struct Base { virtual int g(); virtual ~Base() {} };
struct A : Base {
    // OK: declares three member virtual functions, two of them pure
    virtual int f() = 0, g() override = 0, h();
    // OK: destructor can be pure too
    ~A() = 0;
    // Error: pure-specifier on a function definition
    virtual int b()=0 {}
};

struct Abstract {
    virtual void f() = 0; // pure virtual
    virtual void g() {}  // non-pure virtual
    ~Abstract() {
        g(); // OK: calls Abstract::g()
        // f(); // undefined behavior
        Abstract::f(); // OK: non-virtual call
    }
};
 
// definition of the pure virtual function
void Abstract::f() { std::cout << "A::f()\n"; }
 
struct Concrete : Abstract {
    void f() override {
        Abstract::f(); // OK: calls pure virtual function
    }
    void g() override {}
    ~Concrete() {
        g(); // OK: calls Concrete::g()
        f(); // OK: calls Concrete::f()
    }
};

struct A
{
    virtual void foo();
    void bar();
};
 
struct B : A
{
    void foo() const override; // Error: B::foo does not override A::foo
                               // (signature mismatch)
    void foo() override; // OK: B::foo overrides A::foo
    void bar() override; // Error: A::bar is not virtual
};

struct Base
{
    virtual void foo();
};
 
struct A : Base
{
    void foo() final; // Base::foo is overridden and A::foo is the final override
    void bar() final; // Error: bar cannot be final as it is non-virtual
};
 
struct B final : A // struct B is final
{
    void foo() override; // Error: foo cannot be overridden as it is final in A
};
 
struct C : B // Error: B is final
{
};

class Y {
    int data; // private member
    // the non-member function operator<< will have access to Y's private members
    friend std::ostream& operator<<(std::ostream& out, const Y& o);
    friend char* X::foo(int); // members of other classes can be friends too
    friend X::X(char), X::~X(); // constructors and destructors can be friends
};
// friend declaration does not declare a member function
// this operator<< still needs to be defined, as a non-member
std::ostream& operator<<(std::ostream& out, const Y& y)
{
    return out << y.data; // can access private member Y::data
}

class F {};
int f();
int main()
{
    extern int g();
    class Local { // Local class in the main() function
        friend int f(); // Error, no such function declared in main()
        friend int g(); // OK, there is a declaration for g in main()
        friend class F; // friends a local F (defined later)
        friend class ::F; // friends the global F
    };
    class F {}; // local F
}

#include <iostream>
struct S {
    // will usually occupy 2 bytes:
    // 3 bits: value of b1
    // 2 bits: unused
    // 6 bits: value of b2
    // 2 bits: value of b3
    // 3 bits: unused
    unsigned char b1 : 3, : 2, b2 : 6, b3 : 2;
};
int main()
{
    std::cout << sizeof(S) << '\n'; // usually prints 2
}

#include <iostream>
struct S {
    // will usually occupy 2 bytes:
    // 3 bits: value of b1
    // 5 bits: unused
    // 6 bits: value of b2
    // 2 bits: value of b3
    unsigned char b1 : 3;
    unsigned char :0; // start a new byte
    unsigned char b2 : 6;
    unsigned char b3 : 2;
};
int main()
{
    std::cout << sizeof(S) << '\n'; // usually prints 2
}

class T
{
    int x;
 
    void foo()
    {
        x = 6;       // same as this->x = 6;
        this->x = 5; // explicit use of this->
    }
 
    void foo() const
    {
//        x = 7; // Error: *this is constant
    }
 
    void foo(int x) // parameter x shadows the member with the same name
    {
        this->x = x; // unqualified x refers to the parameter
                     // 'this->' required for disambiguation
    }
 
    int y;
    T(int x) : x(x), // uses parameter x to initialize member x
               y(this->x) // uses member x to initialize member y
    {}
 
    T& operator= ( const T& b )
    {
        x = b.x;
        return *this; // many overloaded operators return *this
    }
};
 
class Outer {
    int a[sizeof(*this)]; // error: not inside a member function
    unsigned int sz = sizeof(*this); // OK: in default member initializer
    void f() {
        int b[sizeof(*this)]; // OK
        struct Inner {
            int c[sizeof(*this)]; // error: not inside a member function of Inner
        };
    }
}

#include <fstream>
#include <string>
#include <mutex>
 
struct Base {
    int n;
};   
 
struct Class : public Base
{
    unsigned char x;
    unsigned char y;
    std::mutex m;
    std::lock_guard<std::mutex> lg;
    std::fstream f;
    std::string s;
 
    Class ( int x )
      : Base { 123 }, // initialize base class
        x ( x ),      // x (member) is initialized with x (parameter)
        y { 0 },      // y initialized to 0
        f{"test.cc", std::ios::app}, // this takes place after m and lg are initialized
        s(__func__),   //__func__ is available because init-list is a part of constructor
        lg ( m ),      // lg uses m, which is already initialized
        m{}            // m is initialized before lg even though it appears last here
    {}                // empty compound statement
 
    Class ( double a )
      : y ( a+1 ),
        x ( y ), // x will be initialized before y, its value here is indeterminate
        lg ( m )
    {} // base class initializer does not appear in the list, it is
       // default-initialized (not the same as if Base() were used, which is value-init)
 
    Class()
    try // function-try block begins before the function body, which includes init list
      : Class( 0.0 ) //delegate constructor
    {
        // ...
    }
    catch (...)
    {
        // exception occurred on initialization
    }
};
 
int main() {
    Class c;
    Class c1(1);
    Class c2(0.1);
}

struct A
{
    int x;
    A(int x = 1): x(x) {} // user-defined default constructor
};
 
struct B: A
{
    // B::B() is implicitly-defined, calls A::A()
};
 
struct C
{
    A a;
    // C::C() is implicitly-defined, calls A::A()
};
 
struct D: A
{
    D(int y): A(y) {}
    // D::D() is not declared because another constructor exists
};
 
struct E: A
{
    E(int y): A(y) {}
    E() = default; // explicitly defaulted, calls A::A()
};
 
struct F
{
    int& ref; // reference member
    const int c; // const member
    // F::F() is implicitly defined as deleted
};
 
int main()
{
    A a;
    B b;
    C c;
//  D d; // compile error
    E e;
//  F f; // compile error
}

#include <iostream>
 
struct A
{
    int i;
 
    A ( int i ) : i ( i ) 
    {
        std::cout << "ctor a" << i << '\n';
    }
 
    ~A()
    {
        std::cout << "dtor a" << i << '\n';
    }
};
 
A a0(0);
 
int main()
{
    A a1(1);
    A* p;
 
    { // nested scope
        A a2(2);
        p = new A(3);
    } // a2 out of scope
 
    delete p; // calls the destructor of a3
}

struct A
{
    int n;
    A(int n = 1) : n(n) { }
    A(const A& a) : n(a.n) { } // user-defined copy ctor
};
 
struct B : A
{
    // implicit default ctor B::B()
    // implicit copy ctor B::B(const B&)
};
 
struct C : B
{
     C() : B() { }
 private:
     C(const C&); // non-copyable, C++98 style
};
 
int main()
{
    A a1(7);
    A a2(a1); // calls the copy ctor
    B b;
    B b2 = b;
    A a3 = b; // conversion to A& and copy ctor
    volatile A va(10);
    // A a4 = va; // compile error
 
    C c;
    // C c2 = c; // compile error
}

#include <iostream>
#include <memory>
#include <string>
#include <algorithm>
 
struct A
{
    int n;
    std::string s1;
    // user-defined copy assignment, copy-and-swap form
    A& operator=(A other)
    {
        std::cout << "copy assignment of A\n";
        std::swap(n, other.n);
        std::swap(s1, other.s1);
        return *this;
    }
};
 
struct B : A
{
    std::string s2;
    // implicitly-defined copy assignment
};
 
struct C
{
    std::unique_ptr<int[]> data;
    std::size_t size;
    // non-copy-and-swap assignment
    C& operator=(const C& other)
    {
        // check for self-assignment
        if(&other == this)
            return *this;
        // reuse storage when possible
        if(size != other.size)
        {
            data.reset(new int[other.size]);
            size = other.size;
        }
        std::copy(&other.data[0], &other.data[0] + size, &data[0]);
        return *this;
    }
    // note: copy-and-swap would always cause a reallocation
};
 
int main()
{
    A a1, a2;
    std::cout << "a1 = a2 calls ";
    a1 = a2; // user-defined copy assignment
 
    B b1, b2;
    b2.s1 = "foo";
    b2.s2 = "bar";
    std::cout << "b1 = b2 calls ";
    b1 = b2; // implicitly-defined copy assignment
    std::cout << "b1.s1 = " << b1.s1 << " b1.s2 = " << b1.s2 << '\n';
}

#include <string>
#include <iostream>
#include <iomanip>
#include <utility>
 
struct A
{
    std::string s;
    int k;
    A() : s("test"), k(-1) { }
    A(const A& o) : s(o.s), k(o.k) { std::cout << "move failed!\n"; }
    A(A&& o) noexcept :
           s(std::move(o.s)),       // explicit move of a member of class type
           k(std::exchange(o.k, 0)) // explicit move of a member of non-class type
    { }
};
 
A f(A a)
{
    return a;
}
 
struct B : A
{
    std::string s2;
    int n;
    // implicit move constructor B::(B&&)
    // calls A's move constructor
    // calls s2's move constructor
    // and makes a bitwise copy of n
};
 
struct C : B
{
    ~C() { } // destructor prevents implicit move constructor C::(C&&)
};
 
struct D : B
{
    D() { }
    ~D() { }          // destructor would prevent implicit move constructor D::(D&&)
    D(D&&) = default; // forces a move constructor anyway
};
 
int main()
{
    std::cout << "Trying to move A\n";
    A a1 = f(A()); // return by value move-constructs the target from the function parameter
    std::cout << "Before move, a1.s = " << std::quoted(a1.s) << " a1.k = " << a1.k << '\n';
    A a2 = std::move(a1); // move-constructs from xvalue
    std::cout << "After move, a1.s = " << std::quoted(a1.s) << " a1.k = " << a1.k << '\n';
 
    std::cout << "Trying to move B\n";
    B b1;
    std::cout << "Before move, b1.s = " << std::quoted(b1.s) << "\n";
    B b2 = std::move(b1); // calls implicit move constructor
    std::cout << "After move, b1.s = " << std::quoted(b1.s) << "\n";
 
    std::cout << "Trying to move C\n";
    C c1;
    C c2 = std::move(c1); // calls copy constructor
 
    std::cout << "Trying to move D\n";
    D d1;
    D d2 = std::move(d1);
}

#include <string>
#include <iostream>
#include <utility>
 
struct A
{
    std::string s;
    A() : s("test") { }
    A(const A& o) : s(o.s) { std::cout << "move failed!\n"; }
    A(A&& o) : s(std::move(o.s)) { }
    A& operator=(const A& other)
    {
         s = other.s;
         std::cout << "copy assigned\n";
         return *this;
    }
    A& operator=(A&& other)
    {
         s = std::move(other.s);
         std::cout << "move assigned\n";
         return *this;
    }
};
 
A f(A a) { return a; }
 
struct B : A
{
     std::string s2; 
     int n;
     // implicit move assignment operator B& B::operator=(B&&)
     // calls A's move assignment operator
     // calls s2's move assignment operator
     // and makes a bitwise copy of n
};
 
struct C : B
{
    ~C() { } // destructor prevents implicit move assignment
};
 
struct D : B
{
    D() { }
    ~D() { } // destructor would prevent implicit move assignment
    D& operator=(D&&) = default; // force a move assignment anyway 
};
 
int main()
{
    A a1, a2;
    std::cout << "Trying to move-assign A from rvalue temporary\n";
    a1 = f(A()); // move-assignment from rvalue temporary
    std::cout << "Trying to move-assign A from xvalue\n";
    a2 = std::move(a1); // move-assignment from xvalue
 
    std::cout << "Trying to move-assign B\n";
    B b1, b2;
    std::cout << "Before move, b1.s = \"" << b1.s << "\"\n";
    b2 = std::move(b1); // calls implicit move assignment
    std::cout << "After move, b1.s = \"" << b1.s << "\"\n";
 
    std::cout << "Trying to move-assign C\n";
    C c1, c2;
    c2 = std::move(c1); // calls the copy assignment operator
 
    std::cout << "Trying to move-assign D\n";
    D d1, d2;
    d2 = std::move(d1);
}

struct A
{
    A() { }         // converting constructor (since C++11)  
    A(int) { }      // converting constructor
    A(int, int) { } // converting constructor (since C++11)
};
 
struct B
{
    explicit B() { }
    explicit B(int) { }
    explicit B(int, int) { }
};
 
int main()
{
    A a1 = 1;      // OK: copy-initialization selects A::A(int)
    A a2(2);       // OK: direct-initialization selects A::A(int)
    A a3{4, 5};    // OK: direct-list-initialization selects A::A(int, int)
    A a4 = {4, 5}; // OK: copy-list-initialization selects A::A(int, int)
    A a5 = (A)1;   // OK: explicit cast performs static_cast, direct-initialization
 
//  B b1 = 1;      // error: copy-initialization does not consider B::B(int)
    B b2(2);       // OK: direct-initialization selects B::B(int)
    B b3{4, 5};    // OK: direct-list-initialization selects B::B(int, int)
//  B b4 = {4, 5}; // error: copy-list-initialization selected an explicit constructor
                   //        B::B(int, int)
    B b5 = (B)1;   // OK: explicit cast performs static_cast, direct-initialization
    B b6;          // OK, default-initialization
    B b7{};        // OK, direct-list-initialization
//  B b8 = {};     // error: copy-list-initialization selected an explicit constructor
                   //        B::B()
}

class A{
public:
    void fun()&{
        cout<<"can called only by lvalue";
    }
    void fun()&&{
        cout<<"can called only by rvalue";
    }
};