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
#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
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
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
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
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
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; }
};
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
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
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
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 examplelambda-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
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
void f() {
int x = 0;
auto g = [x](int x) { return 0; } // error: parameter and simple-capture have the same name
} — end 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
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
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
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
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
// 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
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
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
template<class... Args>
void f(Args... args) {
auto lm = [&, args...] { return g(args...); };
lm();
} — end example