutable and x is an lvaluedecltype(r)r1=y1;// r1 has type float& (transformation not considered)decltype((r))r2=y2;// r2 has type float const&};}Any entity captured by a lambda (implicitly or explicitly) is odr-used by the lambda expression (therefore, implicit capture by a nested lambda triggers implicit capture in the enclosing lambda).
All implicitly-captured variables must be declared within the reaching scope of the lambda.
If a lambda captures the enclosing object (as this or *this), either the nearest enclosing function must be a non-static member function or the lambda must be in a
:
structs2{doubleohseven=.007;autof()// nearest enclosing function for the following two lambdas{return[this]// capture the enclosing s2 by reference{return[*this]// capture the enclosing s2 by copy (C++17){returnohseven;// OK}}();}autog(){return[]// capture nothing{return[*this]{};// error: *this not captured by outer lambda expression}();}};If a lambda expression (or a specialization of a generic lambda's function call operator)(since C++14) ODR-uses *this or any variable with automatic storage duration, it must be captured by the lambda expression.
voidf1(inti){intconstN=20;autom1=[=]{intconstM=30;autom2=[i]{intx[N][M];// N and M are not odr-used // (ok that they are not captured)x[0][0]=i;// i is explicitly captured by m2// and implicitly captured by m1};};structs1// local class within f1(){intf;voidwork(intn)// non-static member function{intm=n*n;intj=40;autom3=[this,m]{autom4=[&,j]// error: j is not captured by m3{intx=n;// error: n is implicitly captured by m4// but not captured by m3x+=m;// OK: m is implicitly captured by m4// and explicitly captured by m3x+=i;// error: i is outside of the reaching scope// (which ends at work())x+=f;// OK: this is captured implicitly by m4// and explicitly captured by m3};};}};}Class members cannot be captured explicitly by a capture without initializer (as mentioned above, only
are permitted in the capture-list ):
classS{intx=0;voidf(){inti=0;// auto l1 = [i, x] { use(i, x); }; // error: x is not a variableautol2=[i,x=x]{use(i,x);};// OK, copy capturei=1;x=1;l2();// calls use(0,0)autol3=[i,&x=x]{use(i,x);};// OK, reference capturei=2;x=2;l3();// calls use(1,2)}};When a lambda captures a member using implicit by-copy capture, it does not make a copy of that member variable: the use of a member variable m is treated as an expression (*this).m, and *this is always implicitly captured by reference:
classS{intx=0;voidf(){inti=0;autol1=[=]{use(i,x);};// captures a copy of i and// a copy of the this pointeri=1;x=1;l1();// calls use(0, 1), as if// i by copy and x by referenceautol2=[i,this]{use(i,x);};// same as above, made expliciti=2;x=2;l2();// calls use(1, 2), as if// i by copy and x by referenceautol3=[&]{use(i,x);};// captures i by reference and// a copy of the this pointeri=3;x=2;l3();// calls use(3, 2), as if// i and x are both by referenceautol4=[i,*this]{use(i,x);};// makes a copy of *this,// including a copy of xi=4;x=4;l4();// calls use(3, 2), as if// i and x are both by copy}};If a lambda expression appears in a
, it cannot explicitly or implicitly capture anything, unless all captures have initializers which satisfy the constraints of an expression appearing in a default argument(since C++14):
voidf2(){inti=1;voidg1(int=[i]{returni;}());// error: captures somethingvoidg2(int=[i]{return0;}());// error: captures somethingvoidg3(int=[=]{returni;}());// error: captures somethingvoidg4(int=[=]{return0;}());// OK: capture-lessvoidg5(int=[]{returnsizeofi;}());// OK: capture-less// C++14voidg6(int=[x=1]{returnx;}());// OK: 1 can appear// in a default argumentvoidg7(int=[x=i]{returnx;}());// error: i cannot appear// in a default argument}Members of
members cannot be captured.
can only be captured by copy.
If a nested lambda m2 captures something that is also captured by the immediately enclosing lambda m1, then m2's capture is transformed as follows:
if the enclosing lambda m1 captures by copy, m2 is capturing the non-static member of m1's closure type, not the original variable or *this; if m1 is not mutable, the non-static data member is considered to be const-qualified.
if the enclosing lambda m1 captures by reference, m2 is capturing the original variable or *this.
Run this code
#include <iostream> int main() { int a = 1, b = 1, c = 1; auto m1 = [a, &b, &c]() mutable { auto m2 = [a, b, &c]() mutable { std::cout << a << b << c << '\n'; a = 4; b = 4; c = 4; }; a = 3; b = 3; c = 3; m2(); }; a = 2; b = 2; c = 2; m1(); // calls m2() and prints 123 std::cout << a << b << c << '\n'; // prints 234 }If a lambda captures anything, the type of the explicit object parameter (if any) of the function call operator can only be
the closure type,
a class type publicly and unambiguously derived from the closure type, or
a reference to a possibly cv-qualified such type.
struct C { template<typename T> C(T); }; void func(int i) { int x = [=](this auto&&) { return i; }(); // OK int y = [=](this C) { return i; }(); // error int z = [](this C) { return 42; }(); // OK auto lambda = [n = 42] (this auto self) { return n; }; using Closure = decltype(lambda); struct D : private Closure { D(Closure l) : Closure(l) {} using Closure::operator(); friend Closure; }; D{lambda}(); // error }(since C++23)Notes
Feature-test macro ValueStdFeature
(C++11)Lambda expressions
(C++14)Generic lambda expressions
(C++20)Explicit template parameter list for generic lambdas
(C++14)Lambda init-capture
(C++20)Allow pack expansion in lambda init-capture
(C++17)Lambda capture of *this by value as [=, *this]
(C++17)constexpr lambda
(C++23)static operator() for capture-less lambdas The rule for implicit lambda capture is slightly changed by defect report
. As of 2023-10, some major implementations have not completely implemented the DR, and thus the old rule, which detects
, is still used in some cases.
Old rule before P0588R1If captures has a capture-default and does not explicitly capture the enclosing object (as this or *this), or an automatic variable that is
in the lambda body, or a
whose corresponding variable has atomic storage duration(since C++20), it captures the entity implicitly if the entity is
named in a
expression within an expression that depends on a template parameter of a generic lambda, or
(since C++14)
by the body of the lambda.
Example
This example shows how to pass a lambda to a generic algorithm and how objects resulting from a lambda expression can be stored in
objects.
Run this code
#include <algorithm> #include <functional> #include <iostream> #include <vector> int main() { std::vector<int> c{1, 2, 3, 4, 5, 6, 7}; int x = 5; c.erase(std::remove_if(c.begin(), c.end(), [x](int n) { return n < x; }), c.end()); std::cout << "c: "; std::for_each(c.begin(), c.end(), [](int i) { std::cout << i << ' '; }); std::cout << '\n'; // the type of a closure cannot be named, but can be inferred with auto // since C++14, lambda could own default arguments auto func1 = [](int i = 6) { return i + 4; }; std::cout << "func1: " << func1() << '\n'; // like all callable objects, closures can be captured in std::function // (this may incur unnecessary overhead) std::function<int(int)> func2 = [](int i) { return i + 4; }; std::cout << "func2: " << func2(6) << '\n'; constexpr int fib_max {8}; std::cout << "Emulate `recursive lambda` calls:\nFibonacci numbers: "; auto nth_fibonacci = [](int n) { std::function<int(int, int, int)> fib = [&](int n, int a, int b) { return n ? fib(n - 1, a + b, a) : b; }; return fib(n, 0, 1); }; for (int i{1}; i <= fib_max; ++i) std::cout << nth_fibonacci(i) << (i < fib_max ? ", " : "\n"); std::cout << "Alternative approach to lambda recursion:\nFibonacci numbers: "; auto nth_fibonacci2 = [](auto self, int n, int a = 0, int b = 1) -> int { return n ? self(self, n - 1, a + b, a) : b; }; for (int i{1}; i <= fib_max; ++i) std::cout << nth_fibonacci2(nth_fibonacci2, i) << (i < fib_max ? ", " : "\n"); #ifdef __cpp_explicit_this_parameter std::cout << "C++23 approach to lambda recursion:\n"; auto nth_fibonacci3 = [](this auto self, int n, int a = 0, int b = 1) -> int { return n ? self(n - 1, a + b, a) : b; }; for (int i{1}; i <= fib_max; ++i) std::cout << nth_fibonacci3(i) << (i < fib_max ? ", " : "\n"); #endif }Possible output:
c: 5 6 7 func1: 10 func2: 10 Emulate `recursive lambda` calls: Fibonacci numbers: 0, 1, 1, 2, 3, 5, 8, 13 Alternative approach to lambda recursion: Fibonacci numbers: 0, 1, 1, 2, 3, 5, 8, 13Defect 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 default argument was not allowed in the
parameter list of a lambda expression allowed
(
) C++11 the return type could only be deduced for lambda
bodies containing only one return statement improved the return
type deduction
C++11 it is not clear that whether the captured member of the
enclosing non-mutable lambda is considered const or not considered const
C++11 the language linkage of the returned function type of
the closure type's conversion function was not specified it has C++
language linkage
C++11 lambda expressions could appear in
function and function template signatures not allowed
C++11 members of anonymous unions could be captured not allowed
C++11 the conversion function for capture-less lambdas
had unspecified exception specification conversion function
is noexcept
C++11 the semantic of __func__ in lambda body was not clear it refers to the closure
class's operator()
C++14 it was unclear whether the members of the closure types of generic
lambdas can be explicitly instantiated or explicitly specialized neither is allowed
C++11 closure had a deleted default constructor
and implicit copy/move constructors no default and defaulted
copy/move constructors
C++11 as for the effect of invoking the result of the
conversion function, it was unspecified on which
object calling its operator() has the same effect on a default-constructed
instance of the closure type
C++11 the parameter list of the closure type's operator()
could refer to the parameter list given in trailingcan only refer
to params
C++11 for a reference captured by reference, it was unspecified
which entity the identifier of the capture refers to it refers to the originally
referenced entity
C++11 the behavior of capturing rvalue references
to functions by copy was not clear made clear
C++11 the behavior was unspecified if a capture
has the same name as a parameter the program is ill-
formed in this case
C++14 lambda expressions appearing in default arguments had
to be capture-less even if all captures are initialized with
expressions which can appear in default arguments allow such lambda
expressions with captures
C++17 each specifier could have multiple
occurrences in the specifier sequence each specifier can only
appear at most once in
the specifier sequence
C++23 a lambdas with explicit object parameter could have a
conversion function to an undesired function pointer type it does not have such
a conversion funtion
C++23 operator() with explicit parameter could be instantiated for
a derived class when the inheritance was not public or ambiguous made ill-formed
C++11 the rule for implicit lambda capture detected odr-use the detection is simplified See also
(C++11)specifies a type deduced from an expression
(C++11)
copyable wrapper of any copy constructible callable object
(class template)
(C++23)
move-only wrapper of any callable object that supports qualifiers in a given call signature
(class template)
External links
- a function which is defined within another (enclosing) function.