7 Expressions [expr]

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

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);        // error

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());        // error: two() is not a constant expression
static_assert(add(one)(one)() == monoid(2)());  // OK

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

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)

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);  // error

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*
    };
  }
};

struct S2 { void f(int i); };
void S2::f(int i) {
  [&, i]{ };        // OK
  [&, this, i]{ };  // OK, equivalent to [&, i]
  [&, &i]{ };       // error: i preceded by & when & is the default
  [=, *this]{ };    // OK
  [=, this]{ };     // OK, equivalent to [=]
  [i, i]{ };        // error: i repeated
  [this, *this]{ }; // error: this appears twice
}

void f() {
  int x = 0;
  auto g = [x](int x) { return 0; };    // error: parameter and simple-capture have the same name
}

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

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 g1 = [=](auto a) {
    f(x);                       // OK: calls #1, captures x
  };

  auto g2 = [=](auto a) {
    int selector[sizeof(a) == 1 ? 1 : 2]{};
    f(x, selector);             // OK: captures x, might call #1 or #2
  };

  auto g3 = [=](auto a) {
    typeid(a + x);              // captures x regardless of whether a + x is an unevaluated operand
  };
}

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 odr-usable due to intervening lambda m3
          int x = n;        // error: n is odr-used but not odr-usable due to intervening lambda m3
          x += m;           // OK: m implicitly captured by m4 and explicitly captured by m3
          x += i;           // error: i is odr-used but not odr-usable
                            // due to intervening function and class scopes
          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
    }();
  }
};

void f2() {
  int i = 1;
  void g1(int = ([i]{ return i; })());          // error
  void g2(int = ([i]{ return 0; })());          // error
  void g3(int = ([=]{ return i; })());          // error
  void g4(int = ([=]{ return 0; })());          // OK
  void g5(int = ([]{ return sizeof i; })());    // OK
  void g6(int = ([x=1]{ return x; })());        // OK
  void g7(int = ([x=i]{ return x; })());        // error
}

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
  };
}

// 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);

auto h(int &r) {
  return [&] {
    ++r;            // Valid after h returns if the lifetime of the
                    // object to which r is bound has not ended
  };
}

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;

template<class... Args>
void f(Args... args) {
  auto lm = [&, args...] { return g(args...); };
  lm();

  auto lm2 = [...xs=std::move(args)] { return g(xs...); };
  lm2();
}