Lambda expressions (since C++11)

struct S2 { void f(int i); };
void S2::f(int i)
{
    [&]{};          // 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]
}

struct S2 { void f(int i); };
void S2::f(int i)
{
    [=]{};        // 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 [=]
}

struct S2 { void f(int i); };
void S2::f(int i)
{
    [i, i] {};        // Error: i repeated
    [this, *this] {}; // Error: "this" repeated (C++17)
}

void f(const int*);
void g()
{
    const int N = 10;
    [=]
    { 
        int arr[N]; // not an odr-use: refers to g's const int N
        f(&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
    }();
}

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&
    };
}

struct s2
{
    double ohseven = .007;
    auto f() // 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)
            {
                return ohseven; // OK
            }
        }();
    }
 
    auto g()
    {
        return [] // capture nothing
        { 
            return [*this]{}; // error: *this not captured by outer lambda-expression
        }();
    }
};

void f1(int i)
{
    int const N = 20;
    auto m1 = [=]
    {
        int const M = 30;
        auto m2 = [i]
        {
            int x[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
        };
    };
 
    struct s1 // local class within f1()
    {
        int f;
        void work(int n) // non-static member function
        {
            int m = n * n;
            int j = 40;
            auto m3 = [this, m]
            {
                auto m4 = [&, j] // error: j is not captured by m3
                {
                    int x = n; // error: n is implicitly captured by m4
                               // but not captured by m3
                    x += m;    // OK: m is implicitly captured by m4
                               // and explicitly captured by m3
                    x += 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 S
{
    int x = 0;
 
    void f()
    {
        int i = 0;
    //  auto l1 = [i, x]{ use(i, x); };    // error: x is not a variable
        auto l2 = [i, x=x]{ use(i, x); };  // OK, copy capture
        i = 1; x = 1; l2(); // calls use(0,0)
        auto l3 = [i, &x=x]{ use(i, x); }; // OK, reference capture
        i = 2; x = 2; l3(); // calls use(1,2)
    }
};

class S
{
    int x = 0;
 
    void f()
    {
        int i = 0;
 
        auto l1 = [=]{ use(i, x); }; // captures a copy of i and
                                     // a copy of the this pointer
        i = 1; x = 1; l1();          // calls use(0, 1), as if
                                     // i by copy and x by reference
 
        auto l2 = [i, this]{ use(i, x); }; // same as above, made explicit
        i = 2; x = 2; l2();          // calls use(1, 2), as if
                                     // i by copy and x by reference
 
        auto l3 = [&]{ use(i, x); }; // captures i by reference and
                                     // a copy of the this pointer
        i = 3; x = 2; l3();          // calls use(3, 2), as if
                                     // i and x are both by reference
 
        auto l4 = [i, *this]{ use(i, x); }; // makes a copy of *this,
                                            // including a copy of x
        i = 4; x = 4; l4();          // calls use(3, 2), as if
                                     // i and x are both by copy
    }
};