8 Expressions [expr]

8.1 Primary expressions [expr.prim]

8.1.5 Lambda expressions [expr.prim.lambda]

lambda-expression:
	lambda-introducer lambda-declarator compound-statement
lambda-introducer:
	[ lambda-capture ]
lambda-declarator:
	( parameter-declaration-clause ) decl-specifier-seq
	noexcept-specifier attribute-specifier-seq trailing-return-type
Lambda expressions provide a concise way to create simple function objects.
[Example
:
#include <algorithm>
#include <cmath>
void abssort(float* x, unsigned N) {
  std::sort(x, x + N, [](float a, float b) { return std::abs(a) < std::abs(b); });
}
end example
]
A lambda-expression is a prvalue whose result object is called the closure object.
A lambda-expression shall not appear in an unevaluated operand, in a template-argument, in an alias-declaration, in a typedef declaration, or in the declaration of a function or function template outside its function body and default arguments.
[Note
:
The intention is to prevent lambdas from appearing in a signature.
end note
]
[Note
:
A closure object behaves like a function object.
end note
]
In the decl-specifier-seq of the lambda-declarator, each decl-specifier shall either be mutable or constexpr.
If a lambda-expression does not include a lambda-declarator, it is as if the lambda-declarator were ().
The lambda return type is auto, which is replaced by the type specified by the trailing-return-type if provided and/or deduced from return statements as described in [dcl.spec.auto].
[Example
:
auto x1 = [](int i){ return i; };     // OK: return type is int
auto x2 = []{ return { 1, 2 }; };     // error: deducing return type from braced-init-list
int j;
auto x3 = []()->auto&& { return j; }; // OK: return type is int&
end example
]

8.1.5.1 Closure types [expr.prim.lambda.closure]

The type of a lambda-expression (which is also the type of the closure object) is a unique, unnamed non-union class type, called the closure type, whose properties are described below.
The closure type is declared in the smallest block scope, class scope, or namespace scope that contains the corresponding lambda-expression.
[Note
:
This determines the set of namespaces and classes associated with the closure type ([basic.lookup.argdep]).
The parameter types of a lambda-declarator do not affect these associated namespaces and classes.
end note
]
The closure type is not an aggregate type ([dcl.init.aggr]).
An implementation may define the closure type differently from what is described below provided this does not alter the observable behavior of the program other than by changing:
An implementation shall not add members of rvalue reference type to the closure type.
The closure type for a non-generic lambda-expression has a public inline function call operator whose parameters and return type are described by the lambda-expression's parameter-declaration-clause and trailing-return-type respectively.
For a generic lambda, the closure type has a public inline function call operator member template whose template-parameter-list consists of one invented type template-parameter for each occurrence of auto in the lambda's parameter-declaration-clause, in order of appearance.
The invented type template-parameter is a parameter pack if the corresponding parameter-declaration declares a function parameter pack ([dcl.fct]).
The return type and function parameters of the function call operator template are derived from the lambda-expression's trailing-return-type and parameter-declaration-clause by replacing each occurrence of auto in the decl-specifiers of the parameter-declaration-clause with the name of the corresponding invented template-parameter.
[Example
:
auto glambda = [](auto a, auto&& b) { return a < b; };
bool b = glambda(3, 3.14);                             // OK

auto vglambda = [](auto printer) {
  return [=](auto&& ... ts) {                          // OK: ts is a function parameter pack
    printer(std::forward<decltype(ts)>(ts)...);

    return [=]() {
      printer(ts ...);
    };
  };
};
auto p = vglambda( [](auto v1, auto v2, auto v3)
                   { std::cout << v1 << v2 << v3; } );
auto q = p(1, 'a', 3.14);                              // OK: outputs 1a3.14
q();                                                   // OK: outputs 1a3.14
end example
]
The function call operator or operator template is declared const ([class.mfct.non-static]) if and only if the lambda-expression's parameter-declaration-clause is not followed by mutable.
It is neither virtual nor declared volatile.
Any noexcept-specifier specified on a lambda-expression applies to the corresponding function call operator or operator template.
An attribute-specifier-seq in a lambda-declarator appertains to the type of the corresponding function call operator or operator template.
The function call operator or any given operator template specialization is a constexpr function if either the corresponding lambda-expression's parameter-declaration-clause is followed by constexpr, or it satisfies the requirements for a constexpr function.
[Note
:
Names referenced in the lambda-declarator are looked up in the context in which the lambda-expression appears.
end note
]
[Example
:
auto ID = [](auto a) { return a; };
static_assert(ID(3) == 3); // OK

struct NonLiteral {
  NonLiteral(int n) : n(n) { }
  int n;
};
static_assert(ID(NonLiteral{3}).n == 3); // ill-formed
end example
]
[Example
:
auto monoid = [](auto v) { return [=] { return v; }; };
auto add = [](auto m1) constexpr {
  auto ret = m1();
  return [=](auto m2) mutable {
    auto m1val = m1();
    auto plus = [=](auto m2val) mutable constexpr
                   { return m1val += m2val; };
    ret = plus(m2());
    return monoid(ret);
  };
};
constexpr auto zero = monoid(0);
constexpr auto one = monoid(1);
static_assert(add(one)(zero)() == one()); // OK

// Since two below is not declared constexpr, an evaluation of its constexpr member function call operator
// cannot perform an lvalue-to-rvalue conversion on one of its subobjects (that represents its capture)
// in a constant expression.
auto two = monoid(2);
assert(two() == 2); // OK, not a constant expression.
static_assert(add(one)(one)() == two()); // ill-formed: two() is not a constant expression
static_assert(add(one)(one)() == monoid(2)()); // OK
end example
]
The closure type for a non-generic lambda-expression with no lambda-capture has a conversion function to pointer to function with C++ language linkage having the same parameter and return types as the closure type's function call operator.
The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification.
The value returned by this conversion function is the address of a function F that, when invoked, has the same effect as invoking the closure type's function call operator.
F is a constexpr function if the function call operator is a constexpr function.
For a generic lambda with no lambda-capture, the closure type has a conversion function template to pointer to function.
The conversion function template has the same invented template-parameter-list, and the pointer to function has the same parameter types, as the function call operator template.
The return type of the pointer to function shall behave as if it were a decltype-specifier denoting the return type of the corresponding function call operator template specialization.
[Note
:
If the generic lambda has no trailing-return-type or the trailing-return-type contains a placeholder type, return type deduction of the corresponding function call operator template specialization has to be done.
The corresponding specialization is that instantiation of the function call operator template with the same template arguments as those deduced for the conversion function template.
Consider the following:
auto glambda = [](auto a) { return a; };
int (*fp)(int) = glambda;
The behavior of the conversion function of glambda above is like that of the following conversion function:
struct Closure {
  template<class T> auto operator()(T t) const { ... }
  template<class T> static auto lambda_call_operator_invoker(T a) {
    // forwards execution to operator()(a) and therefore has
    // the same return type deduced
    ...
  }
  template<class T> using fptr_t =
     decltype(lambda_call_operator_invoker(declval<T>())) (*)(T);

  template<class T> operator fptr_t<T>() const
    { return &lambda_call_operator_invoker; }
};
end note
]
[Example
:
void f1(int (*)(int))   { }
void f2(char (*)(int))  { }

void g(int (*)(int))    { }  // #1
void g(char (*)(char))  { }  // #2

void h(int (*)(int))    { }  // #3
void h(char (*)(int))   { }  // #4

auto glambda = [](auto a) { return a; };
f1(glambda);  // OK
f2(glambda);  // error: ID is not convertible
g(glambda);   // error: ambiguous
h(glambda);   // OK: calls #3 since it is convertible from ID
int& (*fpi)(int*) = [](auto* a) -> auto& { return *a; }; // OK
end example
]
The value returned by any given specialization of this conversion function template is the address of a function F that, when invoked, has the same effect as invoking the generic lambda's corresponding function call operator template specialization.
F is a constexpr function if the corresponding specialization is a constexpr function.
[Note
:
This will result in the implicit instantiation of the generic lambda's body.
The instantiated generic lambda's return type and parameter types shall match the return type and parameter types of the pointer to function.
end note
]
[Example
:
auto GL = [](auto a) { std::cout << a; return a; };
int (*GL_int)(int) = GL;  // OK: through conversion function template
GL_int(3);                // OK: same as GL(3)
end example
]
The conversion function or conversion function template is public, constexpr, non-virtual, non-explicit, const, and has a non-throwing exception specification.
[Example
:
auto Fwd = [](int (*fp)(int), auto a) { return fp(a); };
auto C = [](auto a) { return a; };

static_assert(Fwd(C,3) == 3); // OK

// No specialization of the function call operator template can be constexpr (due to the local static).
auto NC = [](auto a) { static int s; return a; };
static_assert(Fwd(NC,3) == 3); // ill-formed
end example
]
The lambda-expression's compound-statement yields the function-body ([dcl.fct.def]) of the function call operator, but for purposes of name lookup, determining the type and value of this and transforming id-expressions referring to non-static class members into class member access expressions using (*this) ([class.mfct.non-static]), the compound-statement is considered in the context of the lambda-expression.
[Example
:
struct S1 {
  int x, y;
  int operator()(int);
  void f() {
    [=]()->int {
      return operator()(this->x + y); // equivalent to S1​::​operator()(this->x + (*this).y)
                                      // this has type S1*
    };
  }
};
end example
]
Further, a variable _­_­func_­_­ is implicitly defined at the beginning of the compound-statement of the lambda-expression, with semantics as described in [dcl.fct.def.general].
The closure type associated with a lambda-expression has no default constructor and a deleted copy assignment operator.
It has a defaulted copy constructor and a defaulted move constructor ([class.copy]).
[Note
:
These special member functions are implicitly defined as usual, and might therefore be defined as deleted.
end note
]
The closure type associated with a lambda-expression has an implicitly-declared destructor ([class.dtor]).
A member of a closure type shall not be explicitly instantiated ([temp.explicit]), explicitly specialized ([temp.expl.spec]), or named in a friend declaration ([class.friend]).

8.1.5.2 Captures [expr.prim.lambda.capture]

lambda-capture:
	capture-default
	capture-list
	capture-default , capture-list
capture-default:
	&
	=
capture-list:
	capture ...
	capture-list , capture ...
capture:
	simple-capture
	init-capture
simple-capture:
	identifier
	& identifier
	this
	* this
init-capture:
	identifier initializer
	& identifier initializer
The body of a lambda-expression may refer to variables with automatic storage duration and the *this object (if any) of enclosing block scopes by capturing those entities, as described below.
If a lambda-capture includes a capture-default that is &, no identifier in a simple-capture of that lambda-capture shall be preceded by &.
If a lambda-capture includes a capture-default that is =, each simple-capture of that lambda-capture shall be of the form “& identifier” or “* this.
[Note
:
The form [&,this] is redundant but accepted for compatibility with ISO C++ 2014.
end note
]
Ignoring appearances in initializers of init-captures, an identifier or this shall not appear more than once in a lambda-capture.
[Example
:
struct S2 { void f(int i); };
void S2::f(int i) {
  [&, i]{ };        // OK
  [&, &i]{ };       // error: i preceded by & when & is the default
  [=, *this]{ };    // OK
  [=, this]{ };     // error: this when = is the default
  [i, i]{ };        // error: i repeated
  [this, *this]{ }; // error: this appears twice
}
end example
]
A lambda-expression whose smallest enclosing scope is a block scope is a local lambda expression; any other lambda-expression shall not have a capture-default or simple-capture in its lambda-introducer.
The reaching scope of a local lambda expression is the set of enclosing scopes up to and including the innermost enclosing function and its parameters.
[Note
:
This reaching scope includes any intervening lambda-expressions.
end note
]
The identifier in a simple-capture is looked up using the usual rules for unqualified name lookup; each such lookup shall find an entity.
An entity that is designated by a simple-capture is said to be explicitly captured, and shall be *this (when the simple-capture is “this” or “* this”) or a variable with automatic storage duration declared in the reaching scope of the local lambda expression.
If an identifier in a simple-capture appears as the declarator-id of a parameter of the lambda-declarator's parameter-declaration-clause, the program is ill-formed.
[Example
:
void f() {
  int x = 0;
  auto g = [x](int x) { return 0; }    // error: parameter and simple-capture have the same name
}
end example
]
An init-capture behaves as if it declares and explicitly captures a variable of the form “auto init-capture ;” whose declarative region is the lambda-expression's compound-statement, except that:
  • if the capture is by copy (see below), the non-static data member declared for the capture and the variable are treated as two different ways of referring to the same object, which has the lifetime of the non-static data member, and no additional copy and destruction is performed, and
  • if the capture is by reference, the variable's lifetime ends when the closure object's lifetime ends.
[Note
:
This enables an init-capture like “x = std​::​move(x)”; the second “x” must bind to a declaration in the surrounding context.
end note
]
[Example
:
int x = 4;
auto y = [&r = x, x = x+1]()->int {
            r += 2;
            return x+2;
         }();  // Updates ​::​x to 6, and initializes y to 7.

auto z = [a = 42](int a) { return 1; } // error: parameter and local variable have the same name
end example
]
A lambda-expression with an associated capture-default that does not explicitly capture *this or a variable with automatic storage duration (this excludes any id-expression that has been found to refer to an init-capture's associated non-static data member), is said to implicitly capture the entity (i.e., *this or a variable) if the compound-statement:
[Example
:
void f(int, const int (&)[2] = {})    { }   // #1
void f(const int&, const int (&)[1])  { }   // #2
void test() {
  const int x = 17;
  auto g = [](auto a) {
    f(x);                       // OK: calls #1, does not capture x
  };

  auto g2 = [=](auto a) {
    int selector[sizeof(a) == 1 ? 1 : 2]{};
    f(x, selector);             // OK: is a dependent expression, so captures x
  };
}
end example
]
All such implicitly captured entities shall be declared within the reaching scope of the lambda expression.
[Note
:
The implicit capture of an entity by a nested lambda-expression can cause its implicit capture by the containing lambda-expression (see below).
Implicit odr-uses of this can result in implicit capture.
end note
]
An entity is captured if it is captured explicitly or implicitly.
An entity captured by a lambda-expression is odr-used in the scope containing the lambda-expression.
If *this is captured by a local lambda expression, its nearest enclosing function shall be a non-static member function.
If a lambda-expression or an instantiation of the function call operator template of a generic lambda odr-uses this or a variable with automatic storage duration from its reaching scope, that entity shall be captured by the lambda-expression.
If a lambda-expression captures an entity and that entity is not defined or captured in the immediately enclosing lambda expression or function, the program is ill-formed.
[Example
:
void f1(int i) {
  int const N = 20;
  auto m1 = [=]{
    int const M = 30;
    auto m2 = [i]{
      int x[N][M];          // OK: N and M are not odr-used
      x[0][0] = i;          // OK: i is explicitly captured by m2 and implicitly captured by m1
    };
  };
  struct s1 {
    int f;
    void work(int n) {
      int m = n*n;
      int j = 40;
      auto m3 = [this,m] {
        auto m4 = [&,j] {   // error: j not captured by m3
          int x = n;        // error: n implicitly captured by m4 but not captured by m3
          x += m;           // OK: m implicitly captured by m4 and explicitly captured by m3
          x += i;           // error: i is outside of the reaching scope
          x += f;           // OK: this captured implicitly by m4 and explicitly by m3
        };
      };
    }
  };
}

struct s2 {
  double ohseven = .007;
  auto f() {
    return [this] {
      return [*this] {
          return ohseven;   // OK
      }
    }();
  }
  auto g() {
    return [] {
      return [*this] { };   // error: *this not captured by outer lambda-expression
    }();
  }
};
end example
]
A lambda-expression appearing in a default argument shall not implicitly or explicitly capture any entity.
[Example
:
void f2() {
  int i = 1;
  void g1(int = ([i]{ return i; })());          // ill-formed
  void g2(int = ([i]{ return 0; })());          // ill-formed
  void g3(int = ([=]{ return i; })());          // ill-formed
  void g4(int = ([=]{ return 0; })());          // OK
  void g5(int = ([]{ return sizeof i; })());    // OK
}
end example
]
An entity is captured by copy if
For each entity captured by copy, an unnamed non-static data member is declared in the closure type.
The declaration order of these members is unspecified.
The type of such a data member is the referenced type if the entity is a reference to an object, an lvalue reference to the referenced function type if the entity is a reference to a function, or the type of the corresponding captured entity otherwise.
A member of an anonymous union shall not be captured by copy.
Every id-expression within the compound-statement of a lambda-expression that is an odr-use of an entity captured by copy is transformed into an access to the corresponding unnamed data member of the closure type.
[Note
:
An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type.
Furthermore, such an id-expression does not cause the implicit capture of the entity.
end note
]
If *this is captured by copy, each odr-use of this is transformed into a pointer to the corresponding unnamed data member of the closure type, cast to the type of this.
[Note
:
The cast ensures that the transformed expression is a prvalue.
end note
]
An id-expression within the compound-statement of a lambda-expression that is an odr-use of a reference captured by reference refers to the entity to which the captured reference is bound and not to the captured reference.
[Note
:
The validity of such captures is determined by the lifetime of the object to which the reference refers, not by the lifetime of the reference itself.
end note
]
[Example
:
void f(const int*);
void g() {
  const int N = 10;
  [=] {
    int arr[N];     // OK: not an odr-use, refers to automatic variable
    f(&N);          // OK: causes N to be captured; &N points to
                    // the corresponding member of the closure type
  };
}
auto h(int &r) {
  return [&] {
    ++r;            // Valid after h returns if the lifetime of the
                    // object to which r is bound has not ended
  };
}
end example
]
An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy.
It is unspecified whether additional unnamed non-static data members are declared in the closure type for entities captured by reference.
If declared, such non-static data members shall be of literal type.
[Example
:
// The inner closure type must be a literal type regardless of how reference captures are represented.
static_assert([](int n) { return [&n] { return ++n; }(); }(3) == 4);
end example
]
A bit-field or a member of an anonymous union shall not be captured by reference.
If a lambda-expression m2 captures an entity and that entity is captured by an immediately enclosing lambda-expression m1, then m2's capture is transformed as follows:
  • if m1 captures the entity by copy, m2 captures the corresponding non-static data member of m1's closure type;
  • if m1 captures the entity by reference, m2 captures the same entity captured by m1.
[Example
:
The nested lambda expressions and invocations below will output 123234.
int a = 1, b = 1, c = 1;
auto m1 = [a, &b, &c]() mutable {
  auto m2 = [a, b, &c]() mutable {
    std::cout << a << b << c;
    a = 4; b = 4; c = 4;
  };
  a = 3; b = 3; c = 3;
  m2();
};
a = 2; b = 2; c = 2;
m1();
std::cout << a << b << c;
end example
]
Every occurrence of decltype((x)) where x is a possibly parenthesized id-expression that names an entity of automatic storage duration is treated as if x were transformed into an access to a corresponding data member of the closure type that would have been declared if x were an odr-use of the denoted entity.
[Example
:
void f3() {
  float x, &r = x;
  [=] {                     // x and r are not captured (appearance in a decltype operand is not an odr-use)
    decltype(x) y1;         // y1 has type float
    decltype((x)) y2 = y1;  // y2 has type float const& because this lambda is not mutable and x is an lvalue
    decltype(r) r1 = y1;    // r1 has type float& (transformation not considered)
    decltype((r)) r2 = y2;  // r2 has type float const&
  };
}
end example
]
When the lambda-expression is evaluated, the entities that are captured by copy are used to direct-initialize each corresponding non-static data member of the resulting closure object, and the non-static data members corresponding to the init-captures are initialized as indicated by the corresponding initializer (which may be copy- or direct-initialization).
(For array members, the array elements are direct-initialized in increasing subscript order.)
These initializations are performed in the (unspecified) order in which the non-static data members are declared.
[Note
:
This ensures that the destructions will occur in the reverse order of the constructions.
end note
]
[Note
:
If a non-reference entity is implicitly or explicitly captured by reference, invoking the function call operator of the corresponding lambda-expression after the lifetime of the entity has ended is likely to result in undefined behavior.
end note
]
A simple-capture followed by an ellipsis is a pack expansion.
An init-capture followed by an ellipsis is ill-formed.
[Example
:
template<class... Args>
void f(Args... args) {
  auto lm = [&, args...] { return g(args...); };
  lm();
}
end example
]