using namespace std;

int fun(){
    cout<<"call fun";
    return 0;
}
typedef int (*ff)();
using fu=ff;

int main(int _argc,char* _argv[]){
    int(*f)()=fun;
    f();
    (*f)();
    ff fff=(ff)fun;
    fu fu1=fun;
    fu1();
}

class A{
public:
    virtual int fun(){
        cout<<"A";
        return 0;
    }
};

class B:public A{
public:
    int fun() override {
        cout<<"B";
        return 0;
    }
};

typedef int(A::*f)();

int main(int _argc,char* _argv[]){
    f ff=&A::fun;
    A a{};
    B b{};
    (a.*ff)();//A
    (b.*ff)();//B
}

class A{
public:
    int fun(){
        cout<<"A";
        return 0;
    }
};

class B:public A{
public:
    int fun() {
        cout<<"B";
        return 0;
    }
};

typedef int(A::*f)();

int main(int _argc,char* _argv[]){
    f ff=f(&B::fun);
    A a{};
    B b{};
    (a.*ff)();//B
    (b.*ff)();//B
}

class Foo{
public:
  int f(char* c=0){
    std::cout<<"Foo::f()"<<std::endl;
    return 1;
  }
};
 
class Bar{
public:
  void b(int i=0){
    std::cout<<"Bar::b()"<<std::endl;
  }
};
 
class FooDerived:public Foo{
public:
  int f(char* c=0){
    std::cout<<"FooDerived::f()"<<std::endl;
    return 1;
  }
};
 
int main(int argc, char* argv[]){
  typedef  int (Foo::*FPTR) (char*);
  typedef  void (Bar::*BPTR) (int);
  typedef  int (FooDerived::*FDPTR) (char*);
 
  FPTR fptr = &Foo::f;
  BPTR bptr = &Bar::b;
  FDPTR fdptr = &FooDerived::f;
 
  //Bptr = static_cast<void (Bar::*) (int)> (fptr); //error
  fdptr = static_cast<int (Foo::*) (char*)> (fptr); //OK: contravariance
 
  Bar obj;
  ( obj.*(BPTR) fptr )(1);//call: Foo::f()
}
Output:
Foo::f()

class Foo{
public:
  virtual int f(char* c=0){
    std::cout<<"Foo::f()"<<std::endl;
    return 1;
  }
};
 
class Bar{
public:
  virtual void b(int i=0){
    std::cout<<"Bar::b()"<<std::endl;
  }
};
 
class FooDerived:public Foo{
public:
  int f(char* c=0){
    std::cout<<"FooDerived::f()"<<std::endl;
    return 1;
  }
};
 
int main(int argc, char* argv[]){
  typedef  int (Foo::*FPTR) (char*);
  typedef  void (Bar::*BPTR) (int);
  FPTR fptr=&Foo::f;
  BPTR bptr=&Bar::b;
 
  FooDerived objDer;
  (objDer.*fptr)(0);//call: FooDerived::f(), not Foo::f()
 
  Bar obj;
  ( obj.*(BPTR) fptr )(1);//call: Bar::b() , not Foo::f()
}
Output:
FooDerived::f()
Bar::b()

class A{
public:
    void fun(){
        cout<<"A";
    }
};
class B:public A{
public:
    void fun(){
        cout<<"B";
    }
};
int main(int _argc,char* _argv[]){
    typedef void (A::*f)();
    f f1=(f)&B::fun;
    B b;
    (b.*f1)();// if B did not inherit A (b.*(void (B::*)())f1)();
}

template<class T>
void fun(void(T::*f)(int)){
    T t;
    (t.*f)(12);
}

class A{
    public:
        void fun(int i){
            cout<<i;
        }
    };

int main(int argc,char* argv[]){
    fun(&A::fun);
    return 0;
}

template<int i,typename T>
 int iter(T t){
     return i-1;
 }

 template<typename T>
 struct A{
     T t;
 };

 template<typename T>
 using fun= int(*)(T t);

 template <typename T>
 using attr=T A<T>::*;


int main(int argc, char *argv[])
{
    QCoreApplication a(argc,argv);
    fun<int> f=iter<2,int>;
    attr<int> at=&A<int>::t;

    return a.exec();
}

[&]{};          // 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]
[=]{};          // 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 [=]

auto fun=[](int i)->int {return i;};
std::function<int(int)> fun2=fun;
int(*fun3)(int)=fun;
cout<<fun2(4);

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)
  }
};

#include <vector>
#include <iostream>
#include <algorithm>
#include <functional>
 
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';
}

int x = 1;
auto f() { return x; }        // return type is int
const auto& f() { return x; } // return type is const int&
auto f(bool val)
{
    if (val) return 123; // deduces return type int
    else return 3.14f;   // error: deduces return type float
}
auto f() {}              // returns void
auto g() { return f(); } // returns void
auto* x() {}             // error: cannot deduce auto* from void
struct F
{
    virtual auto f() { return 2; } // error virtual function cannot return type deduction
};

int a = 1, *p = NULL, f(), (*pf)(double);
// decl-specifier-seq is int
// declarator f() declares (but doesn't define)
//                a function taking no arguments and returning int
 
struct S
{
    virtual int f(char) const, g(int) &&; // declares two non-static member functions
    virtual int f(char), x; // compile-time error: virtual (in decl-specifier-seq)
                            // is only allowed in declarations of non-static
                            // member functions
};
int f(int a, int *p, int (*(*x)(double))[3]);
int f(int a = 7, int *p = nullptr, int (*(*x)(double))[3] = nullptr);
int f(int, int *, int (*(*)(double))[3]);
int f(int = 7, int * = nullptr, int (*(*)(double))[3] = nullptr);

int fun0(){
    std::vector<int> v{1,2,3,4};
    return v.size();
}
void fun(int i=fun0()){

}

int main()
{
    std::cout << "Test\n"; // There is no operator<< in global namespace, but ADL
                           // examines std namespace because the left argument is in
                           // std and finds std::operator<<(std::ostream&, const char*)
    operator<<(std::cout, "Test\n"); // same, using function call notation
 
    // however,
    std::cout << endl; // Error: 'endl' is not declared in this namespace.
                       // This is not a function call to endl(), so ADL does not apply
 
    endl(std::cout); // OK: this is a function call: ADL examines std namespace
                     // because the argument of endl is in std, and finds std::endl
 
    (endl)(std::cout); // Error: 'endl' is not declared in this namespace.
                       // The sub-expression (endl) is not a function call expression
}

using std::swap;
swap(obj1, obj2);

namespace A {
      struct X;
      struct Y;
      void f(int);
      void g(X);
}
 
namespace B {
    void f(int i) {
        f(i);   // calls B::f (endless recursion)
    }
    void g(A::X x) {
        g(x);   // Error: ambiguous between B::g (ordinary lookup)
                //        and A::g (argument-dependent lookup)
    }
    void h(A::Y y) {
        h(y);   // calls B::h (endless recursion): ADL examines the A namespace
                // but finds no A::h, so only B::h from ordinary lookup is used
    }
}

struct B { void f(int); };
struct A { operator B&(); };
A a;
a.B::f(1); // Error: user-defined conversions cannot be applied
           // to the implicit object parameter
static_cast<B&>(a).f(1); // OK

int f1(int);
int f2(float);
struct A {
    using fp1 = int(*)(int);
    operator fp1() { return f1; } // conversion function to pointer to function
    using fp2 = int(*)(float);
    operator fp2() { return f2; } // conversion function to pointer to function
} a;
int i = a(1); // calls f1 via pointer returned from conversion function

struct A {
    operator int(); // user-defined conversion
};
A operator+(const A&, const A&); // non-member user-defined operator
void m()
{
    A a, b;
    a + b; // member-candidates: none
           // non-member candidates: operator+(a,b)
           // built-in candidates: int(a) + int(b)
           // overload resolution chooses operator+(a,b)
}

struct Y { operator int*(); };  // Y is convertible to int*
int *a = Y() + 100.0; // error: no operator+ between pointer and double

struct A { A(int); };
struct B { B(A); };
B b{ {0} }; // list-init of B
// candidates: B(const B&), B(B&&), B(A)
// {0} -> B&& not viable: would have to call B(A)
// {0} -> const B&: not viable: would have to bind to rvalue, would have to call B(A)
// {0} -> A viable. Calls A(int): user-defined conversion to A is not banned

template<class T> struct A {
    using value_type = T;
    A(value_type);                  // #1
    A(const A&);                    // #2
    A(T, T, int);                   // #3
    template<class U> A(int, T, U); // #4
};                                 
A x (1, 2, 3);  // uses #3, generated from a non-template constructor
A a (42); // uses #6 to deduce A<int> and #1 to initialize
A b = a;  // uses #5 to deduce A<int> and #2 to initialize
A b2 = a;  // uses #7 to deduce A<A<int>> and #1 to initialize

void Fcn(const int*, short); // overload #1
void Fcn(int*, int); // overload #2
int i;
short s = 0;
void f() 
{
    Fcn(&i, 1L);  // 1st argument: &i -> int* is better than &i -> const int*
                  // 2nd argument: 1L -> short and 1L -> int are equivalent
                  // calls Fcn(int*, int)
 
    Fcn(&i,'c');  // 1st argument: &i -> int* is better than &i -> const int*
                  // 2nd argument: 'c' -> int is better than 'c' -> short
                  // calls Fcn(int*, int)
 
    Fcn(&i, s);   // 1st argument: &i -> int* is better than &i -> const int*
                  // 2nd argument: s -> short is better than s -> int
                  // no winner, compilation error
}

struct Base {};
struct Derived : Base {} d;
int f(Base&);    // overload #1
int f(Derived&); // overload #2
int i = f(d); // d -> Derived& has rank Exact Match
              // d -> Base& has rank Conversion
              // calls f(Derived&)

int i;
int f1();
int g(const int&);  // overload #1
int g(const int&&); // overload #2
int j = g(i);    // lvalue int -> const int& is the only valid conversion
int k = g(f1()); // rvalue int -> const int&& better than rvalue int -> const int&

int f(void(&)());  // overload #1
int f(void(&&)()); // overload #2
void g();
int i1 = f(g);     // calls #1

int f(const int &); // overload #1
int f(int &);       // overload #2 (both references)
int g(const int &); // overload #1
int g(int);         // overload #2
int i;
int j = f(i); // lvalue i -> int& is better than lvalue int -> const int&
              // calls f(int&)
int k = g(i); // lvalue i -> const int& ranks Exact Match
              // lvalue i -> rvalue int ranks Exact Match
              // ambiguous overload: compilation error

int f(const int*);
int f(int*);
int i;
int j = f(&i); // &i -> int* is better than &i -> const int*, calls f(int*)

struct A {
    operator short(); // user-defined conversion function
} a;
int f(int);   // overload #1
int f(float); // overload #2
int i = f(a); // A -> short, followed by short -> int (rank Promotion)
              // A -> short, followed by short -> float (rank Conversion)
              // calls f(int)

void f1(int);                                 // #1
void f1(std::initializer_list<long>);         // #2
void g1() { f1({42}); }                       // chooses #2
 
void f2(std::pair<const char*, const char*>); // #3
void f2(std::initializer_list<std::string>);  // #4
void g2() { f2({"foo","bar"}); }              // chooses #4

void f(int    (&&)[] );    // overload #1
void f(double (&&)[] );    // overload #2
void f(int    (&&)[2]);    // overload #3
 
f({1});          // #1: Better than #2 due to conversion, better than #3 due to bounds
f({1.0});        // #2: double -> double is better than double -> int
f({1.0, 2.0});   // #2: double -> double is better than double -> int
f({1, 2});       // #3: -> int[2] is better than -> int[], 
                 //     and int -> int is better than int -> double

struct A { int x, y; };
struct B { int y, x; };
 
void f(A a, int); // #1
void f(B b, ...); // #2
void g(A a); // #3
void g(B b); // #4
 
void h() 
{
    f({.x = 1, .y = 2}, 0); // OK; calls #1
    f({.y = 2, .x = 1}, 0); // error: selects #1, initialization of a fails
                            // due to non-matching member order
    g({.x = 1, .y = 2}); // error: ambiguous between #3 and #4
}

struct A { A(std::initializer_list<int>); };
void f(A);
struct B { B(int, double); };
void g(B);
g({'a','b'});  // calls g(B(int,double)), user-defined conversion
// g({1.0, 1,0}); // error: double->int is narrowing, not allowed in list-init
void f(B);
// f({'a','b'}); // f(A) and f(B) both user-defined conversions

struct A { int m1; double m2;};
void f(A);
f({'a','b'});  // calls f(A(int,double)), user-defined conversion

std::string str = "Hello, ";
str.operator+=("world");                       // same as str += "world";
operator<<(operator<<(std::cout, str) , '\n'); // same as std::cout << str << '\n';
                                               // (since C++17) except for sequencing

T& operator=(const T& other) // copy assignment
{
    if (this != &other) { // self-assignment check expected
        if (other.size != size) {         // storage cannot be reused
            delete[] mArray;              // destroy storage in this
            size = 0;
            mArray = nullptr;             // preserve invariants in case next line throws
            mArray = new int[other.size]; // create storage in this
            size = other.size;
        } 
        std::copy(other.mArray, other.mArray + other.size, mArray);
    }
    return *this;
}
T& operator=(T&& other) noexcept // move assignment
{
    if(this != &other) { // no-op on self-move-assignment (delete[]/size=0 also ok)
        delete[] mArray;                               // delete this storage
        mArray = std::exchange(other.mArray, nullptr); // leave moved-from in valid state
        size = std::exchange(other.size, 0);
    }
    return *this;
}
T& T::operator=(T arg) noexcept // copy/move constructor is called to construct arg
{
    std::swap(size, arg.size); // resources are exchanged between *this and arg
    std::swap(mArray, arg.mArray);
    return *this;
}

class Fraction
{
    int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
    int n, d;
public:
    Fraction(int n, int d = 1) : n(n/gcd(n, d)), d(d/gcd(n, d)) { }
    int num() const { return n; }
    int den() const { return d; }
    Fraction& operator*=(const Fraction& rhs)
    {
        int new_n = n * rhs.n/gcd(n * rhs.n, d * rhs.d);
        d = d * rhs.d/gcd(n * rhs.n, d * rhs.d);
        n = new_n;
        return *this;
    }
};
std::ostream& operator<<(std::ostream& out, const Fraction& f)
{
   return out << f.num() << '/' << f.den() ;
}
bool operator==(const Fraction& lhs, const Fraction& rhs)
{
    return lhs.num() == rhs.num() && lhs.den() == rhs.den();
}
bool operator!=(const Fraction& lhs, const Fraction& rhs)
{
    return !(lhs == rhs);
}
Fraction operator*(Fraction lhs, const Fraction& rhs)
{
    return lhs *= rhs;
}
 
int main()
{
   Fraction f1(3, 8), f2(1, 2), f3(10, 2);
   std::cout << f1 << " * " << f2 << " = " << f1 * f2 << '\n'
             << f2 << " * " << f3 << " = " << f2 * f3 << '\n'
             <<  2 << " * " << f1 << " = " <<  2 * f1 << '\n';
}

int f(int) { return 1; }
int f(double) { return 2; }
 
void g( int(&f1)(int), int(*f2)(double) ) {}
 
template< int(*F)(int) >
struct Templ {};
 
struct Foo {
    int mf(int) { return 3; }
    int mf(double) { return 4; }
};
 
struct Emp {
    void operator<<(int (*)(double)) {}
};
 
int main()
{
    // 1. initialization
    int (*pf)(double) = f; // selects int f(double)
    int (&rf)(int) = f; // selects int f(int)
    int (Foo::*mpf)(int) = &Foo::mf; // selects int mf(int)
 
    // 2. assignment
    pf = nullptr;
    pf = &f; // selects int f(double)
 
    // 3. function argument
    g(f, f); // selects int f(int) for the 1st argument
             // and int f(double) for the second
 
    // 4. user-defined operator
    Emp{} << f; //selects int f(double)
 
    // 5. return value
    auto foo = []() -> int (*)(int) {
        return f; // selects int f(int)
    };
 
    // 6. cast
    auto p = static_cast<int(*)(int)>(f); // selects int f(int)
 
    // 7. template argument
    Templ<f> t;  // selects int f(int)
}

enum A{
    One=0,
    Two=1,
    Three=3
};
class AA{
public:
    enum A{
        One=0,
        Two=1,
        Three=3
    };
};
int main(int _argc,char* _argv[]){
    int l=::A::One;
    l=::One;
    ::A a=(::A)1;
    AA::A aa=AA::One;
    aa=AA::A::One;
}

enum num : char { one = '0' };
std::cout << num::one; // '0', not 48

enum Foo { a, b, c = 10, d, e = 1, f, g = f + c };
//a = 0, b = 1, c = 10, d = 11, e = 1, f = 2, g = 12

int a[2];            // array of 2 int
int* p1 = a;         // a decays to a pointer to the first element of a
 
int b[2][3];         // array of 2 arrays of 3 int
// int** p2 = b;     // error: b does not decay to int**
int (*p2)[3] = b;    // b decays to a pointer to the first 3-element row of b
 
int c[2][3][4];      // array of 2 arrays of 3 arrays of 4 int
// int*** p3 = c;    // error: c does not decay to int***
int (*p3)[3][4] = c; // c decays to a pointer to the first 3 × 4-element plane of c
int l[]{1,2,3,4,5,6,7};//or = {1,2,3,4,5,6,7};
int m=*(l+4);
//l++ ++l errors
int* k=new int[5]{1,2,3,4,5};

void g(int (&a)[3])
{
    std::cout << a[0] << '\n';
}
 
void f(int* p)
{
    std::cout << *p << '\n';
}
 
int main()
{
    int a[3] = {1, 2, 3};
    int* p = a;
 
    std::cout << sizeof a << '\n'  // prints size of array
              << sizeof p << '\n'; // prints size of a pointer
 
    // where arrays are acceptable, but pointers aren't, only arrays may be used
    g(a); // okay: function takes an array by reference
//  g(p); // error
 
    for(int n: a)              // okay: arrays can be used in range-for loops
        std::cout << n << ' '; // prints elements of the array
//  for(int n: p)              // error
//      std::cout << n << ' ';
 
    // where pointers are acceptable, but arrays aren't, both may be used:
    f(a); // okay: function takes a pointer
    f(p); // okay: function takes a pointer
 
    std::cout << *a << '\n' // prints the first element
              << *p << '\n' // same
              << *(a + 1) << ' ' << a[1] << '\n'  // prints the second element
              << *(p + 1) << ' ' << p[1] << '\n'; // same
}

namespace Q {
  namespace V { // original-namespace-definition for V
    void f(); // declaration of Q::V::f
  }
  void V::f() {} // OK
  void V::g() {} // Error: g() is not yet a member of V
  namespace V { // extension-namespace-definition for V
    void g(); // declaration of Q::V::g
  }
}
namespace R { // not a enclosing namespace for Q
   void Q::V::g() {} // Error: cannot define Q::V::g inside R
}
void Q::V::g() {} // OK: global namespace encloses Q

namespace D {
   int d1;
   void f(char);
}
using namespace D; // introduces D::d1, D::f, D::d2, D::f,
                   //  E::e, and E::f into global namespace!
 
int d1; // OK: no conflict with D::d1 when declaring
namespace E {
    int e;
    void f(int);
}
namespace D { // namespace extension
    int d2;
    using namespace E; // transitive using-directive
    void f(int);
}
void f() {
    d1++; // error: ambiguous ::d1 or D::d1?
    ::d1++; // OK
    D::d1++; // OK
    d2++; // OK, d2 is D::d2
    e++; // OK: e is E::e due to transitive using
    f(1); // error: ambiguous: D::f(int) or E::f(int)?
    f('a'); // OK: the only f(char) is D::f(char)
}

namespace vec {
 
    template< typename T >
    class vector {
        // ...
    };
 
} // of vec
 
int main()
{
    std::vector<int> v1; // Standard vector.
    vec::vector<int> v2; // User defined vector.
 
    v1 = v2; // Error: v1 and v2 are different object's type.
 
    {
        using namespace std;
        vector<int> v3; // Same as std::vector
        v1 = v3; // OK
    }
 
    {
        using vec::vector;
        vector<int> v4; // Same as vec::vector
        v2 = v4; // OK
    }
 
    return 0;
}

namespace foo {
    namespace bar {
         namespace baz {
             int qux = 42;
         }
    }
}
 
namespace fbz = foo::bar::baz;
 
int main()
{
    std::cout << fbz::qux << '\n';
}

int& a[3]; // error
int&* p;   // error
int& &r;   // error

typedef int&  lref;
typedef int&& rref;
int n;
lref&  r1 = n; // type of r1 is int&
lref&& r2 = n; // type of r2 is int&
rref&  r3 = n; // type of r3 is int&
rref&& r4 = 1; // type of r4 is int&&

void f(int& x) {
    std::cout << "lvalue reference overload f(" << x << ")\n";
}
 
void f(const int& x) {
    std::cout << "lvalue reference to const overload f(" << x << ")\n";
}
 
void f(int&& x) {
    std::cout << "rvalue reference overload f(" << x << ")\n";
}
 
int main() {
    int i = 1;
    const int ci = 2;
    f(i);  // calls f(int&)
    f(ci); // calls f(const int&)
    f(3);  // calls f(int&&)
           // would call f(const int&) if f(int&&) overload wasn't provided
    f(std::move(i)); // calls f(int&&)
 
    // rvalue reference variables are lvalues when used in expressions
    int&& x = 1;
    f(x);            // calls f(int& x)
    f(std::move(x)); // calls f(int&& x)
}

template<class T>
int f(T&& x) {                    // x is a forwarding reference
    return g(std::forward<T>(x)); // and so can be forwarded
}
 
int main() {
    int i;
    f(i); // argument is lvalue, calls f<int&>(int&), std::forward<int&>(x) is lvalue
    f(0); // argument is rvalue, calls f<int>(int&&), std::forward<int>(x) is rvalue
}
 
template<class T>
int g(const T&& x); // x is not a forwarding reference: const T is not cv-unqualified
 
template<class T> struct A {
    template<class U>
    A(T&& x, U&& y, int* p); // x is not a forwarding reference: T is not a
                             // type template parameter of the constructor,
                             // but y is a forwarding reference
};

struct C {
   int x, y;
} c;
 
int* px = &c.x;   // value of px is "pointer to c.x"
int* pxe= px + 1; // value of pxe is "pointer past the end of c.x"
int* py = &c.y;   // value of py is "pointer to c.y"
 
assert(pxe == py); // == tests if two pointers represent the same address
                   // may or may not fire
 
*pxe = 1; // undefined behavior even if the assertion does not fire

int n;
int* np = &n; // pointer to int
int* const* npp = &np; // non-const pointer to const pointer to non-const int
 
int a[2];
int (*ap)[2] = &a; // pointer to array of int
 
struct S { int n; };
S s = {1};
int* sp = &s.n; // pointer to the int that is a member of s

int n;
int* p = &n;     // pointer to n
int& r = *p;     // reference is bound to the lvalue expression that identifies n
r = 7;           // stores the int 7 in n
std::cout << *p; // lvalue-to-rvalue implicit conversion reads the value from n
int a[2];
int* p1 = a; // pointer to the first element a[0] (an int) of the array a
 
int b[6][3][8];
int (*p2)[3][8] = b; // pointer to the first element b[0] of the array b,
                     // which is an array of 3 arrays of 8 ints

struct Base {};
struct Derived : Base {};
 
Derived d;
Base* p = &d;

const int* const i=new int ;
const int* const* ii=&i;//non-const pointer to const pointer to const object
const int n1=12;
const int* n2=&n1;//int* is error 

int n = 1;
int* p1 = &n;
void* pv = p1;
int* p2 = static_cast<int*>(pv);
std::cout << *p2 << '\n'; // prints 1

int f();
int (*p)() = f;  // pointer p is pointing to f
int (&r)() = *p; // the lvalue that identifies f is bound to a reference
r();             // function f invoked through lvalue reference
(*p)();          // function f invoked through the function lvalue
p();  

struct C { int m; };
 
int main()
{
    int C::* p = &C::m;          // pointer to data member m of class C
    C c = {7};
    std::cout << c.*p << '\n';   // prints 7
    C* cp = &c;
    cp->m = 10;
    std::cout << cp->*p << '\n'; // prints 10
}

struct Base { int m; };
struct Derived : Base {};
 
int main()
{
    int Base::* bp = &Base::m;
    int Derived::* dp = bp;
    Derived d;
    d.m = 1;
    std::cout << d.*dp << ' ' << d.*bp << '\n'; // prints 1 1
}

struct A
{
    int m;
    // const pointer to non-const member
    int A::* const p;
};
 
int main()
{
    // non-const pointer to data member which is a const pointer to non-const member
    int A::* const A::* p1 = &A::p;
 
    const A a = {1, &A::m};
    std::cout << a.*(a.*p1) << '\n'; // prints 1
 
    // regular non-const pointer to a const pointer-to-member
    int A::* const* p2 = &a.p;
    std::cout << a.**p2 << '\n'; // prints 1
}

struct Base
{
    void f(int n) { std::cout << n << '\n'; }
};
struct Derived : Base {};
 
int main()
{
    void (Base::* bp)(int) = &Base::f;
    void (Derived::* dp)(int) = bp;
    Derived d;
    (d.*dp)(1);
    (d.*bp)(2);
}

struct Base {};
struct Derived : Base
{
    void f(int n) { std::cout << n << '\n'; }
};
 
int main()
{
    void (Derived::* dp)(int) = &Derived::f;
    void (Base::* bp)(int) = static_cast<void (Base::*)(int)>(dp);
 
    Derived d;
    (d.*bp)(1); // okay: prints 1
 
    Base b;
    (b.*bp)(2); // undefined behavior
}

int main()
{
    int n1 = 0;           // non-const object
    const int n2 = 0;     // const object
    int const n3 = 0;     // const object (same as n2)
    volatile int n4 = 0;  // volatile object
    const struct
    {
        int n1;
        mutable int n2;
    } x = {0, 0};      // const object with mutable member
 
    n1 = 1; // ok, modifiable object
//  n2 = 2; // error: non-modifiable object
    n4 = 3; // ok, treated as a side-effect
//  x.n1 = 4; // error: member of a const object is const
    x.n2 = 4; // ok, mutable member of a const object isn't const
 
    const int& r1 = n1; // reference to const bound to non-const object
//  r1 = 2; // error: attempt to modify through reference to const
    const_cast<int&>(r1) = 2; // ok, modifies non-const object n1
 
    const int& r2 = n2; // reference to const bound to const object
//  r2 = 2; // error: attempt to modify through reference to const
//  const_cast<int&>(r2) = 2; // undefined behavior: attempt to modify const object n2
}

class Test { 
public: 
  int x; 
  mutable int y; 
  Test() { x = 4; y = 10; } 
}; 
int main() 
{ 
    const Test t1; 
    t1.y = 20; 
    cout << t1.y; 
    return 0; 
}

class Customer 
{ 
    char name[25]; 
    mutable char placedorder[50]; 
    int tableno; 
    mutable int bill; 
public: 
    Customer(char* s, char* m, int a, int p) 
    { 
        strcpy(name, s); 
        strcpy(placedorder, m); 
        tableno = a; 
        bill = p; 
    } 
    void changePlacedOrder(char* p) const
    { 
        strcpy(placedorder, p); 
    } 
    void changeBill(int s) const
    { 
        bill = s; 
    } 
    void display() const
    { 
        cout << "Customer name is: " << name << endl; 
        cout << "Food ordered by customer is: " << placedorder << endl; 
        cout << "table no is: " << tableno << endl; 
        cout << "Total payable amount: " << bill << endl; 
    } 
}; 

#include <iostream>
#include <stdexcept>
 
// C++11 constexpr functions use recursion rather than iteration
// (C++14 constexpr functions may use local variables and loops)
constexpr int factorial(int n)
{
    return n <= 1 ? 1 : (n * factorial(n - 1));
}
 
// literal class
class conststr {
    const char* p;
    std::size_t sz;
public:
    template<std::size_t N>
    constexpr conststr(const char(&a)[N]): p(a), sz(N - 1) {}
 
    // constexpr functions signal errors by throwing exceptions
    // in C++11, they must do so from the conditional operator ?:
    constexpr char operator[](std::size_t n) const
    {
        return n < sz ? p[n] : throw std::out_of_range("");
    }
    constexpr std::size_t size() const { return sz; }
};
 
// C++11 constexpr functions had to put everything in a single return statement
// (C++14 doesn't have that requirement)
constexpr std::size_t countlower(conststr s, std::size_t n = 0,
                                             std::size_t c = 0)
{
    return n == s.size() ? c :
           'a' <= s[n] && s[n] <= 'z' ? countlower(s, n + 1, c + 1) :
                                       countlower(s, n + 1, c);
}
 
// output function that requires a compile-time constant, for testing
template<int n>
struct constN
{
    constN() { std::cout << n << '\n'; }
};
 
int main()
{
    std::cout << "4! = " ;
    constN<factorial(4)> out1; // computed at compile time
 
    volatile int k = 8; // disallow optimization using volatile
    std::cout << k << "! = " << factorial(k) << '\n'; // computed at run time
 
    std::cout << "the number of lowercase letters in \"Hello, world!\" is ";
    constN<countlower("Hello, world!")> out2; // implicitly converted to conststr
}

struct A { double x; };
const A* a;
 
decltype(a->x) y;       // type of y is double (declared type)
decltype((a->x)) z = y; // type of z is const double& (lvalue expression)
 
template<typename T, typename U>
auto add(T t, U u) -> decltype(t + u) // return type depends on template parameters
                                      // return type can be deduced since C++14
{
    return t+u;
}
 
int main() 
{
    int i = 33;
    decltype(i) j = i * 2;
 
    std::cout << "i = " << i << ", "
              << "j = " << j << '\n';
 
    auto f = [](int a, int b) -> int
    {
        return a * b;
    };
 
    decltype(f) g = f; // the type of a lambda function is unique and unnamed
    i = f(2, 2);
    j = g(3, 3);
 
    std::cout << "i = " << i << ", "
              << "j = " << j << '\n';
}

#include <iostream>
#include <utility>

template<class T, class U>
auto add(T t, U u) { return t + u; } // the return type is the type of operator+(T, U)

// perfect forwarding of a function call must use decltype(auto)
// in case the function it calls returns by reference
template<class F, class... Args>
decltype(auto) PerfectForward(F fun, Args&&... args)
{
    return fun(std::forward<Args>(args)...);
}

template<auto n> // C++17 auto parameter declaration
auto f() -> std::pair<decltype(n), decltype(n)> // auto can't deduce from brace-init-list
{
    return {n, n};
}

int main()
{
    auto a = 1 + 2;            // type of a is int
    auto b = add(1, 1.2);      // type of b is double
    static_assert(std::is_same_v<decltype(a), int>);
    static_assert(std::is_same_v<decltype(b), double>);

    auto c0 = a;             // type of c0 is int, holding a copy of a
    decltype(auto) c1 = a;   // type of c1 is int, holding a copy of a
    decltype(auto) c2 = (a); // type of c2 is int&, an alias of a
    std::cout << "a, before modification through c2 = " << a << '\n';
    ++c2;
    std::cout << "a, after modification through c2 = " << a << '\n';

    auto [v, w] = f<0>(); //structured binding declaration
    std::cout<<v<<w;

    auto d = {1, 2}; // OK: type of d is std::initializer_list<int>
    auto n = {5};    // OK: type of n is std::initializer_list<int>
//  auto e{1, 2};    // Error as of C++17, std::initializer_list<int> before
    auto m{5};       // OK: type of m is int as of C++17, initializer_list<int> before
//  decltype(auto) z = { 1, 2 } // Error: {1, 2} is not an expression

    // auto is commonly used for unnamed types such as the types of lambda expressions
    auto lambda = [](int x) { return x + 3; };

//  auto int x; // valid C++98, error as of C++11
//  auto x;     // valid C, error in C++
}

int i;
int&& f();
auto x3a = i;                  // decltype(x3a) is int
decltype(auto) x3d = i;        // decltype(x3d) is int
auto x4a = (i);                // decltype(x4a) is int
decltype(auto) x4d = (i);      // decltype(x4d) is int&
auto x5a = f();                // decltype(x5a) is int
decltype(auto) x5d = f();      // decltype(x5d) is int&&
auto x6a = { 1, 2 };           // decltype(x6a) is std::initializer_list<int>
decltype(auto) x6d = { 1, 2 }; // error, { 1, 2 } is not an expression
auto *x7a = &i;                // decltype(x7a) is int*
decltype(auto)*x7d = &i;       // error, declared type is not plain decltype(auto)

template<class Fun, class... Args>
decltype(auto) Example(Fun fun, Args&&... args) 
{ 
    return fun(std::forward<Args>(args)...); 
}

template<int i> 
struct Int {};

constexpr auto iter(Int<0>) -> Int<0>;

template<int i>
constexpr auto iter(Int<i>) -> decltype(auto) 
{ return iter(Int<i-1>{}); }

int main() { decltype(iter(Int<10>{})) a; }

enum A { A_START = 0 };

void A(enum A a) {}

int main() {
   enum A a;
   A( a );
}

template <typename T>
struct Node {
    struct Node* Next; // OK: lookup of Node finds the injected-class-name
    struct Data* Data; // OK: declares type Data at global scope
                       // and also declares the data member Data
    friend class ::List; // error: cannot introduce a qualified name
    enum Kind* kind; // error: cannot introduce an enum
};
 
Data* p; // OK: struct Data has been declared