______________________________________________________________________

  12   Special member functions                      [special]

  ______________________________________________________________________

1 The default constructor  (_class.ctor_),  copy  constructor  and  copy
  assignment  operator (_class.copy_), and destructor (_class.dtor_) are
  special member functions.  The implementation will implicitly  declare
  these  member  functions  for  a  class type when the program does not
  explicitly declare them, except as noted in _class.ctor_.  The  imple­
  mentation  will  implicitly define them if they are used, as specified
  in _class.ctor_, _class.dtor_ and _class.copy_.  Programs may  explic­
  itly refer to implicitly declared special member functions.  [Example:
          struct A { }; // implicitly-declared A::operator=
          struct B : A {
                  B& operator=(const B &);
          };
          B& B::operator=(const B& s) {
                  this->A::operator=(s); // well-formed
          }
   --end example] [Note: the special member  functions  affect  the  way
  objects of class type are created, copied, and destroyed, and how val­
  ues can be converted to values of other  types.   Often  such  special
  member functions are called implicitly.  ]

2 Special member functions obey the usual access rules (_class.access_).
  [Example: declaring a constructor protected ensures that only  derived
  classes and friends can create objects using it.  ]

  12.1  Constructors                                        [class.ctor]

1 Constructors do not have names.  A special declarator syntax using the
  constructor's class name followed by  a  parameter  list  is  used  to
  declare or define the constructor.  [Example:
          class C {
          public:
                  C();       // declares the constructor
          };

          C::C() { }         // defines the constructor
   --end example]

2 A  constructor  is  used  to  initialize  objects  of  its class type.
  Because constructors do not have names, they are  never  found  during
  name  lookup; however an explicit type conversion using the functional
  notation (_expr.type.conv_) will cause a constructor to be  called  to
  initialize  an  object.  [Note: for initialization of objects of class
  type see _class.init_.  ]

3 A  constructor  shall  not  be  virtual  (_class.virtual_)  or  static
  (_class.static_).   A constructor can be invoked for a const, volatile
  or const volatile object.  A constructor shall not be declared  const,
  volatile, or const volatile (_class.this_).  const and volatile seman­
  tics (_dcl.type.cv_) are not applied on an object under  construction.
  Such semantics only come into effect once the constructor for the most
  derived object (_intro.object_) ends.

4 A default constructor for a class X is a constructor of class  X  that
  can  be called without an argument.  If there is no user-declared con­
  structor for class X, a default constructor  is  implicitly  declared.
  An  implicitly-declared  default constructor is a public member of its
  class.  A constructor is  trivial  if  it  is  an  implicitly-declared
  default constructor and if:

  --its  class has no virtual functions (_class.virtual_) and no virtual
    base classes (_class.mi_), and

  --all the direct base classes of its class have trivial  constructors,
    and

  --for  all  the  nonstatic data members of its class that are of class
    type (or array thereof), each such class has a trivial  constructor.

5 Otherwise, the constructor is non-trivial.

6 An  implicitly-declared  default constructor for a class is implicitly
  defined when it is  used  to  create  an  object  of  its  class  type
  (_intro.object_).   A  program  is ill-formed if the class for which a
  default constructor is implicitly defined has:

  --a nonstatic data member of const type, or

  --a nonstatic data member of reference type, or

  --a nonstatic data member of class type (or  array  thereof)  with  an
    inaccessible default constructor, or

  --a base class with an inaccessible default constructor.

  Before  the  implicitly-declared  default  constructor  for a class is
  implicitly defined, all the implicitly-declared  default  constructors
  for  its  base  classes and its nonstatic data members shall have been
  implicitly defined.

7 Default constructors are called implicitly to create class objects  of
  static    or    automatic    storage   duration   (_basic.stc.static_,
  _basic.stc.auto_) defined without  an  initializer  (_dcl.init_),  are
  called   to   create   class   objects  of  dynamic  storage  duration
  (_basic.stc.dynamic_) created by a new-expression in  which  the  new-
  initializer  is  omitted (_expr.new_), or are called when the explicit
  type conversion syntax (_expr.type.conv_) is used.  A program is  ill-
  formed if the default constructor for an object is implicitly used and

  the constructor is not accessible (_class.access_).

8 [Note: _class.base.init_ describes the order in which constructors for
  base  classes and non-static data members are called and describes how
  arguments can be specified for the calls to these constructors.  ]

9 A copy constructor for a class X is a constructor with a first parame­
  ter  of type X& or of type const X&.  [Note: see _class.copy_ for more
  information on copy constructors.  ]

10A union member shall not be of a class type (or  array  thereof)  that
  has a non-trivial constructor.

11No  return  type (not even void) shall be specified for a constructor.
  A return statement in the body of a constructor shall  not  specify  a
  return value.  The address of a constructor shall not be taken.

12A  constructor  can  be  used  explicitly to create new objects of its
  type, using the syntax
          class-name ( expression-listopt )
  [Example:
          complex zz = complex(1,2.3);
          cprint( complex(7.8,1.2) );
   --end example] An object created in  this  way  is  unnamed.   [Note:
  _class.temporary_  describes  the  lifetime  of  temporary objects.  ]
  [Note:  explicit  constructor  calls  do  not   yield   lvalues,   see
  _basic.lval_.  ]

13[Note:  some language constructs have special semantics when used dur­
  ing construction; see _class.base.init_ and _class.cdtor_.  ]

  12.2  Temporary objects                              [class.temporary]

1 While evaluating an expression, it might be  necessary  or  convenient
  for  an  implementation  to  generate temporary objects to hold values
  resulting from the  evaluation  of  the  expression's  subexpressions.
  During this evaluation, precisely when such temporaries are introduced
  is unspecified.  Even when the creation of  the  temporary  object  is
  avoided,  all  the  semantic  restrictions must be respected as if the
  temporary object was created.  [Example: even if the copy  constructor
  is  not  called,  all the semantic restrictions, such as accessibility
  (_class.access_), shall be satisfied.  ]

2 [Example:
          class X {
              // ...
          public:
              // ...
              X(int);
              X(const X&);
              ~X();
          };

          X f(X);

          void g()
          {
              X a(1);
              X b = f(X(2));
              a = f(a);
          }
  Here, an implementation might use a temporary in  which  to  construct
  X(2)  before  passing  it  to f() using X's copy-constructor; alterna­
  tively, X(2) might be constructed in the space used to hold the  argu­
  ment.   Also,  a temporary might be used to hold the result of f(X(2))
  before copying it to  b  using  X's  copy-constructor;  alternatively,
  f()'s  result  might  be  constructed  in  b.   On the other hand, the
  expression a=f(a) requires a temporary for either the  argument  a  or
  the result of f(a) to avoid undesired aliasing of a.  ]

3 When  an  implementation introduces a temporary object of a class that
  has a non-trivial constructor (_class.ctor_), it shall ensure  that  a
  constructor  is  called  for  the  temporary  object.   Similarly, the
  destructor shall be called for a temporary with a non-trivial destruc­
  tor  (_class.dtor_).  Temporary objects are destroyed as the last step
  in evaluating the full-expression (_intro.execution_) that (lexically)
  contains the point where they were created.  This is true even if that
  evaluation ends in throwing an exception.

4 There are two contexts in which temporaries are destroyed at a differ­
  ent  point  than the end of the full-expression.  The first context is
  when an expression appears as an initializer for a declarator defining
  an  object.   In  that context, the temporary that holds the result of
  the expression shall persist until the object's initialization is com­
  plete.  The object is initialized from a copy of the temporary; during
  this copying, an implementation can call  the  copy  constructor  many
  times;  the temporary is destroyed after it has been copied, before or
  when the initialization completes.  If many temporaries are created by
  the  evaluation  of  the initializer, the temporaries are destroyed in
  reverse order of construction.

5 The second context is when a reference is bound to a  temporary.   The
  temporary to which the reference is bound or the temporary that is the
  complete object to a subobject of which the temporary  is  bound  per­
  sists  for the lifetime of the reference or until the end of the scope
  in which the temporary is created, whichever comes first.  A temporary
  holding  the result of an initializer expression for a declarator that
  declares a reference persists until the end of the scope in which  the
  reference  declaration  occurs.  A temporary bound to a reference in a
  constructor's ctor-initializer (_class.base.init_) persists until  the
  constructor  exits.   A  temporary bound to a reference parameter in a
  function call (_expr.call_) persists until the completion of the  full
  expression  containing  the  call.   A  temporary  bound in a function
  return statement (_stmt.return_) persists until  the  function  exits.
  In  all these cases, the temporaries are destroyed in reverse order of
  construction.  In addition, the destruction of  temporaries  bound  to
  references  shall  take  into  account  the ordering of destruction of
  objects with static or automatic storage duration (_basic.stc.static_,

  _basic.stc.auto_);  that is, if obj1 is an object with static or auto­
  matic storage duration created before the temporary  is  created,  the
  temporary  shall  be destroyed before obj1 is destroyed; if obj2 is an
  object with static or automatic storage  duration  created  after  the
  temporary  is  created, the temporary shall be destroyed after obj2 is
  destroyed.  [Example:
          class C {
                  // ...
          public:
                  C();
                  C(int);
                  friend const C& operator+(const C&, const C&);
                  ~C();
          };
          C obj1;
          const C& cr = C(16)+C(23);
          C obj2;
  the expression C(16)+C(23) creates three temporaries.  A first  tempo­
  rary T1 to hold the result of the expression C(16), a second temporary
  T2 to hold the result of the expression C(23), and a  third  temporary
  T3  to  hold the result of the addition of these two expressions.  The
  temporary T3 is then bound to the reference cr.  Because  addition  is
  commutative,  it is unspecified whether T1 or T2 is created first.  On
  an implementation where T1 is created before T2, it is guaranteed that
  T2 is destroyed before T1.  The temporaries T1 and T2 are bound to the
  reference parameters of operator+; these temporaries are destroyed  at
  the  end of the full expression containing the call to operator+.  The
  temporary T3 bound to the reference cr is destroyed at the end of cr's
  lifetime,  that is, at the end of the program.  In addition, the order
  in which T3 is destroyed takes into account the destruction  order  of
  other  objects with static storage duration.  That is, because obj1 is
  constructed before T3, and T3 is constructed before obj2, it is  guar­
  anteed  that obj2 is destroyed before T3, and it is guaranteed that T3
  is destroyed before obj1.  ]

  12.3  Conversions                                         [class.conv]

1 Type conversions of class objects can be specified by constructors and
  by  conversion  functions.   These conversions are called user-defined
  conversions and are used for implicit type conversions  (_conv_),  for
  initialization   (_dcl.init_),   and  for  explicit  type  conversions
  (_expr.cast_, _expr.static.cast_).

2 User-defined conversions are applied only where they  are  unambiguous
  (_class.member.lookup_,   _class.conv.fct_).    Conversions  obey  the
  access control rules  (_class.access_).   Access  control  is  applied
  after ambiguity resolution (_basic.lookup_).

3 [Note:  See _over.match_ for a discussion of the use of conversions in
  function calls as well as examples below.  ]

  12.3.1  Conversion by constructor                    [class.conv.ctor]

1 A constructor declared without the  function-specifier  explicit  that
  can  be called with a single parameter specifies a conversion from the
  type of its first parameter to the type of its  class.   Such  a  con­
  structor is called a converting constructor.  [Example:
          class X {
              // ...
          public:
              X(int);
              X(const char*, int =0);
          };
          void f(X arg)
          {
              X a = 1;         // a = X(1)
              X b = "Jessie";  // b = X("Jessie",0)
              a = 2;           // a = X(2)
              f(3);            // f(X(3))
          }
   --end example]

2 An explicit constructor constructs objects just like non-explicit con­
  structors, but does so only  where  the  direct-initialization  syntax
  (_dcl.init_)  or  where  casts  (_expr.static.cast_,  _expr.cast_) are
  explicitly used.  [Example:
          class Z {
          public:
                  explicit Z(int);
                  // ...
          };
          Z a1 = 1;        // error: no implicit conversion
          Z a3 = Z(1);     // ok: direct initialization syntax used
          Z a2(1);         // ok: direct initialization syntax used
          Z* p = new Z(1); // ok: direct initialization syntax used
          Z a4 = (Z)1;              // ok: explicit cast used
          Z a5 = static_cast<Z>(1); // ok: explicit cast used
   --end example]

3 A  copy-constructor  (_class.copy_)  is  a  conversion  function.   An
  implicitly-declared  copy constructors is not an explicit constructor;
  it may be called for implicit type conversions.

  12.3.2  Conversion functions                          [class.conv.fct]

1 A member function of a class X with a name of the form
          conversion-function-id:
                  operator conversion-type-id
          conversion-type-id:
                  type-specifier-seq conversion-declaratoropt
          conversion-declarator:
                  ptr-operator conversion-declaratoropt
  specifies a conversion from X to the type specified by the conversion-
  type-id.   Such  member  functions  are  called  conversion functions.

  Classes, enumerations, and typedef-names shall not be declared in  the
  type-specifier-seq.   Neither  parameter  types nor return type can be
  specified.  The type of a conversion function (_dcl.fct_) is "function
  taking no parameter returning conversion-type-id."  A conversion oper­
  ator is never used to convert a (possibly cv-qualified) object to  the
  (possibly  cv-qualified) same object type (or a reference to it), to a
  (possibly cv-qualified) base class of that type  (or  a  reference  to
  it), or to (possibly cv-qualified) void.1)

2 [Example:
          class X {
              // ...
          public:
              operator int();
          };
          void f(X a)
          {
              int i = int(a);
              i = (int)a;
              i = a;
          }
  In   all   three  cases  the  value  assigned  will  be  converted  by
  X::operator int().   --end example]

3 User-defined conversions are not restricted to use in assignments  and
  initializations.  [Example:
          void g(X a, X b)
          {
              int i = (a) ? 1+a : 0;
              int j = (a&&b) ? a+b : i;
              if (a) { // ...
              }
          }
   --end example]

4 The  conversion-type-id  shall  not  represent  a function type nor an
  array type.  The conversion-type-id in a conversion-function-id is the
  longest possible sequence of conversion-declarators.  [Note: this pre­
  vents ambiguities between the declarator operator * and its expression
  counterparts.  [Example:
          &ac.operator int*i; // syntax error:
                              // parsed as: '&(ac.operator int *) i'
                              // not as: '&(ac.operator int)*i'
  The  *  is the pointer declarator and not the multiplication operator.
  ] ]

5 Conversion operators are inherited.

  _________________________
  1)  Even  though  never  directly called to perform a conversion, such
  conversion operators can be declared and can  potentially  be  reached
  through a call to a virtual conversion operator in a base class

6 Conversion functions can be virtual.

7 At most one user-defined conversion (constructor or  conversion  func­
  tion) is implicitly applied to a single value.  [Example:
          class X {
              // ...
          public:
              operator int();
          };
          class Y {
              // ...
          public:
              operator X();
          };
          Y a;
          int b = a;    // error:
                        // a.operator X().operator int() not tried
          int c = X(a); // ok: a.operator X().operator int()
   --end example]

8 User-defined  conversions  are  used implicitly only if they are unam­
  biguous.  A conversion function in a derived class  does  not  hide  a
  conversion  function  in a base class unless the two functions convert
  to the same type.  Function overload resolution (_over.match.best_) is
  used to select the best conversion function to perform the conversion.
  [Example:
          class X {
          public:
              // ...
              operator int();
          };
          class Y : public X {
          public:
              // ...
              operator void*();
          };
          void f(Y& a)
          {
              if (a) {    // calls D::operator void*
                  // ...
              }
          }
   --end example]

  12.4  Destructors                                         [class.dtor]

1 A special declarator syntax using a ~  followed  by  the  destructor's
  class  name followed by an empty parameter list is used to declare the
  destructor in a class definition.  A destructor  is  used  to  destroy
  objects  of  its class type.  A destructor takes no parameters, and no
  return type can be specified for it (not even void).  The address of a
  destructor  shall  not be taken.  A destructor shall not be static.  A
  destructor can be invoked for a  const,  volatile  or  const  volatile

  object.   A  destructor shall not be declared const, volatile or const
  volatile (_class.this_).  const and volatile semantics (_dcl.type.cv_)
  are  not  applied on an object under destruction.  Such semantics stop
  being into effect once the destructor  for  the  most  derived  object
  (_intro.object_) starts.

2 If  a  class has no user-declared destructor, a destructor is declared
  implicitly.  An implicitly-declared destructor is a public  member  of
  its  class.   A  destructor is trivial if it is an implicitly-declared
  destructor and if:

  --all of the direct base classes of its class have trivial destructors
    and

  --for  all  of  the  non-static  data members of its class that are of
    class type (or  array  thereof),  each  such  class  has  a  trivial
    destructor.

3 Otherwise, the destructor is non-trivial.

4 An  implicitly-declared  destructor  is  implicitly defined when it is
  used to destroy an object of its class type (_basic.stc_).  A  program
  is  ill-formed  if  the  class  for  which  a destructor is implicitly
  defined has:

  --a non-static data member of class type (or array  thereof)  with  an
    inaccessible destructor, or

  --a base class with an inaccessible destructor.

  Before  the  implicitly-declared  destructor for a class is implicitly
  defined, all the implicitly-declared destructors for its base  classes
  and its nonstatic data members shall have been implicitly defined.

5 A destructor for class X calls the destructors for X's direct members,
  the destructors for X's direct base classes and, if X is the  type  of
  the  most  derived class (_class.base.init_), its destructor calls the
  destructors for X's virtual  base  classes.   Bases  and  members  are
  destroyed  in the reverse order of the completion of their constructor
  (see _class.base.init_).  A  return  statement  (_stmt.return_)  in  a
  destructor  might  not directly return to the caller; before transfer­
  ring control to the caller, the destructors for the members and  bases
  are  called.   Destructors  for  elements  of  an  array are called in
  reverse order of their construction (see _class.init_).

6 A destructor can be declared virtual (_class.virtual_) or pure virtual
  (_class.abstract_);  if any objects of that class or any derived class
  are created in the program, the destructor shall  be  defined.   If  a
  class  has  a  base  class  with a virtual destructor, its  destructor
  (whether user- or implicitly- declared) is virtual.

7 [Note: some language constructs have special semantics when used  dur­
  ing destruction; see _class.cdtor_.  ]

8 A  union  member  shall not be of a class type (or array thereof) that
  has a non-trivial destructor.

9 Destructors are invoked implicitly (1) for a constructed  object  with
  static  storage  duration  (_basic.stc.static_) at program termination
  (_basic.start.term_), (2) for  a  constructed  object  with  automatic
  storage duration (_basic.stc.auto_) when the block in which the object
  is created exits (_stmt.dcl_), (3) for a constructed temporary  object
  when  the  lifetime  of the temporary object ends (_class.temporary_),
  (4)  for  a  constructed  object   allocated   by   a   new-expression
  (_expr.new_),  through use of a delete-expression (_expr.delete_), (5)
  in  several   situations   due   to   the   handling   of   exceptions
  (_except.handle_).   A  program is ill-formed if the destructor for an
  object is implicitly used and it is not  accessible  (_class.access_).
  Destructors can also be invoked explicitly.

10A  delete-expression invokes the destructor for the referenced object;
  the object's memory location is then passed to a deallocation function
  (_expr.delete_, _class.free_).  [Example:
          class X {
              // ...
          public:
              X(int);
              ~X();
          };
          void g(X*);
          void f()        // common use:
          {
              X* p = new X(111);  // allocate and initialize
              g(p);
              delete p;           // cleanup and deallocate
          }
   --end example]

11In  an  explicit  destructor  call, the destructor name appears as a ~
  followed by a type-name that names the destructor's class  type.   The
  invocation  of  a  destructor is subject to the usual rules for member
  functions (_class.mfct_), that  is,  if  the  object  is  not  of  the
  destructor's class type and not of class derived from the destructor's
  class type, the program has undefined behavior (except  that  invoking
  delete on a null pointer has no effect).  [Example:
          struct B {
                  virtual ~B() { }
          };
          struct D : B {
                  ~D() { }
          };
          D D_object;
          typedef B B_alias;
          B* B_ptr = &D_object;
          D_object.B::~B();  // calls B's destructor
          B_ptr->~B();       // calls D's destructor
          B_ptr->~B_alias(); // calls D's destructor
   --end example]

12[Note:  explicit  calls  of destructors are rarely needed.  One use of
  such calls is for objects placed at specific addresses  using  a  new-
  expression  with the placement option.  Such use of explicit placement
  and destruction of objects can be necessary  to  cope  with  dedicated
  hardware  resources and for writing memory management facilities.  For
  example,
          void* operator new(size_t, void* p) { return p; }

          void f(X* p);

          static char buf[sizeof(X)];
          void g()        // rare, specialized use:
          {
              X* p = new(buf) X(222);  // use buf[]
                                       // and initialize
              f(p);
              p->X::~X();              // cleanup
          }
   --end note]

13Once a destructor is invoked for  an  object,  the  object  no  longer
  exists;  the behavior is undefined if the destructor is invoked for an
  object whose lifetime has  ended  (_basic.life_).   [Example:  if  the
  destructor  for  an  automatic  object  is explicitly invoked, and the
  block is subsequently left in a manner that  would  ordinarily  invoke
  implicit destruction of the object, the behavior is undefined.  ]

14The  notation  for  explicit  call of a destructor can be used for any
  scalar type name.  Using the notation for a type that does not have  a
  destructor  has  no effect.  [Note: allowing this makes it possible to
  write code without having to know if a destructor exists for  a  given
  type.  [Example:
          int* p;
          // ...
          p->int::~int();
   --end example]  --end note]

15
  12.5  Free store                                          [class.free]

1 When an object is created with a new-expression (_expr.new_), an allo­
  cation   function   (operator new()   for   non-array    objects    or
  operator new[]()  for  arrays)  is  (implicitly)  called  to  get  the
  required storage (_basic.stc.dynamic.allocation_).

2 When a new-expression is used to create an object of class T (or array
  thereof),  the  allocation function is looked up in the scope of class
  T; if no allocation function is found, the global allocation  function
  is used (_basic.stc.dynamic.allocation_).

3 When  a  new-expression  is executed, the selected allocation function
  shall be called with the amount of space requested (possibly zero)  as
  its first argument.

4 Any  allocation function for a class T is a static member (even if not
  explicitly declared static).

5 [Example:
          class Arena;  class Array_arena;
          struct B {
              void* operator new(size_t, Arena*);
          };
          struct D1 : B {
          };
          Arena*  ap;  Array_arena* aap;
          void foo(int i)
          {
              new (ap) D1;  // calls B::operator new(size_t, Arena*)
              new D1[i];    // calls ::operator new[](size_t)
              new D1;       // ill-formed: ::operator new(size_t) hidden
          }
   --end example]

6 When an object is deleted with a delete-expression (_expr.delete_),  a
  deallocation  function  (operator delete()  for  non-array  objects or
  operator delete[]() for arrays) is (implicitly) called to reclaim  the
  storage occupied by the object (_basic.stc.dynamic.deallocation_).

7 When  a delete-expression is used to deallocate an array of objects of
  class T, the deallocation function to be called is determined by look­
  ing  up the name of operator delete[] in the scope of class T.  If the
  result of this lookup is ambiguous or  inaccessible,  the  program  is
  ill-formed.   If  no deallocation function is found in that scope, the
  global  deallocation  function  (_basic.stc.dynamic.deallocation_)  is
  used; see _expr.delete_.

8 When  a  delete-expression  is  used  to deallocate an object of class
  type, the deallocation function to be called is determined by  looking
  up  the  name of operator delete in the context of the outermost block
  of the destructor definition  (ignoring  any  names  defined  in  that
  block).2)  If  the  result of the lookup is ambiguous or inaccessible,
  the program is ill-formed.  If no deallocation function  is  found  in
  that       scope,      the      global      deallocation      function
  (_basic.stc.dynamic.deallocation_) is used; see _expr.delete_.

9 When a delete-expression is executed, the selected deallocation  func­
  tion  shall  be  called with the address of the block of storage to be
  reclaimed as its first argument and (if  the  two-parameter  style  is
  used) the size of the block as its second argument.3)
  _________________________
  2)  This  applies to destructor definitions, not mere declarations.  A
  similar look up is not needed for the array version of the delete  op­
  erator  because  _expr.delete_  requires  that  in this situation, the
  static type of the delete-expression's operand be the same as its  dy­
  namic type.
  3)  If  the static type in the delete-expression is different from the
  dynamic type and the destructor is not virtual the size might  be  in­
  correct, but that case is already undefined; see _expr.delete_.

10Any  deallocation  function  for a class X is a static member (even if
  not explicitly declared static).  [Example:
          class X {
              // ...
              void operator delete(void*);
              void operator delete[](void*, size_t);
          };
          class Y {
              // ...
              void operator delete(void*, size_t);
              void operator delete[](void*);
          };
   --end example]

11Since member allocation and deallocation  functions  are  static  they
  cannot  be  virtual.   [Note:  however,  when the cast-expression of a
  delete-expression refers to an object of class type, because the deal­
  location  function  actually  called  is looked up in the scope of the
  destructor, if the destructor is virtual, the effect is the same.  For
  example,
          struct B {
              virtual ~B();
              void operator delete(void*, size_t);
          };
          struct D : B {
              void operator delete(void*);
          };
          void f()
          {
              B* bp = new D;
              delete bp;     //1: uses D::operator delete(void*)
          }
  Here,  storage  for  the non-array object of class D is deallocated by
  D::operator delete(), due to the virtual destructor.  ] [Note: virtual
  destructors  have  no  effect  on  the  deallocation function actually
  called when the cast-expression of a delete-expression  refers  to  an
  array of objects of class type.  For example,
          struct B {
              virtual ~B();
              void operator delete[](void*, size_t);
          };
          struct D : B {
              void operator delete[](void*, size_t);
          };
          void f(int i)
          {
              D* dp = new D[i];
              delete [] dp;  // uses D::operator delete[](void*, size_t)
              B* bp = new D[i];
              delete[] bp;   // undefined behavior
          }
   --end note]

12Access  to  the  deallocation  function is checked statically.  Hence,
  even though a different one might actually be executed, the statically
  visible deallocation function is required to be accessible.  [Example:
  for the call on line //1 above, if B::operator delete() had been  pri­
  vate, the delete expression would have been ill-formed.  ]

  12.6  Initialization                                      [class.init]

1 If  T  is  either a class type or an array of class type, an object of
  type T is default-initialized (_dcl.init_) if:

  --the object has static storage duration and no initializer is  speci­
    fied in its declaration (see _dcl.init_), or

  --the object is created with a new-expression of the form new T() (see
    _expr.new_), or

  --the object is a temporary object created using the functional  nota­
    tion for type conversions T() (see _expr.type.conv_), or

  --the  object is a subobject, either a base of type T or a member m of
    type T, of a class object being created by a constructor that speci­
    fies  a  mem-initializer  of  the form T() or m(), respectively (see
    _class.base.init_).

2 Furthermore, if an object of class type T (or array thereof)

  --has automatic storage duration and no initializer  is  specified  in
    its declaration, or

  --is  created  with  a  new-expression with an omitted new-initializer
    (see _expr.new_), or

  --is a subobject, either a base of type T or a member m of type T  (or
    array thereof), of a class object created by a constructor that does
    not  specify  a  mem-initializer  for  T  or  m,  respectively  (see
    _class.base.init_),

  then  that  object (or, for an array, each element of the array) shall
  be initialized by the default constructor for T (and  the  initializa­
  tion is ill-formed if T has no accessible default constructor).

3 An  object of class type (or array thereof) can be explicitly initial­
  ized; see _class.expl.init_ and _class.base.init_.

4 When an array of class objects is initialized  (either  explicitly  or
  implicitly),  the  constructor shall be called for each element of the
  array,  following  the  subscript  order;  see  _dcl.array_.    [Note:
  destructors  for  the  array  elements  are called in reverse order of
  their construction.  ]

  12.6.1  Explicit initialization                      [class.expl.init]

1 An object of class  type  can  be  initialized  with  a  parenthesized
  expression-list, where the expression-list is construed as an argument
  list for a constructor  that  is  called  to  initialize  the  object.
  Alternatively,  a  single assignment-expression can be specified as an
  initializer using  the  =  form  of  initialization.   Either  direct-
  initialization  semantics  or copy-initialization semantics apply; see
  _dcl.init_.  [Example:
          class complex {
              // ...
          public:
              complex();
              complex(double);
              complex(double,double);
              // ...
          };

          complex sqrt(complex,complex);
          complex a(1);             // initialize by a call of
                                    // complex(double)
          complex b = a;            // initialize by a copy of `a'
          complex c = complex(1,2); // construct complex(1,2)
                                    // using complex(double,double)
                                    // copy it into `c'
          complex d = sqrt(b,c);    // call sqrt(complex,complex)
                                    // and copy the result into `d'
          complex e;                // initialize by a call of
                                    // complex()
          complex f = 3;            // construct complex(3) using
                                    // complex(double)
                                    // copy it into `f'
          complex g = { 1, 2 };     // error; constructor is required
    --end  example]  [Note:  overloading  of  the  assignment   operator
  (_over.ass_) = has no effect on initialization.  ]

2 When  an  aggregate (whether class or array) contains members of class
  type  and  is  initialized  by   a   brace-enclosed   initializer-list
  (_dcl.init.aggr_),   each   such   member   is  copy-initialized  (see
  _dcl.init_) by the corresponding assignment-expression.  If there  are
  fewer  initializers in the initializer-list than members of the aggre­
  gate, each member not explicitly initialized shall be copy-initialized
  (_dcl.init_)  with  an initializer of the form T() (_expr.type.conv_),
  where T represents the  type  of  the  uninitialized  member.   [Note:
  _dcl.init.aggr_  describes  how  assignment-expressions in an initial­
  izer-list are paired with the aggregate members  they  initialize.   ]
  [Example:
          complex v[6] = { 1,complex(1,2),complex(),2 };
  Here,  complex::complex(double)  is  called  for the initialization of
  v[0] and v[3], complex::complex(double,double) is called for the  ini­
  tialization  of v[1], complex::complex() is called for the initializa­
  tion v[2], v[4], and v[5].  For another example,

          class X {
          public:
                  int i;
                  float f;
                  complex c;
          } x = { 99, 88.8, 77.7 };
  Here, x.i is initialized with 99, x.f is initialized  with  88.8,  and
  complex::complex(double)  is  called for the initialization of x.c.  ]
  [Note: braces can be elided in the initializer-list for any aggregate,
  even  if  the  aggregate has members of a class type with user-defined
  type conversions; see _dcl.init.aggr_.  ]

3 [Note: if T is a class type with no default constructor, any  declara­
  tion  of  an  object  of type T (or array thereof) is ill-formed if no
  initializer is explicitly specified (see _class.init_ and _dcl.init_).
  ]

4 [Note:  the  order  in  which objects with static storage duration are
  initialized is described in _basic.start.init_ and _stmt.dcl_.  ]

  12.6.2  Initializing bases and members               [class.base.init]

1 In the definition of a  constructor  for  a  class,  initializers  for
  direct  and  virtual base subobjects and nonstatic data members can be
  specified by a ctor-initializer, which has the form
          ctor-initializer:
                  : mem-initializer-list
          mem-initializer-list:
                  mem-initializer
                  mem-initializer , mem-initializer-list
          mem-initializer:
                  mem-initializer-id ( expression-listopt )
          mem-initializer-id:
                  ::opt nested-name-specifieropt class-name
                  identifier

2 Names in a mem-initializer-id are looked up in the scope of  the  con­
  structor's class and, if not found in that scope, are looked up in the
  scope  containing  the  constructor's  definition.   Unless  the  mem-
  initializer-id  names  a  nonstatic  data  member of the constructor's
  class or a direct or virtual base of that class,  the  mem-initializer
  is  ill-formed.   A  mem-initializer-list  can initialize a base class
  using any name that denotes that base class type.  [Example:
          struct A { A(); };
          typedef A global_A;
          struct B { };
          struct C: public A, public B { C(); };
          C::C(): global_A() { }    // mem-initializer for base A
   --end example] If a mem-initializer-id is ambiguous because it desig­
  nates  both  a  direct non-virtual base class and an inherited virtual
  base class, the mem-initializer is ill-formed.  [Example:

          struct A { A(); };
          struct B: public virtual A { };
          struct C: public A, public B { C(); };
          C::C(): A() { }           // ill-formed: which A?
   --end example] A ctor-initializer may initialize  the  member  of  an
  anonymous  union  that  is  a member of the constructor's class.  If a
  ctor-initializer specifies more than one mem-initializer for the  same
  member,  for  the  same base class or for multiple members of the same
  union (including members of anonymous unions), the ctor-initializer is
  ill-formed.

3 The  expression-list  in  a  mem-initializer is used to initialize the
  base class or nonstatic data member  subobject  denoted  by  the  mem-
  initializer-id.  The semantics of a mem-initializer are as follows:

  --if  the  expression-list of the mem-initializer is omitted, the base
    class or member subobject is default-initialized (see _dcl.init_);

  --otherwise, the subobject indicated by mem-initializer-id is  direct-
    initialized   using   expression-list   as   the   initializer  (see
    _dcl.init_).

  [Example:
          struct B1 { B1(int); /* ... */ };
          struct B2 { B2(int); /* ... */ };
          struct D : B1, B2 {
              D(int);
              B1 b;
              const c;
          };
          D::D(int a) : B2(a+1), B1(a+2), c(a+3), b(a+4)
          { /* ... */ }
          D d(10);
   --end example] There is a sequence  point  (_intro.execution_)  after
  the  initialization of each base and member.  The expression-list of a
  mem-initializer is evaluated as part of the initialization of the cor­
  responding base or member.

4 If  class  X  has a member m of class type M and M has no default con­
  structor, then a definition of a constructor for class X is ill-formed
  if  it  does not specify a mem-initializer for m.  After the call to a
  constructor for class X has completed, if a member  of  X  is  neither
  specified  in  the  constructor's mem-initializers, nor initialized in
  the body of the constructor, nor eligible for  default-initialization,
  the member has indeterminate value.  If a class X has a nonstatic data
  member that is of reference type or of a const type that is not eligi­
  ble  for  default-initialization  (_dcl.init_),  and a constructor for
  class X does not provide a mem-initializer for that member,  the  pro­
  gram is ill-formed.

5 Initialization shall proceed in the following order:

  --First,  and  only  for  the constructor of the most derived class as

    described below, virtual base classes shall be  initialized  in  the
    order  they  appear  on a depth-first left-to-right traversal of the
    directed acyclic graph of base classes, where "left-to-right" is the
    order  of  appearance  of  the base class names in the derived class
    base-specifier-list.

  --Then, direct base classes shall be initialized in declaration  order
    as  they  appear in the base-specifier-list (regardless of the order
    of the mem-initializers).

  --Then, nonstatic data members shall be initialized in the order  they
    were declared in the class definition (again regardless of the order
    of the mem-initializers).

  --Finally, the body of the constructor is executed.

  [Note: the declaration order is mandated to ensure that base and  mem­
  ber  subobjects  are destroyed in the reverse order of initialization.
  ]

6 All sub-objects representing virtual base classes are  initialized  by
  the  constructor  of  the most derived class (_intro.object_).  If the
  constructor of  the  most  derived  class  does  not  specify  a  mem-
  initializer  for  a virtual base class V, then V's default constructor
  is called to initialize the virtual base class subobject.  If  V  does
  not have an accessible default constructor, the initialization is ill-
  formed.  A mem-initializer  naming  a  virtual  base  class  shall  be
  ignored  during  execution of the constructor of any class that is not
  the most derived class.  [Example:
          class V {
          public:
              V();
              V(int);
              // ...
          };
          class A : public virtual V {
          public:
              A();
              A(int);
              // ...
          };
          class B : public virtual V {
          public:
              B();
              B(int);
              // ...
          };
          class C : public A, public B, private virtual V {
          public:
              C();
              C(int);
              // ...
          };

          A::A(int i) : V(i) { /* ... */ }
          B::B(int i) { /* ... */ }
          C::C(int i) { /* ... */ }
          V v(1); // use V(int)
          A a(2); // use V(int)
          B b(3); // use V()
          C c(4); // use V()
   --end example]

7 Names in the expression-list of a mem-initializer are evaluated in the
  scope  of  the constructor for which the mem-initializer is specified.
  [Example:
          class X {
              int a;
              int b;
              int i;
              int j;
          public:
              const int& r;
              X(int i): r(a), b(i), i(i), j(this->i) {}
          };
  initializes X::r to refer to X::a, initializes X::b with the value  of
  the  constructor  parameter  i, initializes X::i with the value of the
  constructor parameter i, and initializes X::j with the value of  X::i;
  this  takes place each time an object of class X is created.  ] [Note:
  because the mem-initializer are evaluated in the  scope  of  the  con­
  structor,  the  this  pointer  can be used in the expression-list of a
  mem-initializer to refer to the object being initialized.  ]

8 Member functions (including virtual member functions, _class.virtual_)
  can  be called for an object under construction.  Similarly, an object
  under  construction  can  be  the  operand  of  the  typeid   operator
  (_expr.typeid_)  or of a dynamic_cast (_expr.dynamic.cast_).  However,
  if these operations are performed in a ctor-initializer (or in a func­
  tion called directly or indirectly from a ctor-initializer) before all
  the mem-initializers for base classes have completed,  the  result  of
  the operation is undefined.  [Example:
          class A {
          public:
                  A(int);
          };
          class B : public A {
                  int j;
          public:
                  int f();
                  B() : A(f()),   // undefined: calls member function
                                  // but base A not yet initialized
                  j(f()) { }      // well-defined: bases are all initialized
          };
          class C {
          public:
                  C(int);
          };

          class D : public B, C {
                  int i;
          public:
                  D() : C(f()),   // undefined: calls member function
                                  // but base C not yet initialized
                  i(f()) {}       // well-defined: bases are all initialized
          };
   --end example]

9 [Note:  _class.cdtor_  describes the result of virtual function calls,
  typeid and dynamic_casts  during  construction  for  the  well-defined
  cases;  that is, describes the polymorphic behavior of an object under
  construction.  ]

  12.7  Construction and destruction                       [class.cdtor]

1 For an object of non-POD class type (_class_), before the  constructor
  begins  execution  and after the destructor finishes execution, refer­
  ring to any nonstatic member or base class of the  object  results  in
  undefined behavior.  [Example:
          struct X { int i; };
          struct Y : X { };
          struct A { int a; };
          struct B : public A { int j; Y y; };
          extern B bobj;
          B* pb = &bobj;          // ok
          int* p1 = &bobj.a;      // undefined, refers to base class member
          int* p2 = &bobj.y.i;    // undefined, refers to member's member
          A* pa = &bobj;          // undefined, upcast to a base class type
          B bobj;                 // definition of bobj
          extern X xobj;
          int* p3 = &xobj.i;      // Ok, X is a POD class
          X xobj;
  For another example,
          struct W { int j; };
          struct X : public virtual W { };
          struct Y {
                  int *p;
                  X x;
                  Y() : p(&x.j)    // undefined, x is not yet constructed
                  { }
          };
   --end example]

2 To explicitly or implicitly convert a pointer (an lvalue) referring to
  an object of class X to a pointer (reference) to a direct or  indirect
  base  class  B of X, the construction of X and the construction of all
  of its direct or indirect bases that  directly  or  indirectly  derive
  from  B  shall have started and the destruction of these classes shall
  not have completed, otherwise  the  conversion  results  in  undefined
  behavior.  To form a pointer to (or access the value of) a direct non­
  static member of an object obj, the construction  of  obj  shall  have
  started  and  its  destruction shall not have completed, otherwise the

  computation of the pointer  value  (or  accessing  the  member  value)
  results in undefined behavior.  [Example:
          struct A { };
          struct B : virtual A { };
          struct C : B { };
          struct D : virtual A { D(A*); };
          struct X { X(A*); };
          struct E : C, D, X {
                  E() : D(this),  // undefined: upcast from E* to A*
                                  // might use path E* -> D* -> A*
                                  // but D is not constructed
                  // D((C*)this), // defined:
                                  // E* -> C* defined because E() has started
                                  // and C* -> A* defined because
                                  // C fully constructed
                  X(this)         // defined: upon construction of X,
                                  // C/B/D/A sublattice is fully constructed
                  { }
          };
   --end example]

3 Member  functions,  including virtual functions (_class.virtual_), can
  be called  during  construction  or  destruction  (_class.base.init_).
  When  a  virtual function is called directly or indirectly from a con­
  structor (including from the mem-initializer for  a  data  member)  or
  from  a  destructor,  and  the object to which the call applies is the
  object under construction or destruction, the function called  is  the
  one  defined in the constructor or destructor's own class or in one of
  its bases, but not a function overriding it in a  class  derived  from
  the  constructor or destructor's class, or overriding it in one of the
  other base classes of the most derived  object  (_intro.object_).   If
  the  virtual  function  call  uses  an  explicit  class  member access
  (_expr.ref_) and the object-expression refers to the object under con­
  struction  or  destruction  but its type is neither the constructor or
  destructor's own class or one of its bases, the result of the call  is
  undefined.  [Example:
          class V {
          public:
                  virtual void f();
                  virtual void g();
          };
          class A : public virtual V {
          public:
                  virtual void f();
          };
          class B : public virtual V {
          public:
                  virtual void g();
                  B(V*, A*);
          };

          class D : public A, B {
          public:
                  virtual void f();
                  virtual void g();
                  D() : B((A*)this, this) { }
          };
          B::B(V* v, A* a) {
                  f();    // calls V::f, not A::f
                  g();    // calls B::g, not D::g
                  v->g(); // v is base of B, the call is well-defined, calls B::g
                  a->f(); // undefined behavior, a's type not a base of B
          }
   --end example]

4 The typeid operator (_expr.typeid_) can be used during construction or
  destruction (_class.base.init_).  When typeid is used in a constructor
  (including  from  the  mem-initializer  for  a  data  member)  or in a
  destructor, or used in a function called (directly or indirectly) from
  a  constructor  or  destructor, if the operand of typeid refers to the
  object under construction or destruction, typeid yields the  type_info
  representing the constructor or destructor's class.  If the operand of
  typeid refers to the object under construction or destruction and  the
  static  type of the operand is neither the constructor or destructor's
  class nor one of its bases, the result of typeid is undefined.

5 Dynamic_casts (_expr.dynamic.cast_) can be used during construction or
  destruction (_class.base.init_). When a dynamic_cast is used in a con­
  structor (including from the mem-initializer for a data member) or  in
  a  destructor,  or  used in a function called (directly or indirectly)
  from a constructor or destructor, if the operand of  the  dynamic_cast
  refers to the object under construction or destruction, this object is
  considered to be a most derived object that has the type of  the  con­
  structor  or  destructor's  class.  If the operand of the dynamic_cast
  refers to the object under construction or destruction and the  static
  type  of  the operand is not a pointer to or object of the constructor
  or destructor's own class  or  one  of  its  bases,  the  dynamic_cast
  results in undefined behavior.

6 [Example:
          class V {
          public:
                  virtual void f();
          };
          class A : public virtual V { };
          class B : public virtual V {
          public:
                  B(V*, A*);
          };
          class D : public A, B {
          public:
                  D() : B((A*)this, this) { }
          };

          B::B(V* v, A* a) {
                  typeid(this);   // type_info for B
                  typeid(*v);     // well-defined: *v has type V, a base of B
                                  // yields type_info for B
                  typeid(*a);     // undefined behavior: type A not a base of B
                  dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B
                                  // results in B*
                  dynamic_cast<B*>(a); // undefined behavior,
                                  // a has type A*, A not a base of B
          }
   --end example]

  12.8  Copying class objects                               [class.copy]

1 A   class  object  can  be  copied  in  two  ways,  by  initialization
  (_class.ctor_, _dcl.init_), including for  function  argument  passing
  (_expr.call_)  and  for  function value return (_stmt.return_), and by
  assignment  (_expr.ass_).   Conceptually,  these  two  operations  are
  implemented  by  a copy constructor (_class.ctor_) and copy assignment
  operator (_over.ass_).

2 A constructor for class X is a copy constructor if its first parameter
  is  of type X&, const X&, volatile X& or const volatile X&, and either
  there are no other  parameters  or  else  all  other  parameters  have
  default  arguments  (_dcl.fct.default_).  [Example: X::X(const X&) and
  X::X(X&, int=1) are copy constructors.
          class X {
              // ...
          public:
              X(int);
              X(const X&, int = 1);
          };
          X a(1);         // calls X(int);
          X b(a, 0);      // calls X(const X&, int);
          X c = b;        // calls X(const X&, int);
   --end example] [Note: all forms of copy constructor may  be  declared
  for a class.  [Example:
          class X {
                  // ...
          public:
                  X(const X&);
                  X(X&); // OK
          };
    --end example]  --end note] [Note: if a class X only has a copy con­
  structor with a parameter of type X&, an initializer of type  const  X
  or  volatile  X  cannot  initialize  an  object of type (possibily cv-
  qualified) X.  [Example:
          struct X {
                  X();    // default constructor
                  X(X&);  // copy constructor with a nonconst parameter
          };
          const X cx;
          X x = cx;       // error -- X::X(X&) cannot copy cx into x

   --end example]  --end note]

3 A declaration of a constructor for a class  X  is  ill-formed  if  its
  first  parameter  is  of  type  (optionally cv-qualified) X and either
  there are no other  parameters  or  else  all  other  parameters  have
  default arguments.

4 If  the  class definition does not explicitly declare a copy construc­
  tor, one is declared implicitly.  Thus, for the class definition
          struct X {
                  X(const X&, int);
          };
  a copy constructor is implicitly-declared.  If the user-declared  con­
  structor is later defined as
          X::X(const X& x, int i =0) { ... }
  then  any  use  of  X's  copy constructor is ill-formed because of the
  ambiguity; no diagnostic is required.

5 The implicitly-declared copy constructor for a class X will  have  the
  form
          X::X(const X&)
  if

  --each  direct  or  virtual  base  class B of X has a copy constructor
    whose first parameter is of type const B& or const volatile B&, and

  --for all the nonstatic data members of X that are of a class  type  M
    (or  array  thereof),  each  such  class type has a copy constructor
    whose first parameter is of type const M& or const volatile M&.4)

  Otherwise, the implicitly declared copy constructor will have the form
          X::X(X&)
  An  implicitly-declared  copy  constructor  is  a public member of its
  class.

  +-------                 BEGIN BOX 1                -------+
  Should the standard require that  the  implicitly-declared  copy  con­
  structor be inline?
  +-------                  END BOX 1                 -------+

6 A copy constructor for class X is trivial if it is implicitly declared
  and if

  --class X has no virtual functions (_class.virtual_)  and  no  virtual
    base classes (_class.mi_), and

  --each direct base class of X has a trivial copy constructor, and
  _________________________
  4) This implies  that  the  reference  parameter  of  the  implicitly-
  declared  copy  constructor  cannot  bind  to  a  volatile lvalue; see
  _diff.special_.

  --for  all  the nonstatic data members of X that are of class type (or
    array thereof), each such class type has a trivial copy constructor;

  otherwise the copy constructor is non-trivial.

7 An implicitly-declared copy constructor is implicitly defined if it is
  used to initialize an object of its class  type  from  a  copy  of  an
  object  of  its  class  type or of a class type derived from its class
  type5).  [Note: the copy constructor is implicitly defined even if the
  implementation elided its use (_class.temporary_).   ]  A  program  is
  ill-formed  if  the  class  for which a copy constructor is implicitly
  defined has:

  --a nonstatic data member of class type (or  array  thereof)  with  an
    inaccessible or ambiguous copy constructor, or

  --a base class with an inaccessible or ambiguous copy constructor.

  Before the implicitly-declared copy constructor for a class is implic­
  itly defined, all implicitly-declared copy constructors for its direct
  and  virtual  base  classes  and its nonstatic data members shall have
  been implicitly defined.

8 The implicitly-defined copy constructor for class X performs a member­
  wise  copy of its subobjects.  The order of copying is the same as the
  order of initialization of bases and members in  a  user-defined  con­
  structor  (see  _class.base.init_).   Each  subobject is copied in the
  manner appropriate to its type:

  --if the subobject is of class type,  the  copy  constructor  for  the
    class is used;

  --if  the subobject is an array, each element is copied, in the manner
    appropriate to the element type;

  --if the subobject is of scalar type, the built-in assignment operator
    is used.

  Virtual base class subobjects shall be copied only once by the implic­
  itly-defined copy constructor (see _class.base.init_).

9 A user-declared copy assignment operator X::operator= is a  non-static
  member  function  of class X with exactly one parameter of type X, X&,
  const X&, volatile X& or const  volatile  X&.   [Note:  an  overloaded
  assignment  operator  must be declared to have only one parameter; see
  _over.ass_.  ] [Note: more than one form of copy  assignment  operator
  may  be  declared  for a class.  ] [Note: if a class X only has a copy
  assignment operator with a parameter of type X&, an expression of type
  const X cannot be assigned to an object of type X.  [Example:

  _________________________
  5) See _dcl.init_ for more details on direct and copy  initialization.

          struct X {
                  X()
                  X& operator=(X&);
          };
          const X cx;
          X x;
          x = cx; // error:
                  // X::operator=(X&) cannot assign cx into x
   --end example]  --end note]

10If  the class definition does not explicitly declare a copy assignment
  operator, one is declared implicitly.   The  implicitly-declared  copy
  assignment operator for a class X will have the form
          X& X::operator=(const X&)
  if

  --each  direct  base class B of X has a copy assignment operator whose
    parameter is of type const B& or const volatile B&, and

  --for all the nonstatic data members of X that are of a class  type  M
    (or array thereof), each such class type has a copy assignment oper­
    ator whose parameter is of type const M& or const volatile M&.6)

  Otherwise, the implicitly declared copy constructor will have the form
          X& X::operator=(X&)
  The  implicitly-declared  copy assignment operator for class X has the
  return type X&; it returns the object for which the assignment  opera­
  tor  is  invoked,  that  is,  the  object assigned to.  An implicitly-
  declared copy assignment operator is a public  member  of  its  class.
  Because  a copy assignment operator is implicitly declared for a class
  if not declared by the user, a base class copy assignment operator  is
  always  hidden  by  the  copy  assignment  operator of a derived class
  (_over.ass_).

  +-------                 BEGIN BOX 2                -------+
  Should the standard require that the implicitly-declared copy  assign­
  ment operator be inline?
  +-------                  END BOX 2                 -------+

11A  copy assignment operator for class X is trivial if it is implicitly
  declared and if

  --each direct base class of X has a trivial copy assignment  operator,
    and

  --for  all  the nonstatic data members of X that are of class type (or
    array thereof), each such class type has a trivial  copy  assignment
  _________________________
  6) This implies  that  the  reference  parameter  of  the  implicitly-
  declared  copy  assignment  operator cannot bind to a volatile lvalue;
  see _diff.special_.

    operator;

  otherwise the copy assignment operator is non-trivial.

12An  implicitly-declared copy assignment operator is implicitly defined
  when an object of its class type is assigned a value of its class type
  or  a value of a class type derived from its class type.  A program is
  ill-formed if the class  for  which  a  copy  assignment  operator  is
  implicitly defined has:

  --a nonstatic data member of const type, or

  --a nonstatic data member of reference type, or

  --a  nonstatic  data  member  of class type (or array thereof) with an
    inaccessible copy assignment operator, or

  --a base class with an inaccessible copy assignment operator.

  Before the implicitly-declared copy assignment operator for a class is
  implicitly  defined, all implicitly-declared copy assignment operators
  for its direct base classes and its nonstatic data members shall  have
  been implicitly defined.

13The  implicitly-defined  copy assignment operator for class X performs
  memberwise assignment of its subobjects.  The direct base classes of X
  are  assigned  first,  in  the order of their declaration in the base-
  specifier-list, and then the immediate nonstatic data members of X are
  assigned,  in the order in which they were declared in the class defi­
  nition.  Each subobject is assigned in the manner appropriate  to  its
  type:

  --if  the subobject is of class type, the copy assignment operator for
    the class is used;

  --if the subobject is an array, each element is assigned, in the  man­
    ner appropriate to the element type;

  --if the subobject is of scalar type, the built-in assignment operator
    is used.

  It is unspecified whether subobjects representing virtual base classes
  are  assigned more than once by the implicitly-defined copy assignment
  operator.  [Example:
          struct V {
          struct A : virtual V { };
          struct B : virtual V { };
          struct C : B, A { };
  it is unspecified whether  the  virtual  base  class  subobject  V  is
  assigned  twice by the implicitly-defined copy assignment operator for
  C.   --end example]

14A program is ill-formed if the copy constructor or the copy assignment
  operator for an object is implicitly used and the special member func­
  tion is not accessible (_class.access_).  [Note:  Copying  one  object
  into  another using the copy constructor or the copy assignment opera­
  tor does not change the layout or size of either object.  ]

15Whenever a class object is copied and the original object and the copy
  have  the  same  type, if the implementation can prove that either the
  original object or the copy will never again be  used  except  as  the
  result  of  an implicit destructor call (_class.dtor_), an implementa­
  tion is permitted to treat the original and the copy as two  different
  ways  of  referring  to the same object and not perform a copy at all.
  In that case, the object is destroyed at the later of times  when  the
  original   and   the  copy  would  have  been  destroyed  without  the
  optimization.7) [Example:
          class Thing {
          public:
                  Thing();
                  ~Thing();
                  Thing(const Thing&);
                  Thing operator=(const Thing&);
                  void fun();
          };
          void f(Thing t) { }
          void g(Thing t) { t.fun(); }

          int main()
          {
                  Thing t1, t2, t3;
                  f(t1);
                  g(t2);
                  g(t3);
                  t3.fun();
          }
  Here t1 does not need to be copied when calling f because f  does  not
  use  its  formal parameter again after copying it. Although g uses its
  parameter, the call to g(t2) does not need to copy t2  because  t2  is
  not used again after it is passed to g.  On the other hand, t3 is used
  after passing it to g so calling g(t3) is required to copy t3.  ]

  _________________________
  7) Because only one object is destroyed instead of two, and  one  copy
  constructor  is  not executed, there is still one object destroyed for
  each one constructed.