10 Classes [class]

A class is a type.
Its name becomes a class-name ([class.name]) within its scope.
class-name:
	identifier
	simple-template-id
An object of a class consists of a (possibly empty) sequence of members and base class objects.
class-specifier:
	class-head { member-specification }
class-head:
	class-key attribute-specifier-seq class-head-name class-virt-specifier base-clause
	class-key attribute-specifier-seq base-clause
class-head-name:
	nested-name-specifier class-name
class-virt-specifier:
	final
class-key:
	class
	struct
	union
A class declaration where the class-name in the class-head-name is a simple-template-id shall be an explicit specialization ([temp.expl.spec]) or a partial specialization ([temp.class.spec]).
A class-specifier whose class-head omits the class-head-name defines an unnamed class.
[Note
:
An unnamed class thus can't be final.
end note
]
A class-name is inserted into the scope in which it is declared immediately after the class-name is seen.
The class-name is also inserted into the scope of the class itself; this is known as the injected-class-name.
For purposes of access checking, the injected-class-name is treated as if it were a public member name.
A class-specifier is commonly referred to as a class definition.
A class is considered defined after the closing brace of its class-specifier has been seen even though its member functions are in general not yet defined.
The optional attribute-specifier-seq appertains to the class; the attributes in the attribute-specifier-seq are thereafter considered attributes of the class whenever it is named.
If a class-head-name contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which the nested-name-specifier refers, or in an element of the inline namespace set ([namespace.def]) of that namespace (i.e., not merely inherited or introduced by a using-declaration), and the class-specifier shall appear in a namespace enclosing the previous declaration.
In such cases, the nested-name-specifier of the class-head-name of the definition shall not begin with a decltype-specifier.
If a class is marked with the class-virt-specifier final and it appears as a class-or-decltype in a base-clause, the program is ill-formed.
Whenever a class-key is followed by a class-head-name, the identifier final, and a colon or left brace, final is interpreted as a class-virt-specifier.
[Example
:
struct A;
struct A final {};      // OK: definition of struct A,
                        // not value-initialization of variable final

struct X {
 struct C { constexpr operator int() { return 5; } };
 struct B final : C{};  // OK: definition of nested class B,
                        // not declaration of a bit-field member final
};
end example
]
[Note
:
Complete objects of class type have nonzero size.
Base class subobjects and members declared with the no_­unique_­address attribute ([dcl.attr.nouniqueaddr]) are not so constrained.
end note
]
[Note
:
Class objects can be assigned ([expr.ass], [over.ass], [class.copy.assign]), passed as arguments to functions ([dcl.init], [class.copy.ctor]), and returned by functions (except objects of classes for which copying or moving has been restricted; see [dcl.fct.def.delete] and [class.access]).
Other plausible operators, such as equality comparison, can be defined by the user; see [over.oper].
end note
]
A union is a class defined with the class-key union; it holds at most one data member at a time ([class.union]).
[Note
:
Aggregates of class type are described in [dcl.init.aggr].
end note
]

10.1 Properties of classes [class.prop]

A trivially copyable class is a class:
  • where each copy constructor, move constructor, copy assignment operator, and move assignment operator ([class.copy.ctor], [class.copy.assign]) is either deleted or trivial,
  • that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator, and
  • that has a trivial, non-deleted destructor.
A trivial class is a class that is trivially copyable and has one or more default constructors, all of which are either trivial or deleted and at least one of which is not deleted.
[Note
:
In particular, a trivially copyable or trivial class does not have virtual functions or virtual base classes.
end note
]
A class S is a standard-layout class if it:
  • has no non-static data members of type non-standard-layout class (or array of such types) or reference,
  • has the same access control for all non-static data members,
  • has no non-standard-layout base classes,
  • has at most one base class subobject of any given type,
  • has all non-static data members and bit-fields in the class and its base classes first declared in the same class, and
  • has no element of the set M(S) of types as a base class, where for any type X, M(X) is defined as follows.108
    [Note
    :
    M(X) is the set of the types of all non-base-class subobjects that may be at a zero offset in X.
    end note
    ]
    • If X is a non-union class type with no (possibly inherited) non-static data members, the set M(X) is empty.
    • If X is a non-union class type with a non-static data member of type that is either of zero size or is the first non-static data member of X (where said member may be an anonymous union), the set M(X) consists of and the elements of .
    • If X is a union type, the set M(X) is the union of all and the set containing all , where each is the type of the non-static data member of X.
    • If X is an array type with element type , the set M(X) consists of and the elements of .
    • If X is a non-class, non-array type, the set M(X) is empty.
[Example
:
   struct B { int i; };         // standard-layout class
   struct C : B { };            // standard-layout class
   struct D : C { };            // standard-layout class
   struct E : D { char : 4; };  // not a standard-layout class

   struct Q {};
   struct S : Q { };
   struct T : Q { };
   struct U : S, T { };         // not a standard-layout class
end example
]
A standard-layout struct is a standard-layout class defined with the class-key struct or the class-key class.
A standard-layout union is a standard-layout class defined with the class-key union.
[Note
:
Standard-layout classes are useful for communicating with code written in other programming languages.
Their layout is specified in [class.mem].
end note
]
[Example
:
struct N {          // neither trivial nor standard-layout
  int i;
  int j;
  virtual ~N();
};

struct T {          // trivial but not standard-layout
  int i;
private:
  int j;
};

struct SL {         // standard-layout but not trivial
  int i;
  int j;
  ~SL();
};

struct POD {        // both trivial and standard-layout
  int i;
  int j;
};
end example
]
This ensures that two subobjects that have the same class type and that belong to the same most derived object are not allocated at the same address ([expr.eq]).

10.2 Class names [class.name]

A class definition introduces a new type.
[Example
:
struct X { int a; };
struct Y { int a; };
X a1;
Y a2;
int a3;
declares three variables of three different types.
This implies that
a1 = a2;                        // error: Y assigned to X
a1 = a3;                        // error: int assigned to X
are type mismatches, and that
int f(X);
int f(Y);
declare an overloaded function f() and not simply a single function f() twice.
For the same reason,
struct S { int a; };
struct S { int a; };            // error, double definition
is ill-formed because it defines S twice.
end example
]
A class declaration introduces the class name into the scope where it is declared and hides any class, variable, function, or other declaration of that name in an enclosing scope.
If a class name is declared in a scope where a variable, function, or enumerator of the same name is also declared, then when both declarations are in scope, the class can be referred to only using an elaborated-type-specifier ([basic.lookup.elab]).
[Example
:
struct stat {
  // ...
};

stat gstat;                     // use plain stat to define variable

int stat(struct stat*);         // redeclare stat as function

void f() {
  struct stat* ps;              // struct prefix needed to name struct stat
  stat(ps);                     // call stat()
}
end example
]
A declaration consisting solely of class-key identifier; is either a redeclaration of the name in the current scope or a forward declaration of the identifier as a class name.
It introduces the class name into the current scope.
[Example
:
struct s { int a; };

void g() {
  struct s;                     // hide global struct s with a block-scope declaration
  s* p;                         // refer to local struct s
  struct s { char* p; };        // define local struct s
  struct s;                     // redeclaration, has no effect
}
end example
]
[Note
:
Such declarations allow definition of classes that refer to each other.
[Example
:
class Vector;

class Matrix {
  // ...
  friend Vector operator*(const Matrix&, const Vector&);
};

class Vector {
  // ...
  friend Vector operator*(const Matrix&, const Vector&);
};
Declaration of friends is described in [class.friend], operator functions in [over.oper].
end example
]
end note
]
[Note
:
An elaborated-type-specifier can also be used as a type-specifier as part of a declaration.
It differs from a class declaration in that if a class of the elaborated name is in scope the elaborated name will refer to it.
end note
]
[Example
:
struct s { int a; };

void g(int s) {
  struct s* p = new struct s;   // global s
  p->a = s;                     // parameter s
}
end example
]
[Note
:
The declaration of a class name takes effect immediately after the identifier is seen in the class definition or elaborated-type-specifier.
For example,
class A * A;
first specifies A to be the name of a class and then redefines it as the name of a pointer to an object of that class.
This means that the elaborated form class A must be used to refer to the class.
Such artistry with names can be confusing and is best avoided.
end note
]
A typedef-name that names a class type, or a cv-qualified version thereof, is also a class-name.
If a typedef-name that names a cv-qualified class type is used where a class-name is required, the cv-qualifiers are ignored.
A typedef-name shall not be used as the identifier in a class-head.

10.3 Class members [class.mem]

member-specification:
	member-declaration member-specification
	access-specifier : member-specification
member-declaration:
	attribute-specifier-seq decl-specifier-seq member-declarator-list ;
	function-definition
	using-declaration
	static_assert-declaration
	template-declaration
	deduction-guide
	alias-declaration
	empty-declaration
member-declarator-list:
	member-declarator
	member-declarator-list , member-declarator
member-declarator:
	declarator virt-specifier-seq pure-specifier
	declarator requires-clause
	declarator brace-or-equal-initializer
	identifier attribute-specifier-seq : constant-expression brace-or-equal-initializer
virt-specifier-seq:
	virt-specifier
	virt-specifier-seq virt-specifier
virt-specifier:
	override
	final
pure-specifier:
	= 0
The member-specification in a class definition declares the full set of members of the class; no member can be added elsewhere.
A direct member of a class X is a member of X that was first declared within the member-specification of X, including anonymous union objects ([class.union.anon]) and direct members thereof.
Members of a class are data members, member functions, nested types, enumerators, and member templates and specializations thereof.
[Note
:
A specialization of a static data member template is a static data member.
A specialization of a member function template is a member function.
A specialization of a member class template is a nested class.
end note
]
For any other member-declaration, each declared entity that is not an unnamed bit-field is a member of the class, and each such member-declaration shall either declare at least one member name of the class or declare at least one unnamed bit-field.
A data member is a non-function member introduced by a member-declarator.
A member function is a member that is a function.
Nested types are classes ([class.name], [class.nest]) and enumerations declared in the class and arbitrary types declared as members by use of a typedef declaration or alias-declaration.
The enumerators of an unscoped enumeration defined in the class are members of the class.
A data member or member function may be declared static in its member-declaration, in which case it is a static member (see [class.static]) (a static data member ([class.static.data]) or static member function ([class.static.mfct]), respectively) of the class.
Any other data member or member function is a non-static member (a non-static data member or non-static member function ([class.mfct.non-static]), respectively).
[Note
:
A non-static data member of non-reference type is a member subobject of a class object.
end note
]
A member shall not be declared twice in the member-specification, except that
[Note
:
A single name can denote several member functions provided their types are sufficiently different ([over]).
end note
]
A complete-class context of a class is a within the member-specification of the class.
[Note
:
A complete-class context of a nested class is also a complete-class context of any enclosing class, if the nested class is defined within the member-specification of the enclosing class.
end note
]
A class is considered a completely-defined object type ([basic.types]) (or complete type) at the closing } of the class-specifier.
The class is regarded as complete within its complete-class contexts; otherwise it is regarded as incomplete within its own class member-specification.
In a member-declarator, an = immediately following the declarator is interpreted as introducing a pure-specifier if the declarator-id has function type, otherwise it is interpreted as introducing a brace-or-equal-initializer.
[Example
:
struct S {
  using T = void();
  T * p = 0;        // OK: brace-or-equal-initializer
  virtual T f = 0;  // OK: pure-specifier
};
end example
]
In a member-declarator for a bit-field, the constant-expression is parsed as the longest sequence of tokens that could syntactically form a constant-expression.
[Example
:
int a;
const int b = 0;
struct S {
  int x1 : 8 = 42;               // OK, "= 42" is brace-or-equal-initializer
  int x2 : 8 { 42 };             // OK, "{ 42 }" is brace-or-equal-initializer
  int y1 : true ? 8 : a = 42;    // OK, brace-or-equal-initializer is absent
  int y2 : true ? 8 : b = 42;    // error: cannot assign to const int
  int y3 : (true ? 8 : b) = 42;  // OK, "= 42" is brace-or-equal-initializer
  int z : 1 || new int { 0 };    // OK, brace-or-equal-initializer is absent
};
end example
]
A brace-or-equal-initializer shall appear only in the declaration of a data member.
(For static data members, see [class.static.data]; for non-static data members, see [class.base.init] and [dcl.init.aggr]).
A brace-or-equal-initializer for a non-static data member specifies a default member initializer for the member, and shall not directly or indirectly cause the implicit definition of a defaulted default constructor for the enclosing class or the exception specification of that constructor.
A member shall not be declared with the extern storage-class-specifier.
Within a class definition, a member shall not be declared with the thread_­local storage-class-specifier unless also declared static.
The decl-specifier-seq may be omitted in constructor, destructor, and conversion function declarations only; when declaring another kind of member the decl-specifier-seq shall contain a type-specifier that is not a cv-qualifier.
The member-declarator-list can be omitted only after a class-specifier or an enum-specifier or in a friend declaration.
A pure-specifier shall be used only in the declaration of a virtual function that is not a friend declaration.
The optional attribute-specifier-seq in a member-declaration appertains to each of the entities declared by the member-declarators; it shall not appear if the optional member-declarator-list is omitted.
A virt-specifier-seq shall contain at most one of each virt-specifier.
A virt-specifier-seq shall appear only in the first declaration of a virtual member function ([class.virtual]).
The type of a non-static data member shall not be an incomplete type ([basic.types]), an abstract class type ([class.abstract]), or a (possibly multi-dimensional) array thereof.
[Note
:
In particular, a class C cannot contain a non-static member of class C, but it can contain a pointer or reference to an object of class C.
end note
]
[Note
:
See [expr.prim.id] for restrictions on the use of non-static data members and non-static member functions.
end note
]
[Note
:
The type of a non-static member function is an ordinary function type, and the type of a non-static data member is an ordinary object type.
There are no special member function types or data member types.
end note
]
[Example
:
A simple example of a class definition is
struct tnode {
  char tword[20];
  int count;
  tnode* left;
  tnode* right;
};
which contains an array of twenty characters, an integer, and two pointers to objects of the same type.
Once this definition has been given, the declaration
tnode s, *sp;
declares s to be a tnode and sp to be a pointer to a tnode.
With these declarations, sp->count refers to the count member of the object to which sp points; s.left refers to the left subtree pointer of the object s; and s.right->tword[0] refers to the initial character of the tword member of the right subtree of s.
end example
]
Non-static data members of a (non-union) class with the same access control are allocated so that later members have higher addresses within a class object.
The order of allocation of non-static data members with different access control is unspecified.
Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other; so might requirements for space for managing virtual functions and virtual base classes.
If T is the name of a class, then each of the following shall have a name different from T:
  • every static data member of class T;
  • every member function of class T
    [Note
    :
    This restriction does not apply to constructors, which do not have names
    end note
    ]
    ;
  • every member of class T that is itself a type;
  • every member template of class T;
  • every enumerator of every member of class T that is an unscoped enumerated type; and
  • every member of every anonymous union that is a member of class T.
In addition, if class T has a user-declared constructor, every non-static data member of class T shall have a name different from T.
The common initial sequence of two standard-layout struct ([class.prop]) types is the longest sequence of non-static data members and bit-fields in declaration order, starting with the first such entity in each of the structs, such that corresponding entities have layout-compatible types, either both entities are declared with the no_­unique_­address attribute ([dcl.attr.nouniqueaddr]) or neither is, and either both entities are bit-fields with the same width or neither is a bit-field.
[Example
:
  struct A { int a; char b; };
  struct B { const int b1; volatile char b2; };
  struct C { int c; unsigned : 0; char b; };
  struct D { int d; char b : 4; };
  struct E { unsigned int e; char b; };
The common initial sequence of A and B comprises all members of either class.
The common initial sequence of A and C and of A and D comprises the first member in each case.
The common initial sequence of A and E is empty.
end example
]
Two standard-layout struct ([class.prop]) types are layout-compatible classes if their common initial sequence comprises all members and bit-fields of both classes ([basic.types]).
Two standard-layout unions are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in any order) have layout-compatible types.
In a standard-layout union with an active member of struct type T1, it is permitted to read a non-static data member m of another union member of struct type T2 provided m is part of the common initial sequence of T1 and T2; the behavior is as if the corresponding member of T1 were nominated.
[Example
:
struct T1 { int a, b; };
struct T2 { int c; double d; };
union U { T1 t1; T2 t2; };
int f() {
  U u = { { 1, 2 } };   // active member is t1
  return u.t2.c;        // OK, as if u.t1.a were nominated
}
end example
]
[Note
:
Reading a volatile object through a glvalue of non-volatile type has undefined behavior ([dcl.type.cv]).
end note
]
If a standard-layout class object has any non-static data members, its address is the same as the address of its first non-static data member if that member is not a bit-field.
Its address is also the same as the address of each of its base class subobjects.
[Note
:
There might therefore be unnamed padding within a standard-layout struct object inserted by an implementation, but not at its beginning, as necessary to achieve appropriate alignment.
end note
]
[Note
:
The object and its first subobject are pointer-interconvertible ([basic.compound], [expr.static.cast]).
end note
]

10.3.1 Member functions [class.mfct]

A member function may be defined in its class definition, in which case it is an inline member function, or it may be defined outside of its class definition if it has already been declared but not defined in its class definition.
A member function definition that appears outside of the class definition shall appear in a namespace scope enclosing the class definition.
Except for member function definitions that appear outside of a class definition, and except for explicit specializations of member functions of class templates and member function templates ([temp.spec]) appearing outside of the class definition, a member function shall not be redeclared.
An inline member function (whether static or non-static) may also be defined outside of its class definition provided either its declaration in the class definition or its definition outside of the class definition declares the function as inline or constexpr.
[Note
:
Member functions of a class in namespace scope have the linkage of that class.
Member functions of a local class have no linkage.
end note
]
[Note
:
There can be at most one definition of a non-inline member function in a program.
There may be more than one inline member function definition in a program.
end note
]
If the definition of a member function is lexically outside its class definition, the member function name shall be qualified by its class name using the ​::​ operator.
[Note
:
A name used in a member function definition (that is, in the parameter-declaration-clause including the default arguments or in the member function body) is looked up as described in [basic.lookup].
end note
]
[Example
:
struct X {
  typedef int T;
  static T count;
  void f(T);
};
void X::f(T t = count) { }
The member function f of class X is defined in global scope; the notation X​::​f specifies that the function f is a member of class X and in the scope of class X.
In the function definition, the parameter type T refers to the typedef member T declared in class X and the default argument count refers to the static data member count declared in class X.
end example
]
[Note
:
A static local variable or local type in a member function always refers to the same entity, whether or not the member function is inline.
end note
]
Previously declared member functions may be mentioned in friend declarations.
Member functions of a local class shall be defined inline in their class definition, if they are defined at all.
[Note
:
A member function can be declared (but not defined) using a typedef for a function type.
The resulting member function has exactly the same type as it would have if the function declarator were provided explicitly, see [dcl.fct].
For example,
typedef void fv();
typedef void fvc() const;
struct S {
  fv memfunc1;      // equivalent to: void memfunc1();
  void memfunc2();
  fvc memfunc3;     // equivalent to: void memfunc3() const;
};
fv  S::* pmfv1 = &S::memfunc1;
fv  S::* pmfv2 = &S::memfunc2;
fvc S::* pmfv3 = &S::memfunc3;
Also see [temp.arg].
end note
]

10.3.2 Non-static member functions [class.mfct.non-static]

A non-static member function may be called for an object of its class type, or for an object of a class derived from its class type, using the class member access syntax ([over.match.call]).
A non-static member function may also be called directly using the function call syntax ([expr.call], [over.match.call]) from within the body of a member function of its class or of a class derived from its class.
If a non-static member function of a class X is called for an object that is not of type X, or of a type derived from X, the behavior is undefined.
When an id-expression that is not part of a class member access syntax and not used to form a pointer to member ([expr.unary.op]) is used in a member of class X in a context where this can be used, if name lookup resolves the name in the id-expression to a non-static non-type member of some class C, and if either the id-expression is potentially evaluated or C is X or a base class of X, the id-expression is transformed into a class member access expression using (*this) as the postfix-expression to the left of the . operator.
[Note
:
If C is not X or a base class of X, the class member access expression is ill-formed.
end note
]
Similarly during name lookup, when an unqualified-id used in the definition of a member function for class X resolves to a static member, an enumerator or a nested type of class X or of a base class of X, the unqualified-id is transformed into a qualified-id in which the nested-name-specifier names the class of the member function.
These transformations do not apply in the template definition context ([temp.dep.type]).
[Example
:
struct tnode {
  char tword[20];
  int count;
  tnode* left;
  tnode* right;
  void set(const char*, tnode* l, tnode* r);
};

void tnode::set(const char* w, tnode* l, tnode* r) {
  count = strlen(w)+1;
  if (sizeof(tword)<=count)
      perror("tnode string too long");
  strcpy(tword,w);
  left = l;
  right = r;
}

void f(tnode n1, tnode n2) {
  n1.set("abc",&n2,0);
  n2.set("def",0,0);
}
In the body of the member function tnode​::​set, the member names tword, count, left, and right refer to members of the object for which the function is called.
Thus, in the call n1.set("abc",&n2,0), tword refers to n1.tword, and in the call n2.set("def",0,0), it refers to n2.tword.
The functions strlen, perror, and strcpy are not members of the class tnode and should be declared elsewhere.109
end example
]
A non-static member function may be declared const, volatile, or const volatile.
These cv-qualifiers affect the type of the this pointer.
They also affect the function type of the member function; a member function declared const is a const member function, a member function declared volatile is a volatile member function and a member function declared const volatile is a const volatile member function.
[Example
:
struct X {
  void g() const;
  void h() const volatile;
};
X​::​g is a const member function and X​::​h is a const volatile member function.
end example
]
A non-static member function may be declared with a ref-qualifier ([dcl.fct]); see [over.match.funcs].
A non-static member function may be declared virtual ([class.virtual]) or pure virtual ([class.abstract]).
See, for example, <cstring> ([c.strings]).

10.3.2.1 The this pointer [class.this]

In the body of a non-static ([class.mfct]) member function, the keyword this is a prvalue whose value is the address of the object for which the function is called.
The type of this in a member function of a class X is X*.
If the member function is declared const, the type of this is const X*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*.
[Note
:
Thus in a const member function, the object for which the function is called is accessed through a const access path.
end note
]
[Example
:
struct s {
  int a;
  int f() const;
  int g() { return a++; }
  int h() const { return a++; } // error
};

int s::f() const { return a; }
The a++ in the body of s​::​h is ill-formed because it tries to modify (a part of) the object for which s​::​h() is called.
This is not allowed in a const member function because this is a pointer to const; that is, *this has const type.
end example
]
Similarly, volatile semantics apply in volatile member functions when accessing the object and its non-static data members.
A cv-qualified member function can be called on an object-expression only if the object-expression is as cv-qualified or less-cv-qualified than the member function.
[Example
:
void k(s& x, const s& y) {
  x.f();
  x.g();
  y.f();
  y.g();                        // error
}
The call y.g() is ill-formed because y is const and s​::​g() is a non-const member function, that is, s​::​g() is less-qualified than the object-expression y.
end example
]
Constructors and destructors shall not be declared const, volatile or const volatile.
[Note
:
However, these functions can be invoked to create and destroy objects with cv-qualified types, see [class.ctor] and [class.dtor].
end note
]

10.3.3 Special member functions [special]

The default constructor ([class.ctor]), copy constructor, move constructor ([class.copy.ctor]), copy assignment operator, move assignment operator ([class.copy.assign]), and destructor ([class.dtor]) are special member functions.
[Note
:
The implementation will implicitly declare these member functions for some class types when the program does not explicitly declare them.
The implementation will implicitly define them if they are odr-used ([basic.def.odr]) or needed for constant evaluation ([expr.const]).
end note
]
An implicitly-declared special member function is declared at the closing } of the class-specifier.
Programs shall not define implicitly-declared special member functions.
Programs may explicitly refer to implicitly-declared special member functions.
[Example
:
A program may explicitly call or form a pointer to member to an implicitly-declared special member function.
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
  return *this;
}
end example
]
[Note
:
The special member functions affect the way objects of class type are created, copied, moved, and destroyed, and how values can be converted to values of other types.
Often such special member functions are called implicitly.
end note
]
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.
end example
]
For a class, its non-static data members, its non-virtual direct base classes, and, if the class is not abstract ([class.abstract]), its virtual base classes are called its potentially constructed subobjects.

10.3.4 Constructors [class.ctor]

Constructors do not have names.
In a declaration of a constructor, the declarator is a function declarator ([dcl.fct]) of the form
ptr-declarator ( parameter-declaration-clause ) noexcept-specifier attribute-specifier-seq
where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:
The class-name shall not be a typedef-name.
In a constructor declaration, each decl-specifier in the optional decl-specifier-seq shall be friend, inline, constexpr, or an explicit-specifier.
[Example
:
struct S {
  S();              // declares the constructor
};

S::S() { }          // defines the constructor
end example
]
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].
end note
]
A constructor can be invoked for a const, volatile or const volatile object.
const and volatile semantics ([dcl.type.cv]) are not applied on an object under construction.
They come into effect when the constructor for the most derived object ([intro.object]) ends.
A default constructor for a class X is a constructor of class X for which each parameter that is not a function parameter pack has a default argument (including the case of a constructor with no parameters).
If there is no user-declared constructor for class X, a non-explicit constructor having no parameters is implicitly declared as defaulted ([dcl.fct.def]).
An implicitly-declared default constructor is an inline public member of its class.
A defaulted default constructor for class X is defined as deleted if:
  • X is a union that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,
  • X is a non-union class that has a variant member M with a non-trivial default constructor and no variant member of the anonymous union containing M has a default member initializer,
  • any non-static data member with no default member initializer ([class.mem]) is of reference type,
  • any non-variant non-static data member of const-qualified type (or array thereof) with no brace-or-equal-initializer does not have a user-provided default constructor,
  • X is a union and all of its variant members are of const-qualified type (or array thereof),
  • X is a non-union class and all members of any anonymous union member are of const-qualified type (or array thereof),
  • any potentially constructed subobject, except for a non-static data member with a brace-or-equal-initializer, has class type M (or array thereof) and either M has no default constructor or overload resolution ([over.match]) as applied to find M's corresponding constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or
  • any potentially constructed subobject has a type with a destructor that is deleted or inaccessible from the defaulted default constructor.
A default constructor is trivial if it is not user-provided and if:
  • its class has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • no non-static data member of its class has a default member initializer ([class.mem]), and
  • all the direct base classes of its class have trivial default constructors, and
  • for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.
Otherwise, the default constructor is non-trivial.
A default constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) to create an object of its class type ([intro.object]), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.
The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with no ctor-initializer and an empty compound-statement.
If that user-written default constructor would be ill-formed, the program is ill-formed.
If that user-written default constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined default constructor is constexpr.
Before the defaulted default constructor for a class is implicitly defined, all the non-user-provided default constructors for its base classes and its non-static data members shall have been implicitly defined.
[Note
:
An implicitly-declared default constructor has an exception specification ([except.spec]).
An explicitly-defaulted definition might have an implicit exception specification, see [dcl.fct.def].
end note
]
Default constructors are called implicitly to create class objects of static, thread, or automatic storage duration ([basic.stc.static], [basic.stc.thread], [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]).
[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.
end note
]
A return statement in the body of a constructor shall not specify a return value.
The address of a constructor shall not be taken.
A functional notation type conversion ([expr.type.conv]) can be used to create new objects of its type.
[Note
:
The syntax looks like an explicit call of the constructor.
end note
]
[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.
end note
]
[Note
:
Explicit constructor calls do not yield lvalues, see [basic.lval].
end note
]
[Note
:
Some language constructs have special semantics when used during construction; see [class.base.init] and [class.cdtor].
end note
]
During the construction of an object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified.
[Example
:
struct C;
void no_opt(C*);

struct C {
  int c;
  C() : c(0) { no_opt(this); }
};

const C cobj;

void no_opt(C* cptr) {
  int i = cobj.c * 100;         // value of cobj.c is unspecified
  cptr->c = 1;
  cout << cobj.c * 100          // value of cobj.c is unspecified
       << '\n';
}

extern struct D d;
struct D {
  D(int a) : a(a), b(d.a) {}
  int a, b;
};
D d = D(1);                     // value of d.b is unspecified
end example
]

10.3.5 Copy/move constructors [class.copy.ctor]

A non-template 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.
struct X {
  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
]
A non-template constructor for class X is a move 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
:
Y​::​Y(Y&&) is a move constructor.
struct Y {
  Y(const Y&);
  Y(Y&&);
};
extern Y f(int);
Y d(f(1));          // calls Y(Y&&)
Y e = d;            // calls Y(const Y&)
end example
]
[Note
:
All forms of copy/move constructor may be declared for a class.
[Example
:
struct X {
  X(const X&);
  X(X&);            // OK
  X(X&&);
  X(const X&&);     // OK, but possibly not sensible
};
end example
]
end note
]
[Note
:
If a class X only has a copy constructor with a parameter of type X&, an initializer of type const X or volatile X cannot initialize an object of type (possibly cv-qualified) X.
[Example
:
struct X {
  X();              // default constructor
  X(X&);            // copy constructor with a non-const parameter
};
const X cx;
X x = cx;           // error: X​::​X(X&) cannot copy cx into x
end example
]
end note
]
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.
A member function template is never instantiated to produce such a constructor signature.
[Example
:
struct S {
  template<typename T> S(T);
  S();
};

S g;

void h() {
  S a(g);           // does not instantiate the member template to produce S​::​S<S>(S);
                    // uses the implicitly declared copy constructor
}
end example
]
If the class definition does not explicitly declare a copy constructor, a non-explicit one is declared implicitly.
If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted ([dcl.fct.def]).
The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.
The implicitly-declared copy constructor for a class X will have the form
X::X(const X&)
if each potentially constructed subobject of a class type M (or array thereof) has a copy constructor whose first parameter is of type const M& or const volatile M&.110
Otherwise, the implicitly-declared copy constructor will have the form
X::X(X&)
If the definition of a class X does not explicitly declare a move constructor, a non-explicit one will be implicitly declared as defaulted if and only if
  • X does not have a user-declared copy constructor,
  • X does not have a user-declared copy assignment operator,
  • X does not have a user-declared move assignment operator, and
  • X does not have a user-declared destructor.
[Note
:
When the move constructor is not implicitly declared or explicitly supplied, expressions that otherwise would have invoked the move constructor may instead invoke a copy constructor.
end note
]
The implicitly-declared move constructor for class X will have the form
X::X(X&&)
An implicitly-declared copy/move constructor is an inline public member of its class.
A defaulted copy/​move constructor for a class X is defined as deleted ([dcl.fct.def.delete]) if X has:
  • a potentially constructed subobject type M (or array thereof) that cannot be copied/moved because overload resolution ([over.match]), as applied to find M's corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,
  • a variant member whose corresponding constructor as selected by overload resolution is non-trivial,
  • any potentially constructed subobject of a type with a destructor that is deleted or inaccessible from the defaulted constructor, or,
  • for the copy constructor, a non-static data member of rvalue reference type.
A defaulted move constructor that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).
[Note
:
A deleted move constructor would otherwise interfere with initialization from an rvalue which can use the copy constructor instead.
end note
]
A copy/move constructor for class X is trivial if it is not user-provided and if:
  • class X has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • the constructor selected to copy/move each direct base class subobject is trivial, and
  • for each non-static data member of X that is of class type (or array thereof), the constructor selected to copy/move that member is trivial;
otherwise the copy/move constructor is non-trivial.
A copy/move constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.
[Note
:
The copy/move constructor is implicitly defined even if the implementation elided its odr-use ([basic.def.odr], [class.temporary]).
end note
]
If the implicitly-defined constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined constructor is constexpr.
Before the defaulted copy/move constructor for a class is implicitly defined, all non-user-provided copy/move constructors for its potentially constructed subobjects shall have been implicitly defined.
[Note
:
An implicitly-declared copy/move constructor has an implied exception specification ([except.spec]).
end note
]
The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its bases and members.
[Note
:
Default member initializers of non-static data members are ignored.
See also the example in [class.base.init].
end note
]
The order of initialization is the same as the order of initialization of bases and members in a user-defined constructor (see [class.base.init]).
Let x be either the parameter of the constructor or, for the move constructor, an xvalue referring to the parameter.
Each base or non-static data member is copied/moved in the manner appropriate to its type:
  • if the member is an array, each element is direct-initialized with the corresponding subobject of x;
  • if a member m has rvalue reference type T&&, it is direct-initialized with static_­cast<T&&>(x.m);
  • otherwise, the base or member is direct-initialized with the corresponding base or member of x.
Virtual base class subobjects shall be initialized only once by the implicitly-defined copy/move constructor (see [class.base.init]).
The implicitly-defined copy/move constructor for a union X copies the object representation ([basic.types]) of X.
This implies that the reference parameter of the implicitly-declared copy constructor cannot bind to a volatile lvalue; see [diff.special].

10.3.6 Copy/move assignment operator [class.copy.assign]

A user-declared copy assignment operator X​::​operator= is a non-static non-template member function of class X with exactly one parameter of type X, X&, const X&, volatile X& or const volatile X&.111
[Note
:
An overloaded assignment operator must be declared to have only one parameter; see [over.ass].
end note
]
[Note
:
More than one form of copy assignment operator may be declared for a class.
end note
]
[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
:
struct X {
  X();
  X& operator=(X&);
};
const X cx;
X x;
void f() {
  x = cx;           // error: X​::​operator=(X&) cannot assign cx into x
}
end example
]
end note
]
If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly.
If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defined as defaulted ([dcl.fct.def]).
The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor.
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&, const volatile B& or B, and
  • for all the non-static data members of X that are of a class type M (or array thereof), each such class type has a copy assignment operator whose parameter is of type const M&, const volatile M& or M.112
Otherwise, the implicitly-declared copy assignment operator will have the form
X& X::operator=(X&)
A user-declared move assignment operator X​::​operator= is a non-static non-template member function of class X with exactly one parameter of type 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].
end note
]
[Note
:
More than one form of move assignment operator may be declared for a class.
end note
]
If the definition of a class X does not explicitly declare a move assignment operator, one will be implicitly declared as defaulted if and only if
  • X does not have a user-declared copy constructor,
  • X does not have a user-declared move constructor,
  • X does not have a user-declared copy assignment operator, and
  • X does not have a user-declared destructor.
[Example
:
The class definition
struct S {
  int a;
  S& operator=(const S&) = default;
};
will not have a default move assignment operator implicitly declared because the copy assignment operator has been user-declared.
The move assignment operator may be explicitly defaulted.
struct S {
  int a;
  S& operator=(const S&) = default;
  S& operator=(S&&) = default;
};
end example
]
The implicitly-declared move assignment operator for a class X will have the form
X& X::operator=(X&&);
The implicitly-declared copy/move assignment operator for class X has the return type X&; it returns the object for which the assignment operator is invoked, that is, the object assigned to.
An implicitly-declared copy/move assignment operator is an inline public member of its class.
A defaulted copy/move assignment operator for class X is defined as deleted if X has:
  • a variant member with a non-trivial corresponding assignment operator and X is a union-like class, or
  • a non-static data member of const non-class type (or array thereof), or
  • a non-static data member of reference type, or
  • a direct non-static data member of class type M (or array thereof) or a direct base class M that cannot be copied/moved because overload resolution ([over.match]), as applied to find M's corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator.
A defaulted move assignment operator that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).
Because a copy/move assignment operator is implicitly declared for a class if not declared by the user, a base class copy/move assignment operator is always hidden by the corresponding assignment operator of a derived class ([over.ass]).
A using-declaration that brings in from a base class an assignment operator with a parameter type that could be that of a copy/move assignment operator for the derived class is not considered an explicit declaration of such an operator and does not suppress the implicit declaration of the derived class operator; the operator introduced by the using-declaration is hidden by the implicitly-declared operator in the derived class.
A copy/move assignment operator for class X is trivial if it is not user-provided and if:
  • class X has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • the assignment operator selected to copy/move each direct base class subobject is trivial, and
  • for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is trivial;
otherwise the copy/move assignment operator is non-trivial.
A copy/move assignment operator for a class X that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) (e.g., when it is selected by overload resolution to assign to an object of its class type), when it is needed for constant evaluation ([expr.const]), or when it is explicitly defaulted after its first declaration.
The implicitly-defined copy/move assignment operator is constexpr if
  • X is a literal type, and
  • the assignment operator selected to copy/move each direct base class subobject is a constexpr function, and
  • for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is a constexpr function.
Before the defaulted copy/move assignment operator for a class is implicitly defined, all non-user-provided copy/move assignment operators for its direct base classes and its non-static data members shall have been implicitly defined.
[Note
:
An implicitly-declared copy/move assignment operator has an implied exception specification ([except.spec]).
end note
]
The implicitly-defined copy/move assignment operator for a non-union class X performs memberwise copy/move 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 non-static data members of X are assigned, in the order in which they were declared in the class definition.
Let x be either the parameter of the function or, for the move operator, an xvalue referring to the parameter.
Each subobject is assigned in the manner appropriate to its type:
  • if the subobject is of class type, as if by a call to operator= with the subobject as the object expression and the corresponding subobject of x as a single function argument (as if by explicit qualification; that is, ignoring any possible virtual overriding functions in more derived classes);
  • if the subobject is an array, each element is assigned, in the manner 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/move 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/move assignment operator for C.
end example
]
The implicitly-defined copy assignment operator for a union X copies the object representation ([basic.types]) of X.
Because a template assignment operator or an assignment operator taking an rvalue reference parameter is never a copy assignment operator, the presence of such an assignment operator does not suppress the implicit declaration of a copy assignment operator.
Such assignment operators participate in overload resolution with other assignment operators, including copy assignment operators, and, if selected, will be used to assign an object.
This implies that the reference parameter of the implicitly-declared copy assignment operator cannot bind to a volatile lvalue; see [diff.special].

10.3.7 Destructors [class.dtor]

In a declaration of a destructor, the declarator is a function declarator ([dcl.fct]) of the form
ptr-declarator ( parameter-declaration-clause ) noexcept-specifier attribute-specifier-seq
where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:
The class-name shall not be a typedef-name.
A destructor shall take no arguments ([dcl.fct]).
Each decl-specifier of the decl-specifier-seq of a destructor declaration (if any) shall be friend, inline, or virtual.
A destructor is used to destroy objects of its class type.
The address of a destructor shall not be taken.
A destructor can be invoked for a const, volatile or const volatile object.
const and volatile semantics ([dcl.type.cv]) are not applied on an object under destruction.
They stop being in effect when the destructor for the most derived object ([intro.object]) starts.
[Note
:
A declaration of a destructor that does not have a noexcept-specifier has the same exception specification as if it had been implicitly declared ([except.spec]).
end note
]
If a class has no user-declared destructor, a destructor is implicitly declared as defaulted ([dcl.fct.def]).
An implicitly-declared destructor is an inline public member of its class.
A defaulted destructor for a class X is defined as deleted if:
  • X is a union-like class that has a variant member with a non-trivial destructor,
  • any potentially constructed subobject has class type M (or array thereof) and M has a deleted destructor or a destructor that is inaccessible from the defaulted destructor,
  • or, for a virtual destructor, lookup of the non-array deallocation function results in an ambiguity or in a function that is deleted or inaccessible from the defaulted destructor.
A destructor is trivial if it is not user-provided and if:
  • the destructor is not virtual,
  • 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.
Otherwise, the destructor is non-trivial.
A destructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) or when it is explicitly defaulted after its first declaration.
Before the defaulted destructor for a class is implicitly defined, all the non-user-provided destructors for its base classes and its non-static data members shall have been implicitly defined.
After executing the body of the destructor and destroying any automatic objects allocated within the body, a destructor for class X calls the destructors for X's direct non-variant non-static data members, the destructors for X's non-virtual 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.
All destructors are called as if they were referenced with a qualified name, that is, ignoring any possible virtual overriding destructors in more derived 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 transferring 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]).
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.
[Note
:
Some language constructs have special semantics when used during destruction; see [class.cdtor].
end note
]
A destructor is invoked implicitly
In each case, the context of the invocation is the context of the construction of the object.
A destructor may also be invoked implicitly through use of a delete-expression for a constructed object allocated by a new-expression; the context of the invocation is the delete-expression.
[Note
:
An array of class type contains several subobjects for each of which the destructor is invoked.
end note
]
A destructor can also be invoked explicitly.
A destructor is potentially invoked if it is invoked or as specified in [expr.new], [dcl.init.aggr], [class.base.init], and [except.throw].
A program is ill-formed if a destructor that is potentially invoked is deleted or not accessible from the context of the invocation.
At the point of definition of a virtual destructor (including an implicit definition ([class.dtor])), the non-array deallocation function is determined as if for the expression delete this appearing in a non-virtual destructor of the destructor's class (see [expr.delete]).
If the lookup fails or if the deallocation function has a deleted definition ([dcl.fct.def]), the program is ill-formed.
[Note
:
This assures that a deallocation function corresponding to the dynamic type of an object is available for the delete-expression ([class.free]).
end note
]
In an explicit destructor call, the destructor is specified by a ~ followed by a type-name or decltype-specifier that denotes 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 a class derived from the destructor's class type (including when the destructor is invoked via a null pointer value), the program has undefined behavior.
[Note
:
Invoking delete on a null pointer does not call the destructor; see [expr.delete].
end note
]
[Example
:
struct B {
  virtual ~B() { }
};
struct D : B {
  ~D() { }
};

D D_object;
typedef B B_alias;
B* B_ptr = &D_object;

void f() {
  D_object.B::~B();             // calls B's destructor
  B_ptr->~B();                  // calls D's destructor
  B_ptr->~B_alias();            // calls D's destructor
  B_ptr->B_alias::~B();         // calls B's destructor
  B_ptr->B_alias::~B_alias();   // calls B's destructor
}
end example
]
[Note
:
An explicit destructor call must always be written using a member access operator ([expr.ref]) or a qualified-id; in particular, the unary-expression ~X() in a member function is not an explicit destructor call ([expr.unary.op]).
end note
]
[Note
:
Explicit calls of destructors are rarely needed.
One use of such calls is for objects placed at specific addresses using a placement new-expression.
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(std::size_t, void* p) { return p; }
struct X {
  X(int);
  ~X();
};
void f(X* p);

void g() {                      // rare, specialized use:
  char* buf = new char[sizeof(X)];
  X* p = new(buf) X(222);       // use buf[] and initialize
  f(p);
  p->X::~X();                   // cleanup
}
end note
]
Once 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.
end example
]
[Note
:
The notation for explicit call of a destructor can be used for any scalar type name ([expr.pseudo]).
Allowing this makes it possible to write code without having to know if a destructor exists for a given type.
For example:
typedef int I;
I* p;
p->I::~I();
end note
]

10.3.8 Conversions [class.conv]

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]).
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]).
[Note
:
See [over.match] for a discussion of the use of conversions in function calls as well as examples below.
end note
]
At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.
[Example
:
struct X {
  operator int();
};

struct Y {
  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
]
User-defined conversions are used implicitly only if they are unambiguous.
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]) selects the best conversion function to perform the conversion.
[Example
:
struct X {
  operator int();
};

struct Y : X {
    operator char();
};

void f(Y& a) {
  if (a) {          // ill-formed: X​::​operator int() or Y​::​operator char()
  }
}
end example
]

10.3.8.1 Conversion by constructor [class.conv.ctor]

A constructor that is not explicit ([dcl.fct.spec]) specifies a conversion from the types of its parameters (if any) to the type of its class.
Such a constructor is called a converting constructor.
[Example
:
struct X {
    X(int);
    X(const char*, int =0);
    X(int, int);
};

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))
  f({1, 2});        // f(X(1,2))
}
end example
]
[Note
:
An explicit constructor constructs objects just like non-explicit constructors, but does so only where the direct-initialization syntax ([dcl.init]) or where casts ([expr.static.cast], [expr.cast]) are explicitly used; see also [over.match.copy].
A default constructor may be an explicit constructor; such a constructor will be used to perform default-initialization or value-initialization ([dcl.init]).
[Example
:
struct Z {
  explicit Z();
  explicit Z(int);
  explicit Z(int, int);
};

Z a;                            // OK: default-initialization performed
Z b{};                          // OK: direct initialization syntax used
Z c = {};                       // error: copy-list-initialization
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
Z a6 = { 3, 4 };                // error: no implicit conversion
end example
]
end note
]
A non-explicit copy/move constructor ([class.copy.ctor]) is a converting constructor.
[Note
:
An implicitly-declared copy/move constructor is not an explicit constructor; it may be called for implicit type conversions.
end note
]

10.3.8.2 Conversion functions [class.conv.fct]

A member function of a class X having no parameters with a name of the form
conversion-function-id:
	operator conversion-type-id
conversion-type-id:
	type-specifier-seq conversion-declarator
conversion-declarator:
	ptr-operator conversion-declarator
specifies a conversion from X to the type specified by the conversion-type-id.
Such functions are called conversion functions.
A decl-specifier in the decl-specifier-seq of a conversion function (if any) shall be neither a defining-type-specifier nor static.
The type of the conversion function ([dcl.fct]) is “function taking no parameter returning conversion-type-id.
A conversion function 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.113
[Example
:
struct X {
  operator int();
  operator auto() -> short;     // error: trailing return type
};

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
]
A conversion function may be explicit ([dcl.fct.spec]), in which case it is only considered as a user-defined conversion for direct-initialization ([dcl.init]).
Otherwise, user-defined conversions are not restricted to use in assignments and initializations.
[Example
:
class Y { };
struct Z {
  explicit operator Y() const;
};

void h(Z z) {
  Y y1(z);          // OK: direct-initialization
  Y y2 = z;         // ill-formed: copy-initialization
  Y y3 = (Y)z;      // OK: cast notation
}

void g(X a, X b) {
  int i = (a) ? 1+a : 0;
  int j = (a&&b) ? a+b : i;
  if (a) {
  }
}
end example
]
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 sequence of tokens that could possibly form a conversion-type-id.
[Note
:
This prevents 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.
end example
]
This rule also prevents ambiguities for attributes.
[Example
:
operator int [[noreturn]] ();   // error: noreturn attribute applied to a type
end example
]
end note
]
Conversion functions are inherited.
Conversion functions can be virtual.
A conversion function template shall not have a deduced return type ([dcl.spec.auto]).
[Example
:
struct S {
  operator auto() const { return 10; }      // OK
  template<class T>
  operator auto() const { return 1.2; }     // error: conversion function template
};
end example
]
These conversions are considered as standard conversions for the purposes of overload resolution ([over.best.ics], [over.ics.ref]) and therefore initialization ([dcl.init]) and explicit casts ([expr.static.cast]).
A conversion to void does not invoke any conversion function ([expr.static.cast]).
Even though never directly called to perform a conversion, such conversion functions can be declared and can potentially be reached through a call to a virtual conversion function in a base class.

10.3.9 Static members [class.static]

A static member s of class X may be referred to using the qualified-id expression X​::​s; it is not necessary to use the class member access syntax ([expr.ref]) to refer to a static member.
A static member may be referred to using the class member access syntax, in which case the object expression is evaluated.
[Example
:
struct process {
  static void reschedule();
};
process& g();

void f() {
  process::reschedule();        // OK: no object necessary
  g().reschedule();             // g() is called
}
end example
]
A static member may be referred to directly in the scope of its class or in the scope of a class derived ([class.derived]) from its class; in this case, the static member is referred to as if a qualified-id expression was used, with the nested-name-specifier of the qualified-id naming the class scope from which the static member is referenced.
[Example
:
int g();
struct X {
  static int g();
};
struct Y : X {
  static int i;
};
int Y::i = g();                 // equivalent to Y​::​g();
end example
]
If an unqualified-id is used in the definition of a static member following the member's declarator-id, and name lookup ([basic.lookup.unqual]) finds that the unqualified-id refers to a static member, enumerator, or nested type of the member's class (or of a base class of the member's class), the unqualified-id is transformed into a qualified-id expression in which the nested-name-specifier names the class scope from which the member is referenced.
[Note
:
See [expr.prim.id] for restrictions on the use of non-static data members and non-static member functions.
end note
]
Static members obey the usual class member access rules ([class.access]).
When used in the declaration of a class member, the static specifier shall only be used in the member declarations that appear within the member-specification of the class definition.
[Note
:
It cannot be specified in member declarations that appear in namespace scope.
end note
]

10.3.9.1 Static member functions [class.static.mfct]

[Note
:
The rules described in [class.mfct] apply to static member functions.
end note
]
[Note
:
A static member function does not have a this pointer ([class.this]).
end note
]
A static member function shall not be virtual.
There shall not be a static and a non-static member function with the same name and the same parameter types ([over.load]).
A static member function shall not be declared const, volatile, or const volatile.

10.3.9.2 Static data members [class.static.data]

A static data member is not part of the subobjects of a class.
If a static data member is declared thread_­local there is one copy of the member per thread.
If a static data member is not declared thread_­local there is one copy of the data member that is shared by all the objects of the class.
The declaration of a non-inline static data member in its class definition is not a definition and may be of an incomplete type other than cv void.
The definition for a static data member that is not defined inline in the class definition shall appear in a namespace scope enclosing the member's class definition.
In the definition at namespace scope, the name of the static data member shall be qualified by its class name using the ​::​ operator.
The initializer expression in the definition of a static data member is in the scope of its class ([basic.scope.class]).
[Example
:
class process {
  static process* run_chain;
  static process* running;
};

process* process::running = get_main();
process* process::run_chain = running;
The static data member run_­chain of class process is defined in global scope; the notation process​::​run_­chain specifies that the member run_­chain is a member of class process and in the scope of class process.
In the static data member definition, the initializer expression refers to the static data member running of class process.
end example
]
[Note
:
Once the static data member has been defined, it exists even if no objects of its class have been created.
[Example
:
In the example above, run_­chain and running exist even if no objects of class process are created by the program.
end example
]
end note
]
If a non-volatile non-inline const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression ([expr.const]).
The member shall still be defined in a namespace scope if it is odr-used ([basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer.
An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer.
If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see [depr.static_constexpr]).
Declarations of other static data members shall not specify a brace-or-equal-initializer.
[Note
:
There shall be exactly one definition of a static data member that is odr-used ([basic.def.odr]) in a program; no diagnostic is required.
end note
]
Unnamed classes and classes contained directly or indirectly within unnamed classes shall not contain static data members.
[Note
:
Static data members of a class in namespace scope have the linkage of that class ([basic.link]).
A local class cannot have static data members ([class.local]).
end note
]
Static data members are initialized and destroyed exactly like non-local variables ([basic.start.static], [basic.start.dynamic], [basic.start.term]).
A static data member shall not be mutable ([dcl.stc]).

10.3.10 Bit-fields [class.bit]

A member-declarator of the form
identifier attribute-specifier-seq : constant-expression brace-or-equal-initializer
specifies a bit-field; its length is set off from the bit-field name by a colon.
The optional attribute-specifier-seq appertains to the entity being declared.
The bit-field attribute is not part of the type of the class member.
The constant-expression shall be an integral constant expression with a value greater than or equal to zero.
The value of the integral constant expression may be larger than the number of bits in the object representation ([basic.types]) of the bit-field's type; in such cases the extra bits are padding bits ([basic.types]).
Allocation of bit-fields within a class object is implementation-defined.
Alignment of bit-fields is implementation-defined.
Bit-fields are packed into some addressable allocation unit.
[Note
:
Bit-fields straddle allocation units on some machines and not on others.
Bit-fields are assigned right-to-left on some machines, left-to-right on others.
end note
]
A declaration for a bit-field that omits the identifier declares an unnamed bit-field.
Unnamed bit-fields are not members and cannot be initialized.
An unnamed bit-field shall not be declared with a cv-qualified type.
[Note
:
An unnamed bit-field is useful for padding to conform to externally-imposed layouts.
end note
]
As a special case, an unnamed bit-field with a width of zero specifies alignment of the next bit-field at an allocation unit boundary.
Only when declaring an unnamed bit-field may the value of the constant-expression be equal to zero.
A bit-field shall not be a static member.
A bit-field shall have integral or enumeration type ([basic.fundamental]).
A bool value can successfully be stored in a bit-field of any nonzero size.
The address-of operator & shall not be applied to a bit-field, so there are no pointers to bit-fields.
A non-const reference shall not be bound to a bit-field ([dcl.init.ref]).
[Note
:
If the initializer for a reference of type const T& is an lvalue that refers to a bit-field, the reference is bound to a temporary initialized to hold the value of the bit-field; the reference is not bound to the bit-field directly.
end note
]
If the value true or false is stored into a bit-field of type bool of any size (including a one bit bit-field), the original bool value and the value of the bit-field shall compare equal.
If the value of an enumerator is stored into a bit-field of the same enumeration type and the number of bits in the bit-field is large enough to hold all the values of that enumeration type ([dcl.enum]), the original enumerator value and the value of the bit-field shall compare equal.
[Example
:
enum BOOL { FALSE=0, TRUE=1 };
struct A {
  BOOL b:1;
};
A a;
void f() {
  a.b = TRUE;
  if (a.b == TRUE)              // yields true
    { /* ... */ }
}
end example
]

10.3.11 Nested class declarations [class.nest]

A class can be declared within another class.
A class declared within another is called a nested class.
The name of a nested class is local to its enclosing class.
The nested class is in the scope of its enclosing class.
[Note
:
See [expr.prim.id] for restrictions on the use of non-static data members and non-static member functions.
end note
]
[Example
:
int x;
int y;

struct enclose {
  int x;
  static int s;

  struct inner {
    void f(int i) {
      int a = sizeof(x);        // OK: operand of sizeof is an unevaluated operand
      x = i;                    // error: assign to enclose​::​x
      s = i;                    // OK: assign to enclose​::​s
      ::x = i;                  // OK: assign to global x
      y = i;                    // OK: assign to global y
    }
    void g(enclose* p, int i) {
      p->x = i;                 // OK: assign to enclose​::​x
    }
  };
};

inner* p = 0;                   // error: inner not in scope
end example
]
Member functions and static data members of a nested class can be defined in a namespace scope enclosing the definition of their class.
[Example
:
struct enclose {
  struct inner {
    static int x;
    void f(int i);
  };
};

int enclose::inner::x = 1;

void enclose::inner::f(int i) { /* ... */ }
end example
]
If class X is defined in a namespace scope, a nested class Y may be declared in class X and later defined in the definition of class X or be later defined in a namespace scope enclosing the definition of class X.
[Example
:
class E {
  class I1;                     // forward declaration of nested class
  class I2;
  class I1 { };                 // definition of nested class
};
class E::I2 { };                // definition of nested class
end example
]
Like a member function, a friend function ([class.friend]) defined within a nested class is in the lexical scope of that class; it obeys the same rules for name binding as a static member function of that class ([class.static]), but it has no special access rights to members of an enclosing class.

10.3.12 Nested type names [class.nested.type]

Type names obey exactly the same scope rules as other names.
In particular, type names defined within a class definition cannot be used outside their class without qualification.
[Example
:
struct X {
  typedef int I;
  class Y { /* ... */ };
  I a;
};

I b;                            // error
Y c;                            // error
X::Y d;                         // OK
X::I e;                         // OK
end example
]

10.4 Unions [class.union]

In a union, a non-static data member is active if its name refers to an object whose lifetime has begun and has not ended ([basic.life]).
At most one of the non-static data members of an object of union type can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time.
[Note
:
One special guarantee is made in order to simplify the use of unions: If a standard-layout union contains several standard-layout structs that share a common initial sequence ([class.mem]), and if a non-static data member of an object of this standard-layout union type is active and is one of the standard-layout structs, it is permitted to inspect the common initial sequence of any of the standard-layout struct members; see [class.mem].
end note
]
The size of a union is sufficient to contain the largest of its non-static data members.
Each non-static data member is allocated as if it were the sole member of a struct.
[Note
:
A union object and its non-static data members are pointer-interconvertible ([basic.compound], [expr.static.cast]).
As a consequence, all non-static data members of a union object have the same address.
end note
]
A union can have member functions (including constructors and destructors), but it shall not have virtual ([class.virtual]) functions.
A union shall not have base classes.
A union shall not be used as a base class.
If a union contains a non-static data member of reference type the program is ill-formed.
[Note
:
Absent default member initializers ([class.mem]), if any non-static data member of a union has a non-trivial default constructor ([class.ctor]), copy constructor, move constructor ([class.copy.ctor]), copy assignment operator, move assignment operator ([class.copy.assign]), or destructor ([class.dtor]), the corresponding member function of the union must be user-provided or it will be implicitly deleted ([dcl.fct.def.delete]) for the union.
end note
]
[Example
:
Consider the following union:
union U {
  int i;
  float f;
  std::string s;
};
Since std​::​string ([string.classes]) declares non-trivial versions of all of the special member functions, U will have an implicitly deleted default constructor, copy/move constructor, copy/move assignment operator, and destructor.
To use U, some or all of these member functions must be user-provided.
end example
]
When the left operand of an assignment operator involves a member access expression ([expr.ref]) that nominates a union member, it may begin the lifetime of that union member, as described below.
For an expression E, define the set S(E) of subexpressions of E as follows:
  • If E is of the form A.B, S(E) contains the elements of S(A), and also contains A.B if B names a union member of a non-class, non-array type, or of a class type with a trivial default constructor that is not deleted, or an array of such types.
  • If E is of the form A[B] and is interpreted as a built-in array subscripting operator, S(E) is S(A) if A is of array type, S(B) if B is of array type, and empty otherwise.
  • Otherwise, S(E) is empty.
In an assignment expression of the form E1 = E2 that uses either the built-in assignment operator ([expr.ass]) or a trivial assignment operator ([class.copy.assign]), for each element X of S(E1), if modification of X would have undefined behavior under [basic.life], an object of the type of X is implicitly created in the nominated storage; no initialization is performed and the beginning of its lifetime is sequenced after the value computation of the left and right operands and before the assignment.
[Note
:
This ends the lifetime of the previously-active member of the union, if any ([basic.life]).
end note
]
[Example
:
union A { int x; int y[4]; };
struct B { A a; };
union C { B b; int k; };
int f() {
  C c;                  // does not start lifetime of any union member
  c.b.a.y[3] = 4;       // OK: S(c.b.a.y[3]) contains c.b and c.b.a.y;
                        // creates objects to hold union members c.b and c.b.a.y
  return c.b.a.y[3];    // OK: c.b.a.y refers to newly created object (see [basic.life])
}

struct X { const int a; int b; };
union Y { X x; int k; };
void g() {
  Y y = { { 1, 2 } };   // OK, y.x is active union member ([class.mem])
  int n = y.x.a;
  y.k = 4;              // OK: ends lifetime of y.x, y.k is active member of union
  y.x.b = n;            // undefined behavior: y.x.b modified outside its lifetime,
                        // S(y.x.b) is empty because X's default constructor is deleted,
                        // so union member y.x's lifetime does not implicitly start
}
end example
]
[Note
:
In general, one must use explicit destructor calls and placement new-expression to change the active member of a union.
end note
]
[Example
:
Consider an object u of a union type U having non-static data members m of type M and n of type N.
If M has a non-trivial destructor and N has a non-trivial constructor (for instance, if they declare or inherit virtual functions), the active member of u can be safely switched from m to n using the destructor and placement new-expression as follows:
u.m.~M();
new (&u.n) N;
end example
]

10.4.1 Anonymous unions [class.union.anon]

A union of the form
union { member-specification } ;
is called an anonymous union; it defines an unnamed type and an unnamed object of that type called an anonymous union object.
Each member-declaration in the member-specification of an anonymous union shall either define a non-static data member or be a static_assert-declaration.
[Note
:
Nested types, anonymous unions, and functions cannot be declared within an anonymous union.
end note
]
The names of the members of an anonymous union shall be distinct from the names of any other entity in the scope in which the anonymous union is declared.
For the purpose of name lookup, after the anonymous union definition, the members of the anonymous union are considered to have been defined in the scope in which the anonymous union is declared.
[Example
:
void f() {
  union { int a; const char* p; };
  a = 1;
  p = "Jennifer";
}
Here a and p are used like ordinary (non-member) variables, but since they are union members they have the same address.
end example
]
Anonymous unions declared in a named namespace or in the global namespace shall be declared static.
Anonymous unions declared at block scope shall be declared with any storage class allowed for a block-scope variable, or with no storage class.
A storage class is not allowed in a declaration of an anonymous union in a class scope.
An anonymous union shall not have private or protected members ([class.access]).
An anonymous union shall not have member functions.
A union for which objects, pointers, or references are declared is not an anonymous union.
[Example
:
void f() {
  union { int aa; char* p; } obj, *ptr = &obj;
  aa = 1;           // error
  ptr->aa = 1;      // OK
}
The assignment to plain aa is ill-formed since the member name is not visible outside the union, and even if it were visible, it is not associated with any particular object.
end example
]
[Note
:
Initialization of unions with no user-declared constructors is described in [dcl.init.aggr].
end note
]
A union-like class is a union or a class that has an anonymous union as a direct member.
A union-like class X has a set of variant members.
If X is a union, a non-static data member of X that is not an anonymous union is a variant member of X.
In addition, a non-static data member of an anonymous union that is a member of X is also a variant member of X.
At most one variant member of a union may have a default member initializer.
[Example
:
union U {
  int x = 0;
  union {
    int k;
  };
  union {
    int z;
    int y = 1;      // error: initialization for second variant member of U
  };
};
end example
]

10.5 Local class declarations [class.local]

A class can be declared within a function definition; such a class is called a local class.
The name of a local class is local to its enclosing scope.
The local class is in the scope of the enclosing scope, and has the same access to names outside the function as does the enclosing function.
[Note
:
A declaration in a local class cannot odr-use ([basic.def.odr]) a local entity from an enclosing scope.
end note
]
[Example
:
int x;
void f() {
  static int s;
  int x;
  const int N = 5;
  extern int q();
  int arr[2];
  auto [y, z] = arr;

  struct local {
    int g() { return x; }       // error: odr-use of non-odr-usable variable x
    int h() { return s; }       // OK
    int k() { return ::x; }     // OK
    int l() { return q(); }     // OK
    int m() { return N; }       // OK: not an odr-use
    int* n() { return &N; }     // error: odr-use of non-odr-usable variable N
    int p() { return y; }       // error: odr-use of non-odr-usable structured binding y
  };
}

local* p = 0;                   // error: local not in scope
end example
]
An enclosing function has no special access to members of the local class; it obeys the usual access rules ([class.access]).
Member functions of a local class shall be defined within their class definition, if they are defined at all.
If class X is a local class a nested class Y may be declared in class X and later defined in the definition of class X or be later defined in the same scope as the definition of class X.
A class nested within a local class is a local class.
A local class shall not have static data members.

10.6 Derived classes [class.derived]

A class-or-decltype shall denote a class type that is not an incompletely defined class ([class]).
The class denoted by the class-or-decltype of a base-specifier is called a direct base class for the class being defined.
During the lookup for a base class name, non-type names are ignored ([basic.scope.hiding]).
If the name found is not a class-name, the program is ill-formed.
A class B is a base class of a class D if it is a direct base class of D or a direct base class of one of D's base classes.
A class is an indirect base class of another if it is a base class but not a direct base class.
A class is said to be (directly or indirectly) derived from its (direct or indirect) base classes.
[Note
:
See [class.access] for the meaning of access-specifier.
end note
]
Unless redeclared in the derived class, members of a base class are also considered to be members of the derived class.
Members of a base class other than constructors are said to be inherited by the derived class.
Constructors of a base class can also be inherited as described in [namespace.udecl].
Inherited members can be referred to in expressions in the same manner as other members of the derived class, unless their names are hidden or ambiguous ([class.member.lookup]).
[Note
:
The scope resolution operator ​::​ ([expr.prim.id.qual]) can be used to refer to a direct or indirect base member explicitly.
This allows access to a name that has been redeclared in the derived class.
A derived class can itself serve as a base class subject to access control; see [class.access.base].
A pointer to a derived class can be implicitly converted to a pointer to an accessible unambiguous base class ([conv.ptr]).
An lvalue of a derived class type can be bound to a reference to an accessible unambiguous base class ([dcl.init.ref]).
end note
]
The base-specifier-list specifies the type of the base class subobjects contained in an object of the derived class type.
[Example
:
struct Base {
  int a, b, c;
};
struct Derived : Base {
  int b;
};
struct Derived2 : Derived {
  int c;
};
Here, an object of class Derived2 will have a subobject of class Derived which in turn will have a subobject of class Base.
end example
]
A base-specifier followed by an ellipsis is a pack expansion ([temp.variadic]).
The order in which the base class subobjects are allocated in the most derived object ([intro.object]) is unspecified.
[Note
:
A derived class and its base class subobjects can be represented by a directed acyclic graph (DAG) where an arrow means “directly derived from”.
An arrow need not have a physical representation in memory.
A DAG of subobjects is often referred to as a “subobject lattice”.
dag Base Base Derived1 Derived1 Derived1->Base Derived2 Derived2 Derived2->Derived1
Figure 2 — Directed acyclic graph
end note
]
[Note
:
Initialization of objects representing base classes can be specified in constructors; see [class.base.init].
end note
]
[Note
:
A base class subobject might have a layout ([basic.stc]) different from the layout of a most derived object of the same type.
A base class subobject might have a polymorphic behavior ([class.cdtor]) different from the polymorphic behavior of a most derived object of the same type.
A base class subobject may be of zero size ([class]); however, two subobjects that have the same class type and that belong to the same most derived object must not be allocated at the same address ([expr.eq]).
end note
]

10.6.1 Multiple base classes [class.mi]

A class can be derived from any number of base classes.
[Note
:
The use of more than one direct base class is often called multiple inheritance.
end note
]
[Example
:
class A { /* ... */ };
class B { /* ... */ };
class C { /* ... */ };
class D : public A, public B, public C { /* ... */ };
end example
]
[Note
:
The order of derivation is not significant except as specified by the semantics of initialization by constructor ([class.base.init]), cleanup ([class.dtor]), and storage layout ([class.mem], [class.access.spec]).
end note
]
A class shall not be specified as a direct base class of a derived class more than once.
[Note
:
A class can be an indirect base class more than once and can be a direct and an indirect base class.
There are limited things that can be done with such a class.
The non-static data members and member functions of the direct base class cannot be referred to in the scope of the derived class.
However, the static members, enumerations and types can be unambiguously referred to.
end note
]
[Example
:
class X { /* ... */ };
class Y : public X, public X { /* ... */ };             // ill-formed
class L { public: int next;  /* ... */ };
class A : public L { /* ... */ };
class B : public L { /* ... */ };
class C : public A, public B { void f(); /* ... */ };   // well-formed
class D : public A, public L { void f(); /* ... */ };   // well-formed
end example
]
A base class specifier that does not contain the keyword virtual specifies a non-virtual base class.
A base class specifier that contains the keyword virtual specifies a virtual base class.
For each distinct occurrence of a non-virtual base class in the class lattice of the most derived class, the most derived object ([intro.object]) shall contain a corresponding distinct base class subobject of that type.
For each distinct base class that is specified virtual, the most derived object shall contain a single base class subobject of that type.
[Note
:
For an object of class type C, each distinct occurrence of a (non-virtual) base class L in the class lattice of C corresponds one-to-one with a distinct L subobject within the object of type C.
Given the class C defined above, an object of class C will have two subobjects of class L as shown in Figure [fig:nonvirt].
nonvirt L1 L L2 L A A A->L1 B B B->L2 C C C->A C->B
Figure 3 — Non-virtual base
In such lattices, explicit qualification can be used to specify which subobject is meant.
The body of function C​::​f could refer to the member next of each L subobject:
void C::f() { A::next = B::next; }      // well-formed
Without the A​::​ or B​::​ qualifiers, the definition of C​::​f above would be ill-formed because of ambiguity ([class.member.lookup]).
end note
]
[Note
:
In contrast, consider the case with a virtual base class:
class V { /* ... */ };
class A : virtual public V { /* ... */ };
class B : virtual public V { /* ... */ };
class C : public A, public B { /* ... */ };
virt V V A A A->V B B B->V C C C->A C->B
Figure 4 — Virtual base
For an object c of class type C, a single subobject of type V is shared by every base class subobject of c that has a virtual base class of type V.
Given the class C defined above, an object of class C will have one subobject of class V, as shown in Figure [fig:virt].
end note
]
[Note
:
A class can have both virtual and non-virtual base classes of a given type.
class B { /* ... */ };
class X : virtual public B { /* ... */ };
class Y : virtual public B { /* ... */ };
class Z : public B { /* ... */ };
class AA : public X, public Y, public Z { /* ... */ };
For an object of class AA, all virtual occurrences of base class B in the class lattice of AA correspond to a single B subobject within the object of type AA, and every other occurrence of a (non-virtual) base class B in the class lattice of AA corresponds one-to-one with a distinct B subobject within the object of type AA.
Given the class AA defined above, class AA has two subobjects of class B: Z's B and the virtual B shared by X and Y, as shown in Figure [fig:virtnonvirt].
virtnonvirt B1 B B2 B AA AA X X AA->X Y Y AA->Y Z Z AA->Z X->B1 Y->B1 Z->B2
Figure 5 — Virtual and non-virtual base
end note
]

10.6.2 Virtual functions [class.virtual]

[Note
:
Virtual functions support dynamic binding and object-oriented programming.
end note
]
A class that declares or inherits a virtual function is called a polymorphic class.
If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list ([dcl.fct]), cv-qualification, and ref-qualifier (or absence of same) as Base​::​vf is declared, then Derived​::​vf is also virtual (whether or not it is so declared) and it overrides114 Base​::​vf.
For convenience we say that any virtual function overrides itself.
A virtual member function C​::​vf of a class object S is a final overrider unless the most derived class ([intro.object]) of which S is a base class subobject (if any) declares or inherits another member function that overrides vf.
In a derived class, if a virtual member function of a base class subobject has more than one final overrider the program is ill-formed.
[Example
:
struct A {
  virtual void f();
};
struct B : virtual A {
  virtual void f();
};
struct C : B , virtual A {
  using A::f;
};

void foo() {
  C c;
  c.f();              // calls B​::​f, the final overrider
  c.C::f();           // calls A​::​f because of the using-declaration
}
end example
]
[Example
:
struct A { virtual void f(); };
struct B : A { };
struct C : A { void f(); };
struct D : B, C { };  // OK: A​::​f and C​::​f are the final overriders
                      // for the B and C subobjects, respectively
end example
]
[Note
:
A virtual member function does not have to be visible to be overridden, for example,
struct B {
  virtual void f();
};
struct D : B {
  void f(int);
};
struct D2 : D {
  void f();
};
the function f(int) in class D hides the virtual function f() in its base class B; D​::​f(int) is not a virtual function.
However, f() declared in class D2 has the same name and the same parameter list as B​::​f(), and therefore is a virtual function that overrides the function B​::​f() even though B​::​f() is not visible in class D2.
end note
]
If a virtual function f in some class B is marked with the virt-specifier final and in a class D derived from B a function D​::​f overrides B​::​f, the program is ill-formed.
[Example
:
struct B {
  virtual void f() const final;
};

struct D : B {
  void f() const;     // error: D​::​f attempts to override final B​::​f
};
end example
]
If a virtual function is marked with the virt-specifier override and does not override a member function of a base class, the program is ill-formed.
[Example
:
struct B {
  virtual void f(int);
};

struct D : B {
  virtual void f(long) override;  // error: wrong signature overriding B​::​f
  virtual void f(int) override;   // OK
};
end example
]
A virtual function shall not have a trailing requires-clause ([dcl.decl]).
[Example
:
struct A {
  virtual void f() requires true; // error: virtual function cannot be constrained ([temp.constr.decl])
};
end example
]
Even though destructors are not inherited, a destructor in a derived class overrides a base class destructor declared virtual; see [class.dtor] and [class.free].
The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions.
If a function D​::​f overrides a function B​::​f, the return types of the functions are covariant if they satisfy the following criteria:
  • both are pointers to classes, both are lvalue references to classes, or both are rvalue references to classes115
  • the class in the return type of B​::​f is the same class as the class in the return type of D​::​f, or is an unambiguous and accessible direct or indirect base class of the class in the return type of D​::​f
  • both pointers or references have the same cv-qualification and the class type in the return type of D​::​f has the same cv-qualification as or less cv-qualification than the class type in the return type of B​::​f.
If the class type in the covariant return type of D​::​f differs from that of B​::​f, the class type in the return type of D​::​f shall be complete at the point of declaration of D​::​f or shall be the class type D.
When the overriding function is called as the final overrider of the overridden function, its result is converted to the type returned by the (statically chosen) overridden function ([expr.call]).
[Example
:
class B { };
class D : private B { friend class Derived; };
struct Base {
  virtual void vf1();
  virtual void vf2();
  virtual void vf3();
  virtual B*   vf4();
  virtual B*   vf5();
  void f();
};

struct No_good : public Base {
  D*  vf4();        // error: B (base class of D) inaccessible
};

class A;
struct Derived : public Base {
    void vf1();     // virtual and overrides Base​::​vf1()
    void vf2(int);  // not virtual, hides Base​::​vf2()
    char vf3();     // error: invalid difference in return type only
    D*   vf4();     // OK: returns pointer to derived class
    A*   vf5();     // error: returns pointer to incomplete class
    void f();
};

void g() {
  Derived d;
  Base* bp = &d;                // standard conversion:
                                // Derived* to Base*
  bp->vf1();                    // calls Derived​::​vf1()
  bp->vf2();                    // calls Base​::​vf2()
  bp->f();                      // calls Base​::​f() (not virtual)
  B*  p = bp->vf4();            // calls Derived​::​vf4() and converts the
                                // result to B*
  Derived*  dp = &d;
  D*  q = dp->vf4();            // calls Derived​::​vf4() and does not
                                // convert the result to B*
  dp->vf2();                    // ill-formed: argument mismatch
}
end example
]
[Note
:
The interpretation of the call of a virtual function depends on the type of the object for which it is called (the dynamic type), whereas the interpretation of a call of a non-virtual member function depends only on the type of the pointer or reference denoting that object (the static type) ([expr.call]).
end note
]
[Note
:
The virtual specifier implies membership, so a virtual function cannot be a non-member ([dcl.fct.spec]) function.
Nor can a virtual function be a static member, since a virtual function call relies on a specific object for determining which function to invoke.
A virtual function declared in one class can be declared a friend ([class.friend]) in another class.
end note
]
A virtual function declared in a class shall be defined, or declared pure ([class.abstract]) in that class, or both; no diagnostic is required ([basic.def.odr]).
[Example
:
Here are some uses of virtual functions with multiple base classes:
struct A {
  virtual void f();
};

struct B1 : A {                 // note non-virtual derivation
  void f();
};

struct B2 : A {
  void f();
};

struct D : B1, B2 {             // D has two separate A subobjects
};

void foo() {
  D   d;
//   A*  ap = &d;                  // would be ill-formed: ambiguous
  B1*  b1p = &d;
  A*   ap = b1p;
  D*   dp = &d;
  ap->f();                      // calls D​::​B1​::​f
  dp->f();                      // ill-formed: ambiguous
}
In class D above there are two occurrences of class A and hence two occurrences of the virtual member function A​::​f.
The final overrider of B1​::​A​::​f is B1​::​f and the final overrider of B2​::​A​::​f is B2​::​f.
end example
]
[Example
:
The following example shows a function that does not have a unique final overrider:
struct A {
  virtual void f();
};

struct VB1 : virtual A {        // note virtual derivation
  void f();
};

struct VB2 : virtual A {
  void f();
};

struct Error : VB1, VB2 {       // ill-formed
};

struct Okay : VB1, VB2 {
  void f();
};
Both VB1​::​f and VB2​::​f override A​::​f but there is no overrider of both of them in class Error.
This example is therefore ill-formed.
Class Okay is well-formed, however, because Okay​::​f is a final overrider.
end example
]
[Example
:
The following example uses the well-formed classes from above.
struct VB1a : virtual A {       // does not declare f
};

struct Da : VB1a, VB2 {
};

void foe() {
  VB1a*  vb1ap = new Da;
  vb1ap->f();                   // calls VB2​::​f
}
end example
]
Explicit qualification with the scope operator ([expr.prim.id.qual]) suppresses the virtual call mechanism.
[Example
:
class B { public: virtual void f(); };
class D : public B { public: void f(); };

void D::f() { /* ... */ B::f(); }
Here, the function call in D​::​f really does call B​::​f and not D​::​f.
end example
]
A function with a deleted definition ([dcl.fct.def]) shall not override a function that does not have a deleted definition.
Likewise, a function that does not have a deleted definition shall not override a function with a deleted definition.
If an overriding function specifies contract conditions ([dcl.attr.contract]), it shall specify the same list of contract conditions as its overridden functions; no diagnostic is required if corresponding conditions will always evaluate to the same value.
Otherwise, it is considered to have the list of contract conditions from one of its overridden functions; the names in the contract conditions are bound, and the semantic constraints are checked, at the point where the contract conditions appear.
Given a virtual function f with a contract condition that odr-uses *this ([basic.def.odr]), the class of which f is a direct member shall be be an unambiguous and accessible base class of any class in which f is overridden.
If a function overrides more than one function, all of the overridden functions shall have the same list of contract conditions ([dcl.attr.contract]); no diagnostic is required if corresponding conditions will always evaluate to the same value.
[Example
:
struct A {
  virtual void g() [[expects: x == 0]];
  int x = 42;
};

int x = 42;
struct B {
  virtual void g() [[expects: x == 0]];
}

struct C : A, B {
  virtual void g();             // error: preconditions of overridden functions are not the same
};
end example
]
A function with the same name but a different parameter list ([over]) as a virtual function is not necessarily virtual and does not override.
The use of the virtual specifier in the declaration of an overriding function is legal but redundant (has empty semantics).
Access control ([class.access]) is not considered in determining overriding.
Multi-level pointers to classes or references to multi-level pointers to classes are not allowed.

10.6.3 Abstract classes [class.abstract]

[Note
:
The abstract class mechanism supports the notion of a general concept, such as a shape, of which only more concrete variants, such as circle and square, can actually be used.
An abstract class can also be used to define an interface for which derived classes provide a variety of implementations.
end note
]
A virtual function is specified as a pure virtual function by using a pure-specifier in the function declaration in the class definition.
[Note
:
Such a function might be inherited: see below.
end note
]
A class is an abstract class if it has at least one pure virtual function.
[Note
:
An abstract class can be used only as a base class of some other class; no objects of an abstract class can be created except as subobjects of a class derived from it ([basic.def], [class.mem]).
end note
]
A pure virtual function need be defined only if called with, or as if with ([class.dtor]), the qualified-id syntax ([expr.prim.id.qual]).
[Example
:
class point { /* ... */ };
class shape {                   // abstract class
  point center;
public:
  point where() { return center; }
  void move(point p) { center=p; draw(); }
  virtual void rotate(int) = 0; // pure virtual
  virtual void draw() = 0;      // pure virtual
};
end example
]
[Note
:
A function declaration cannot provide both a pure-specifier and a definition
end note
]
[Example
:
struct C {
  virtual void f() = 0 { };     // ill-formed
};
end example
]
[Note
:
An abstract class type cannot be used as a parameter or return type of a function being defined ([dcl.fct]) or called ([expr.call]), except as specified in [dcl.type.simple].
Further, an abstract class type cannot be used as the type of an explicit type conversion ([expr.static.cast], [expr.reinterpret.cast], [expr.const.cast]), because the resulting prvalue would be of abstract class type ([basic.lval]).
However, pointers and references to abstract class types can appear in such contexts.
end note
]
A class is abstract if it contains or inherits at least one pure virtual function for which the final overrider is pure virtual.
[Example
:
class ab_circle : public shape {
  int radius;
public:
  void rotate(int) { }
  // ab_­circle​::​draw() is a pure virtual
};
Since shape​::​draw() is a pure virtual function ab_­circle​::​draw() is a pure virtual by default.
The alternative declaration,
class circle : public shape {
  int radius;
public:
  void rotate(int) { }
  void draw();                  // a definition is required somewhere
};
would make class circle non-abstract and a definition of circle​::​draw() must be provided.
end example
]
[Note
:
An abstract class can be derived from a class that is not abstract, and a pure virtual function may override a virtual function which is not pure.
end note
]
Member functions can be called from a constructor (or destructor) of an abstract class; the effect of making a virtual call ([class.virtual]) to a pure virtual function directly or indirectly for the object being created (or destroyed) from such a constructor (or destructor) is undefined.

10.7 Member name lookup [class.member.lookup]

Member name lookup determines the meaning of a name (id-expression) in a class scope ([basic.scope.class]).
Name lookup can result in an ambiguity, in which case the program is ill-formed.
For an id-expression, name lookup begins in the class scope of this; for a qualified-id, name lookup begins in the scope of the nested-name-specifier.
Name lookup takes place before access control ([basic.lookup], [class.access]).
The following steps define the result of name lookup for a member name f in a class scope C.
The lookup set for f in C, called , consists of two component sets: the declaration set, a set of members named f; and the subobject set, a set of subobjects where declarations of these members (possibly including using-declarations) were found.
In the declaration set, using-declarations are replaced by the set of designated members that are not hidden or overridden by members of the derived class ([namespace.udecl]), and type declarations (including injected-class-names) are replaced by the types they designate.
is calculated as follows:
If C contains a declaration of the name f, the declaration set contains every declaration of f declared in C that satisfies the requirements of the language construct in which the lookup occurs.
[Note
:
Looking up a name in an elaborated-type-specifier ([basic.lookup.elab]) or base-specifier, for instance, ignores all non-type declarations, while looking up a name in a nested-name-specifier ([basic.lookup.qual]) ignores function, variable, and enumerator declarations.
As another example, looking up a name in a using-declaration includes the declaration of a class or enumeration that would ordinarily be hidden by another declaration of that name in the same scope.
end note
]
If the resulting declaration set is not empty, the subobject set contains C itself, and calculation is complete.
Otherwise (i.e., C does not contain a declaration of f or the resulting declaration set is empty), is initially empty.
If C has base classes, calculate the lookup set for f in each direct base class subobject , and merge each such lookup set in turn into .
The following steps define the result of merging lookup set into the intermediate :
  • If each of the subobject members of is a base class subobject of at least one of the subobject members of , or if is empty, is unchanged and the merge is complete.
    Conversely, if each of the subobject members of is a base class subobject of at least one of the subobject members of , or if is empty, the new is a copy of .
  • Otherwise, if the declaration sets of and differ, the merge is ambiguous: the new is a lookup set with an invalid declaration set and the union of the subobject sets.
    In subsequent merges, an invalid declaration set is considered different from any other.
  • Otherwise, the new is a lookup set with the shared set of declarations and the union of the subobject sets.
The result of name lookup for f in C is the declaration set of .
If it is an invalid set, the program is ill-formed.
[Example
:
struct A { int x; };                    // S(x,A) = { { A​::​x }, { A } }
struct B { float x; };                  // S(x,B) = { { B​::​x }, { B } }
struct C: public A, public B { };       // S(x,C) = { invalid, { A in C, B in C } }
struct D: public virtual C { };         // S(x,D) = S(x,C)
struct E: public virtual C { char x; }; // S(x,E) = { { E​::​x }, { E } }
struct F: public D, public E { };       // S(x,F) = S(x,E)
int main() {
  F f;
  f.x = 0;                              // OK, lookup finds E​::​x
}
is unambiguous because the A and B base class subobjects of D are also base class subobjects of E, so is discarded in the first merge step.
end example
]
If the name of an overloaded function is unambiguously found, overload resolution ([over.match]) also takes place before access control.
Ambiguities can often be resolved by qualifying a name with its class name.
[Example
:
struct A {
  int f();
};
struct B {
  int f();
};
struct C : A, B {
  int f() { return A::f() + B::f(); }
};
end example
]
[Note
:
A static member, a nested type or an enumerator defined in a base class T can unambiguously be found even if an object has more than one base class subobject of type T.
Two base class subobjects share the non-static member subobjects of their common virtual base classes.
end note
]
[Example
:
struct V {
  int v;
};
struct A {
  int a;
  static int   s;
  enum { e };
};
struct B : A, virtual V { };
struct C : A, virtual V { };
struct D : B, C { };

void f(D* pd) {
  pd->v++;          // OK: only one v (virtual)
  pd->s++;          // OK: only one s (static)
  int i = pd->e;    // OK: only one e (enumerator)
  pd->a++;          // error, ambiguous: two as in D
}
end example
]
[Note
:
When virtual base classes are used, a hidden declaration can be reached along a path through the subobject lattice that does not pass through the hiding declaration.
This is not an ambiguity.
The identical use with non-virtual base classes is an ambiguity; in that case there is no unique instance of the name that hides all the others.
end note
]
[Example
:
struct V { int f();  int x; };
struct W { int g();  int y; };
struct B : virtual V, W {
  int f();  int x;
  int g();  int y;
};
struct C : virtual V, W { };

struct D : B, C { void glorp(); };
virt W1 W V V W2 W B B B->W1 B->V C C C->V C->W2 D D D->B D->C
Figure 6 — Name lookup
The names declared in V and the left-hand instance of W are hidden by those in B, but the names declared in the right-hand instance of W are not hidden at all.
void D::glorp() {
  x++;              // OK: B​::​x hides V​::​x
  f();              // OK: B​::​f() hides V​::​f()
  y++;              // error: B​::​y and C's W​::​y
  g();              // error: B​::​g() and C's W​::​g()
}
end example
]
An explicit or implicit conversion from a pointer to or an expression designating an object of a derived class to a pointer or reference to one of its base classes shall unambiguously refer to a unique object representing the base class.
[Example
:
struct V { };
struct A { };
struct B : A, virtual V { };
struct C : A, virtual V { };
struct D : B, C { };

void g() {
  D d;
  B* pb = &d;
  A* pa = &d;       // error, ambiguous: C's A or B's A?
  V* pv = &d;       // OK: only one V subobject
}
end example
]
[Note
:
Even if the result of name lookup is unambiguous, use of a name found in multiple subobjects might still be ambiguous ([conv.mem], [expr.ref], [class.access.base]).
end note
]
[Example
:
struct B1 {
  void f();
  static void f(int);
  int i;
};
struct B2 {
  void f(double);
};
struct I1: B1 { };
struct I2: B1 { };

struct D: I1, I2, B2 {
  using B1::f;
  using B2::f;
  void g() {
    f();                        // Ambiguous conversion of this
    f(0);                       // Unambiguous (static)
    f(0.0);                     // Unambiguous (only one B2)
    int B1::* mpB1 = &D::i;     // Unambiguous
    int D::* mpD = &D::i;       // Ambiguous conversion
  }
};
end example
]

10.8 Member access control [class.access]

A member of a class can be
  • private; that is, its name can be used only by members and friends of the class in which it is declared.
  • protected; that is, its name can be used only by members and friends of the class in which it is declared, by classes derived from that class, and by their friends (see [class.protected]).
  • public; that is, its name can be used anywhere without access restriction.
A member of a class can also access all the names to which the class has access.
A local class of a member function may access the same names that the member function itself may access.116
Members of a class defined with the keyword class are private by default.
Members of a class defined with the keywords struct or union are public by default.
[Example
:
class X {
  int a;            // X​::​a is private by default
};

struct S {
  int a;            // S​::​a is public by default
};
end example
]
Access control is applied uniformly to all names, whether the names are referred to from declarations or expressions.
[Note
:
Access control applies to names nominated by friend declarations ([class.friend]) and using-declarations.
end note
]
In the case of overloaded function names, access control is applied to the function selected by overload resolution.
[Note
:
Because access control applies to names, if access control is applied to a typedef name, only the accessibility of the typedef name itself is considered.
The accessibility of the entity referred to by the typedef is not considered.
For example,
class A {
  class B { };
public:
  typedef B BB;
};

void f() {
  A::BB x;          // OK, typedef name A​::​BB is public
  A::B y;           // access error, A​::​B is private
}
end note
]
[Note
:
Access to members and base classes is controlled, not their visibility ([basic.scope.hiding]).
Names of members are still visible, and implicit conversions to base classes are still considered, when those members and base classes are inaccessible.
end note
]
The interpretation of a given construct is established without regard to access control.
If the interpretation established makes use of inaccessible member names or base classes, the construct is ill-formed.
All access controls in [class.access] affect the ability to access a class member name from the declaration of a particular entity, including parts of the declaration preceding the name of the entity being declared and, if the entity is a class, the definitions of members of the class appearing outside the class's member-specification.
[Note
:
This access also applies to implicit references to constructors, conversion functions, and destructors.
end note
]
[Example
:
class A {
  typedef int I;    // private member
  I f();
  friend I g(I);
  static I x;
  template<int> struct Q;
  template<int> friend struct R;
protected:
    struct B { };
};

A::I A::f() { return 0; }
A::I g(A::I p = A::x);
A::I g(A::I p) { return 0; }
A::I A::x = 0;
template<A::I> struct A::Q { };
template<A::I> struct R { };

struct D: A::B, A { };
Here, all the uses of A​::​I are well-formed because A​::​f, A​::​x, and A​::​Q are members of class A and g and R are friends of class A.
This implies, for example, that access checking on the first use of A​::​I must be deferred until it is determined that this use of A​::​I is as the return type of a member of class A.
Similarly, the use of A​::​B as a base-specifier is well-formed because D is derived from A, so checking of base-specifiers must be deferred until the entire base-specifier-list has been seen.
end example
]
The names in a default argument ([dcl.fct.default]) are bound at the point of declaration, and access is checked at that point rather than at any points of use of the default argument.
Access checking for default arguments in function templates and in member functions of class templates is performed as described in [temp.inst].
The names in a default template-argument ([temp.param]) have their access checked in the context in which they appear rather than at any points of use of the default template-argument.
[Example
:
class B { };
template <class T> class C {
protected:
  typedef T TT;
};

template <class U, class V = typename U::TT>
class D : public U { };

D <C<B> >* d;       // access error, C​::​TT is protected
end example
]
Access permissions are thus transitive and cumulative to nested and local classes.

10.8.1 Access specifiers [class.access.spec]

Member declarations can be labeled by an access-specifier ([class.derived]):
access-specifier : member-specification
An access-specifier specifies the access rules for members following it until the end of the class or until another access-specifier is encountered.
[Example
:
class X {
  int a;            // X​::​a is private by default: class used
public:
  int b;            // X​::​b is public
  int c;            // X​::​c is public
};
end example
]
Any number of access specifiers is allowed and no particular order is required.
[Example
:
struct S {
  int a;            // S​::​a is public by default: struct used
protected:
  int b;            // S​::​b is protected
private:
  int c;            // S​::​c is private
public:
  int d;            // S​::​d is public
};
end example
]
[Note
:
The effect of access control on the order of allocation of data members is described in [class.mem].
end note
]
When a member is redeclared within its class definition, the access specified at its redeclaration shall be the same as at its initial declaration.
[Example
:
struct S {
  class A;
  enum E : int;
private:
  class A { };        // error: cannot change access
  enum E: int { e0 }; // error: cannot change access
};
end example
]
[Note
:
In a derived class, the lookup of a base class name will find the injected-class-name instead of the name of the base class in the scope in which it was declared.
The injected-class-name might be less accessible than the name of the base class in the scope in which it was declared.
end note
]
[Example
:
class A { };
class B : private A { };
class C : public B {
  A* p;             // error: injected-class-name A is inaccessible
  ::A* q;           // OK
};
end example
]

10.8.2 Accessibility of base classes and base class members [class.access.base]

If a class is declared to be a base class ([class.derived]) for another class using the public access specifier, the public members of the base class are accessible as public members of the derived class and protected members of the base class are accessible as protected members of the derived class.
If a class is declared to be a base class for another class using the protected access specifier, the public and protected members of the base class are accessible as protected members of the derived class.
If a class is declared to be a base class for another class using the private access specifier, the public and protected members of the base class are accessible as private members of the derived class.117
In the absence of an access-specifier for a base class, public is assumed when the derived class is defined with the class-key struct and private is assumed when the class is defined with the class-key class.
[Example
:
class B { /* ... */ };
class D1 : private B { /* ... */ };
class D2 : public B { /* ... */ };
class D3 : B { /* ... */ };             // B private by default
struct D4 : public B { /* ... */ };
struct D5 : private B { /* ... */ };
struct D6 : B { /* ... */ };            // B public by default
class D7 : protected B { /* ... */ };
struct D8 : protected B { /* ... */ };
Here B is a public base of D2, D4, and D6, a private base of D1, D3, and D5, and a protected base of D7 and D8.
end example
]
[Note
:
A member of a private base class might be inaccessible as an inherited member name, but accessible directly.
Because of the rules on pointer conversions ([conv.ptr]) and explicit casts ([expr.cast]), a conversion from a pointer to a derived class to a pointer to an inaccessible base class might be ill-formed if an implicit conversion is used, but well-formed if an explicit cast is used.
For example,
class B {
public:
  int mi;                       // non-static member
  static int si;                // static member
};
class D : private B {
};
class DD : public D {
  void f();
};

void DD::f() {
  mi = 3;                       // error: mi is private in D
  si = 3;                       // error: si is private in D
  ::B  b;
  b.mi = 3;                     // OK (b.mi is different from this->mi)
  b.si = 3;                     // OK (b.si is different from this->si)
  ::B::si = 3;                  // OK
  ::B* bp1 = this;              // error: B is a private base class
  ::B* bp2 = (::B*)this;        // OK with cast
  bp2->mi = 3;                  // OK: access through a pointer to B.
}
end note
]
A base class B of N is accessible at R, if
  • an invented public member of B would be a public member of N, or
  • R occurs in a member or friend of class N, and an invented public member of B would be a private or protected member of N, or
  • R occurs in a member or friend of a class P derived from N, and an invented public member of B would be a private or protected member of P, or
  • there exists a class S such that B is a base class of S accessible at R and S is a base class of N accessible at R.
[Example
:
class B {
public:
  int m;
};

class S: private B {
  friend class N;
};

class N: private S {
  void f() {
    B* p = this;    // OK because class S satisfies the fourth condition above: B is a base class of N
                    // accessible in f() because B is an accessible base class of S and S is an accessible
                    // base class of N.
  }
};
end example
]
If a base class is accessible, one can implicitly convert a pointer to a derived class to a pointer to that base class ([conv.ptr], [conv.mem]).
[Note
:
It follows that members and friends of a class X can implicitly convert an X* to a pointer to a private or protected immediate base class of X.
end note
]
The access to a member is affected by the class in which the member is named.
This naming class is the class in which the member name was looked up and found.
[Note
:
This class can be explicit, e.g., when a qualified-id is used, or implicit, e.g., when a class member access operator ([expr.ref]) is used (including cases where an implicit “this->” is added).
If both a class member access operator and a qualified-id are used to name the member (as in p->T​::​m), the class naming the member is the class denoted by the nested-name-specifier of the qualified-id (that is, T).
end note
]
A member m is accessible at the point R when named in class N if
  • m as a member of N is public, or
  • m as a member of N is private, and R occurs in a member or friend of class N, or
  • m as a member of N is protected, and R occurs in a member or friend of class N, or in a member of a class P derived from N, where m as a member of P is public, private, or protected, or
  • there exists a base class B of N that is accessible at R, and m is accessible at R when named in class B.
    [Example
    :
    class B;
    class A {
    private:
      int i;
      friend void f(B*);
    };
    class B : public A { };
    void f(B* p) {
      p->i = 1;         // OK: B* can be implicitly converted to A*, and f has access to i in A
    }
    end example
    ]
If a class member access operator, including an implicit “this->”, is used to access a non-static data member or non-static member function, the reference is ill-formed if the left operand (considered as a pointer in the “.” operator case) cannot be implicitly converted to a pointer to the naming class of the right operand.
[Note
:
This requirement is in addition to the requirement that the member be accessible as named.
end note
]
As specified previously in [class.access], private members of a base class remain inaccessible even to derived classes unless friend declarations within the base class definition are used to grant access explicitly.

10.8.3 Friends [class.friend]

A friend of a class is a function or class that is given permission to use the private and protected member names from the class.
A class specifies its friends, if any, by way of friend declarations.
Such declarations give special access rights to the friends, but they do not make the nominated friends members of the befriending class.
[Example
:
The following example illustrates the differences between members and friends:
class X {
  int a;
  friend void friend_set(X*, int);
public:
  void member_set(int);
};

void friend_set(X* p, int i) { p->a = i; }
void X::member_set(int i) { a = i; }

void f() {
  X obj;
  friend_set(&obj,10);
  obj.member_set(10);
}
end example
]
Declaring a class to be a friend implies that the names of private and protected members from the class granting friendship can be accessed in the base-specifiers and member declarations of the befriended class.
[Example
:
class A {
  class B { };
  friend class X;
};

struct X : A::B {               // OK: A​::​B accessible to friend
  A::B mx;                      // OK: A​::​B accessible to member of friend
  class Y {
    A::B my;                    // OK: A​::​B accessible to nested member of friend
  };
};
end example
]
[Example
:
class X {
  enum { a=100 };
  friend class Y;
};

class Y {
  int v[X::a];                  // OK, Y is a friend of X
};

class Z {
  int v[X::a];                  // error: X​::​a is private
};
end example
]
A class shall not be defined in a friend declaration.
[Example
:
class A {
  friend class B { };           // error: cannot define class in friend declaration
};
end example
]
A friend declaration that does not declare a function shall have one of the following forms:
friend elaborated-type-specifier ;
friend simple-type-specifier ;
friend typename-specifier ;
[Note
:
A friend declaration may be the declaration in a template-declaration ([temp], [temp.friend]).
end note
]
If the type specifier in a friend declaration designates a (possibly cv-qualified) class type, that class is declared as a friend; otherwise, the friend declaration is ignored.
[Example
:
class C;
typedef C Ct;

class X1 {
  friend C;                     // OK: class C is a friend
};

class X2 {
  friend Ct;                    // OK: class C is a friend
  friend D;                     // error: no type-name D in scope
  friend class D;               // OK: elaborated-type-specifier declares new class
};

template <typename T> class R {
  friend T;
};

R<C> rc;                        // class C is a friend of R<C>
R<int> Ri;                      // OK: "friend int;" is ignored
end example
]
A function first declared in a friend declaration has the linkage of the namespace of which it is a member ([basic.link]).
Otherwise, the function retains its previous linkage ([dcl.stc]).
When a friend declaration refers to an overloaded name or operator, only the function specified by the parameter types becomes a friend.
A member function of a class X can be a friend of a class Y.
[Example
:
class Y {
  friend char* X::foo(int);
  friend X::X(char);            // constructors can be friends
  friend X::~X();               // destructors can be friends
};
end example
]
A function can be defined in a friend declaration of a class if and only if the class is a non-local class ([class.local]), the function name is unqualified, and the function has namespace scope.
[Example
:
class M {
  friend void f() { }           // definition of global f, a friend of M,
                                // not the definition of a member function
};
end example
]
Such a function is implicitly an inline function ([dcl.inline]).
A friend function defined in a class is in the (lexical) scope of the class in which it is defined.
A friend function defined outside the class is not ([basic.lookup.unqual]).
No storage-class-specifier shall appear in the decl-specifier-seq of a friend declaration.
A name nominated by a friend declaration shall be accessible in the scope of the class containing the friend declaration.
The meaning of the friend declaration is the same whether the friend declaration appears in the private, protected, or public ([class.mem]) portion of the class member-specification.
Friendship is neither inherited nor transitive.
[Example
:
class A {
  friend class B;
  int a;
};

class B {
  friend class C;
};

class C  {
  void f(A* p) {
    p->a++;         // error: C is not a friend of A despite being a friend of a friend
  }
};

class D : public B  {
  void f(A* p) {
    p->a++;         // error: D is not a friend of A despite being derived from a friend
  }
};
end example
]
If a friend declaration appears in a local class ([class.local]) and the name specified is an unqualified name, a prior declaration is looked up without considering scopes that are outside the innermost enclosing non-class scope.
For a friend function declaration, if there is no prior declaration, the program is ill-formed.
For a friend class declaration, if there is no prior declaration, the class that is specified belongs to the innermost enclosing non-class scope, but if it is subsequently referenced, its name is not found by name lookup until a matching declaration is provided in the innermost enclosing non-class scope.
[Example
:
class X;
void a();
void f() {
  class Y;
  extern void b();
  class A {
  friend class X;   // OK, but X is a local class, not ​::​X
  friend class Y;   // OK
  friend class Z;   // OK, introduces local class Z
  friend void a();  // error, ​::​a is not considered
  friend void b();  // OK
  friend void c();  // error
  };
  X* px;            // OK, but ​::​X is found
  Z* pz;            // error, no Z is found
}
end example
]

10.8.4 Protected member access [class.protected]

An additional access check beyond those described earlier in [class.access] is applied when a non-static data member or non-static member function is a protected member of its naming class ([class.access.base]).118
As described earlier, access to a protected member is granted because the reference occurs in a friend or member of some class C.
If the access is to form a pointer to member ([expr.unary.op]), the nested-name-specifier shall denote C or a class derived from C.
All other accesses involve a (possibly implicit) object expression ([expr.ref]).
In this case, the class of the object expression shall be C or a class derived from C.
[Example
:
class B {
protected:
  int i;
  static int j;
};

class D1 : public B {
};

class D2 : public B {
  friend void fr(B*,D1*,D2*);
  void mem(B*,D1*);
};

void fr(B* pb, D1* p1, D2* p2) {
  pb->i = 1;                    // ill-formed
  p1->i = 2;                    // ill-formed
  p2->i = 3;                    // OK (access through a D2)
  p2->B::i = 4;                 // OK (access through a D2, even though naming class is B)
  int B::* pmi_B = &B::i;       // ill-formed
  int B::* pmi_B2 = &D2::i;     // OK (type of &D2​::​i is int B​::​*)
  B::j = 5;                     // ill-formed (not a friend of naming class B)
  D2::j = 6;                    // OK (because refers to static member)
}

void D2::mem(B* pb, D1* p1) {
  pb->i = 1;                    // ill-formed
  p1->i = 2;                    // ill-formed
  i = 3;                        // OK (access through this)
  B::i = 4;                     // OK (access through this, qualification ignored)
  int B::* pmi_B = &B::i;       // ill-formed
  int B::* pmi_B2 = &D2::i;     // OK
  j = 5;                        // OK (because j refers to static member)
  B::j = 6;                     // OK (because B​::​j refers to static member)
}

void g(B* pb, D1* p1, D2* p2) {
  pb->i = 1;                    // ill-formed
  p1->i = 2;                    // ill-formed
  p2->i = 3;                    // ill-formed
}
end example
]
This additional check does not apply to other members, e.g., static data members or enumerator member constants.

10.8.5 Access to virtual functions [class.access.virt]

The access rules ([class.access]) for a virtual function are determined by its declaration and are not affected by the rules for a function that later overrides it.
[Example
:
class B {
public:
  virtual int f();
};

class D : public B {
private:
  int f();
};

void f() {
  D d;
  B* pb = &d;
  D* pd = &d;

  pb->f();                      // OK: B​::​f() is public, D​::​f() is invoked
  pd->f();                      // error: D​::​f() is private
}
end example
]
Access is checked at the call point using the type of the expression used to denote the object for which the member function is called (B* in the example above).
The access of the member function in the class in which it was defined (D in the example above) is in general not known.

10.8.6 Multiple access [class.paths]

If a name can be reached by several paths through a multiple inheritance graph, the access is that of the path that gives most access.
[Example
:
class W { public: void f(); };
class A : private virtual W { };
class B : public virtual W { };
class C : public A, public B {
  void f() { W::f(); }          // OK
};
Since W​::​f() is available to C​::​f() along the public path through B, access is allowed.
end example
]

10.8.7 Nested classes [class.access.nest]

A nested class is a member and as such has the same access rights as any other member.
The members of an enclosing class have no special access to members of a nested class; the usual access rules ([class.access]) shall be obeyed.
[Example
:
class E {
  int x;
  class B { };

  class I {
    B b;                        // OK: E​::​I can access E​::​B
    int y;
    void f(E* p, int i) {
      p->x = i;                 // OK: E​::​I can access E​::​x
    }
  };

  int g(I* p) {
    return p->y;                // error: I​::​y is private
  }
};
end example
]

10.9 Initialization [class.init]

When no initializer is specified for an object of (possibly cv-qualified) class type (or array thereof), or the initializer has the form (), the object is initialized as specified in [dcl.init].
An object of class type (or array thereof) can be explicitly initialized; see [class.expl.init] and [class.base.init].
When an array of class objects is initialized (either explicitly or implicitly) and the elements are initialized by constructor, 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.
end note
]

10.9.1 Explicit initialization [class.expl.init]

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
:
struct complex {
  complex();
  complex(double);
  complex(double,double);
};

complex sqrt(complex,complex);

complex a(1);                   // initialized by calling complex(double) with argument 1
complex b = a;                  // initialized as a copy of a
complex c = complex(1,2);       // initialized by calling complex(double,double) with arguments 1 and 2
complex d = sqrt(b,c);          // initialized by calling sqrt(complex,complex) with d as its result object
complex e;                      // initialized by calling complex()
complex f = 3;                  // initialized by calling complex(double) with argument 3
complex g = { 1, 2 };           // initialized by calling complex(double, double) with arguments 1 and 2
end example
]
[Note
:
Overloading of the assignment operator ([over.ass]) has no effect on initialization.
end note
]
An object of class type can also be initialized by a braced-init-list.
List-initialization semantics apply; see [dcl.init] and [dcl.init.list].
[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 initialization of v[1], complex​::​complex() is called for the initialization v[2], v[4], and v[5].
For another example,
struct X {
  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.
end example
]
[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].
end note
]
[Note
:
If T is a class type with no default constructor, any declaration of an object of type T (or array thereof) is ill-formed if no initializer is explicitly specified (see [class.init] and [dcl.init]).
end note
]
[Note
:
The order in which objects with static or thread storage duration are initialized is described in [basic.start.dynamic] and [stmt.dcl].
end note
]

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

In the definition of a constructor for a class, initializers for direct and virtual base class subobjects and non-static 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-list , mem-initializer ...
mem-initializer:
	mem-initializer-id ( expression-list )
	mem-initializer-id braced-init-list
mem-initializer-id:
	class-or-decltype
	identifier
In a mem-initializer-id an initial unqualified identifier is looked up in the scope of the constructor's class and, if not found in that scope, it is looked up in the scope containing the constructor's definition.
[Note
:
If the constructor's class contains a member with the same name as a direct or virtual base class of the class, a mem-initializer-id naming the member or base class and composed of a single identifier refers to the class member.
A mem-initializer-id for the hidden base class may be specified using a qualified name.
end note
]
Unless the mem-initializer-id names the constructor's class, a non-static 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 class-or-decltype 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 designates 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 a variant member of the constructor's class.
If a ctor-initializer specifies more than one mem-initializer for the same member or for the same base class, the ctor-initializer is ill-formed.
A mem-initializer-list can delegate to another constructor of the constructor's class using any class-or-decltype that denotes the constructor's class itself.
If a mem-initializer-id designates the constructor's class, it shall be the only mem-initializer; the constructor is a delegating constructor, and the constructor selected by the mem-initializer is the target constructor.
The target constructor is selected by overload resolution.
Once the target constructor returns, the body of the delegating constructor is executed.
If a constructor delegates to itself directly or indirectly, the program is ill-formed, no diagnostic required.
[Example
:
struct C {
  C( int ) { }                  // #1: non-delegating constructor
  C(): C(42) { }                // #2: delegates to #1
  C( char c ) : C(42.0) { }     // #3: ill-formed due to recursion with #4
  C( double d ) : C('a') { }    // #4: ill-formed due to recursion with #3
};
end example
]
The expression-list or braced-init-list in a mem-initializer is used to initialize the designated subobject (or, in the case of a delegating constructor, the complete class object) according to the initialization rules of [dcl.init] for direct-initialization.
[Example
:
struct B1 { B1(int); /* ... */ };
struct B2 { B2(int); /* ... */ };
struct D : B1, B2 {
  D(int);
  B1 b;
  const int c;
};

D::D(int a) : B2(a+1), B1(a+2), c(a+3), b(a+4) { /* ... */ }
D d(10);
end example
]
[Note
:
The initialization performed by each mem-initializer constitutes a full-expression ([intro.execution]).
Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization.
end note
]
A mem-initializer where the mem-initializer-id denotes a virtual base class is ignored during execution of a constructor of any class that is not the most derived class.
A temporary expression bound to a reference member in a mem-initializer is ill-formed.
[Example
:
struct A {
  A() : v(42) { }   // error
  const int& v;
};
end example
]
In a non-delegating constructor, if a given potentially constructed subobject is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then
  • if the entity is a non-static data member that has a default member initializer ([class.mem]) and either
    the entity is initialized from its default member initializer as specified in [dcl.init];
  • otherwise, if the entity is an anonymous union or a variant member ([class.union.anon]), no initialization is performed;
  • otherwise, the entity is default-initialized ([dcl.init]).
[Note
:
An abstract class ([class.abstract]) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted.
end note
]
An attempt to initialize more than one non-static data member of a union renders the program ill-formed.
[Note
:
After the call to a constructor for class X for an object with automatic or dynamic storage duration has completed, if the constructor was not invoked as part of value-initialization and a member of X is neither initialized nor given a value during execution of the compound-statement of the body of the constructor, the member has an indeterminate value.
end note
]
[Example
:
struct A {
  A();
};

struct B {
  B(int);
};

struct C {
  C() { }               // initializes members as follows:
  A a;                  // OK: calls A​::​A()
  const B b;            // error: B has no default constructor
  int i;                // OK: i has indeterminate value
  int j = 5;            // OK: j has the value 5
};
end example
]
If a given non-static data member has both a default member initializer and a mem-initializer, the initialization specified by the mem-initializer is performed, and the non-static data member's default member initializer is ignored.
[Example
:
Given
struct A {
  int i = /* some integer expression with side effects */ ;
  A(int arg) : i(arg) { }
  // ...
};
the A(int) constructor will simply initialize i to the value of arg, and the side effects in i's default member initializer will not take place.
end example
]
A temporary expression bound to a reference member from a default member initializer is ill-formed.
[Example
:
struct A {
  A() = default;        // OK
  A(int v) : v(v) { }   // OK
  const int& v = 42;    // OK
};
A a1;                   // error: ill-formed binding of temporary to reference
A a2(1);                // OK, unfortunately
end example
]
In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked ([class.dtor]).
[Note
:
This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown ([except.ctor]).
end note
]
In a non-delegating constructor, initialization proceeds in the following order:
  • First, and only for the constructor of the most derived class ([intro.object]), virtual base classes are 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 classes in the derived class base-specifier-list.
  • Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).
  • Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).
  • Finally, the compound-statement of the constructor body is executed.
[Note
:
The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization.
end note
]
[Example
:
struct V {
  V();
  V(int);
};

struct A : virtual V {
  A();
  A(int);
};

struct B : virtual V {
  B();
  B(int);
};

struct C : A, B, virtual V {
  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
]
Names in the expression-list or braced-init-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.
end example
]
[Note
:
Because the mem-initializer are evaluated in the scope of the constructor, the this pointer can be used in the expression-list of a mem-initializer to refer to the object being initialized.
end note
]
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 function called directly or indirectly from a ctor-initializer) before all the mem-initializers for base classes have completed, the program has undefined behavior.
[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
]
[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.
end note
]
A mem-initializer followed by an ellipsis is a pack expansion ([temp.variadic]) that initializes the base classes specified by a pack expansion in the base-specifier-list for the class.
[Example
:
template<class... Mixins>
class X : public Mixins... {
public:
  X(const Mixins&... mixins) : Mixins(mixins)... { }
};
end example
]

10.9.3 Initialization by inherited constructor [class.inhctor.init]

When a constructor for type B is invoked to initialize an object of a different type D (that is, when the constructor was inherited ([namespace.udecl])), initialization proceeds as if a defaulted default constructor were used to initialize the D object and each base class subobject from which the constructor was inherited, except that the B subobject is initialized by the invocation of the inherited constructor.
The complete initialization is considered to be a single function call; in particular, the initialization of the inherited constructor's parameters is sequenced before the initialization of any part of the D object.
[Example
:
struct B1 {
  B1(int, ...) { }
};

struct B2 {
  B2(double) { }
};

int get();

struct D1 : B1 {
  using B1::B1;     // inherits B1(int, ...)
  int x;
  int y = get();
};

void test() {
  D1 d(2, 3, 4);    // OK: B1 is initialized by calling B1(2, 3, 4),
                    // then d.x is default-initialized (no initialization is performed),
                    // then d.y is initialized by calling get()
  D1 e;             // error: D1 has a deleted default constructor
}

struct D2 : B2 {
  using B2::B2;
  B1 b;
};

D2 f(1.0);          // error: B1 has a deleted default constructor

struct W { W(int); };
struct X : virtual W { using W::W; X() = delete; };
struct Y : X { using X::X; };
struct Z : Y, virtual W { using Y::Y; };
Z z(0);             // OK: initialization of Y does not invoke default constructor of X

template<class T> struct Log : T {
  using T::T;       // inherits all constructors from class T
  ~Log() { std::clog << "Destroying wrapper" << std::endl; }
};
Class template Log wraps any class and forwards all of its constructors, while writing a message to the standard log whenever an object of class Log is destroyed.
end example
]
If the constructor was inherited from multiple base class subobjects of type B, the program is ill-formed.
[Example
:
struct A { A(int); };
struct B : A { using A::A; };

struct C1 : B { using B::B; };
struct C2 : B { using B::B; };

struct D1 : C1, C2 {
  using C1::C1;
  using C2::C2;
};

struct V1 : virtual B { using B::B; };
struct V2 : virtual B { using B::B; };

struct D2 : V1, V2 {
  using V1::V1;
  using V2::V2;
};

D1 d1(0);           // ill-formed: ambiguous
D2 d2(0);           // OK: initializes virtual B base class, which initializes the A base class
                    // then initializes the V1 and V2 base classes as if by a defaulted default constructor

struct M { M(); M(int); };
struct N : M { using M::M; };
struct O : M {};
struct P : N, O { using N::N; using O::O; };
P p(0);             // OK: use M(0) to initialize N's base class,
                    // use M() to initialize O's base class
end example
]
When an object is initialized by an inherited constructor, initialization of the object is complete when the initialization of all subobjects is complete.

10.9.4 Construction and destruction [class.cdtor]

For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior.
For an object with a non-trivial destructor, referring to any non-static member or base class of the object after the destructor finishes execution results in undefined behavior.
[Example
:
struct X { int i; };
struct Y : X { Y(); };                  // non-trivial
struct A { int a; };
struct B : public A { int j; Y y; };    // non-trivial

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 trivial 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
]
To explicitly or implicitly convert a pointer (a glvalue) 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)” would be defined: E*  C* is defined because E() has started,
                    // and C*  A* is defined because C is fully constructed

  X(this) {}        // defined: upon construction of X, C/B/D/A sublattice is fully constructed
};
end example
]
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 constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class.
If the virtual function call uses an explicit class member access ([expr.ref]) and the object expression refers to the complete object of x or one of that object's base class subobjects but not x or one of its base class subobjects, the behavior is undefined.
[Example
:
struct V {
  virtual void f();
  virtual void g();
};

struct A : virtual V {
  virtual void f();
};

struct B : virtual V {
  virtual void g();
  B(V*, A*);
};

struct D : A, B {
  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
]
The typeid operator ([expr.typeid]) can be used during construction or destruction ([class.base.init]).
When typeid is used in a constructor (including the mem-initializer or default member initializer ([class.mem]) for a non-static 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 std​::​type_­info object 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 behavior is undefined.
dynamic_­casts ([expr.dynamic.cast]) can be used during construction or destruction ([class.base.init]).
When a dynamic_­cast is used in a constructor (including the mem-initializer or default member initializer for a non-static 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 constructor 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.
[Example
:
struct V {
  virtual void f();
};

struct A : virtual V { };

struct B : virtual V {
  B(V*, A*);
};

struct D : A, B {
  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
]

10.9.5 Copy/move elision [class.copy.elision]

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects.
In such cases, the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same object.
If the first parameter of the selected constructor is an rvalue reference to the object's type, the destruction of that object occurs when the target would have been destroyed; otherwise, the destruction occurs at the later of the times when the two objects would have been destroyed without the optimization.119
This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):
  • in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function parameter or a variable introduced by the exception-declaration of a handler ([except.handle])) with the same type (ignoring cv-qualification) as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function call's return object
  • in a throw-expression, when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one), the copy/move operation from the operand to the exception object ([except.throw]) can be omitted by constructing the automatic object directly into the exception object
  • when the exception-declaration of an exception handler ([except]) declares an object of the same type (except for cv-qualification) as the exception object ([except.throw]), the copy operation can be omitted by treating the exception-declaration as an alias for the exception object if the meaning of the program will be unchanged except for the execution of constructors and destructors for the object declared by the exception-declaration.
    [Note
    :
    There cannot be a move from the exception object because it is always an lvalue.
    end note
    ]
Copy elision is required where an expression is evaluated in a context requiring a constant expression ([expr.const]) and in constant initialization ([basic.start.static]).
[Note
:
Copy elision might not be performed if the same expression is evaluated in another context.
end note
]
[Example
:
class Thing {
public:
  Thing();
  ~Thing();
  Thing(const Thing&);
};

Thing f() {
  Thing t;
  return t;
}

Thing t2 = f();

struct A {
  void *p;
  constexpr A(): p(this) {}
};

constexpr A g() {
  A a;
  return a;
}

constexpr A a;          // well-formed, a.p points to a
constexpr A b = g();    // well-formed, b.p points to b

void h() {
  A c = g();            // well-formed, c.p may point to c or to an ephemeral temporary
}
Here the criteria for elision can eliminate the copying of the local automatic object t into the result object for the function call f(), which is the global object t2.
Effectively, the construction of the local object t can be viewed as directly initializing the global object t2, and that object's destruction will occur at program exit.
Adding a move constructor to Thing has the same effect, but it is the move construction from the local automatic object to t2 that is elided.
end example
]
In the following copy-initialization contexts, a move operation might be used instead of a copy operation: overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue.
If the first overload resolution fails or was not performed, or if the type of the first parameter of the selected constructor is not an rvalue reference to the object's type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue.
[Note
:
This two-stage overload resolution must be performed regardless of whether copy elision will occur.
It determines the constructor to be called if elision is not performed, and the selected constructor must be accessible even if the call is elided.
end note
]
[Example
:
class Thing {
public:
  Thing();
  ~Thing();
  Thing(Thing&&);
private:
  Thing(const Thing&);
};

Thing f(bool b) {
  Thing t;
  if (b)
    throw t;            // OK: Thing(Thing&&) used (or elided) to throw t
  return t;             // OK: Thing(Thing&&) used (or elided) to return t
}

Thing t2 = f(false);    // OK: no extra copy/move performed, t2 constructed by call to f

struct Weird {
  Weird();
  Weird(Weird&);
};

Weird g() {
  Weird w;
  return w;             // OK: first overload resolution fails, second overload resolution selects Weird(Weird&)
}
end example
]
Because only one object is destroyed instead of two, and one copy/move constructor is not executed, there is still one object destroyed for each one constructed.

10.10 Comparisons [class.compare]

10.10.1 Defaulted comparison operator functions [class.compare.default]

A defaulted comparison operator function ([expr.spaceship], [expr.rel], [expr.eq]) for some class C shall be a non-template function declared in the member-specification of C that is
  • a non-static member of C having one parameter of type const C&, or
  • a friend of C having two parameters of type const C&.
A three-way comparison operator for a class type C is a structural comparison operator if it is defined as defaulted in the definition of C, and all three-way comparison operators it invokes are structural comparison operators.
A type T has strong structural equality if, for a glvalue x of type const T, x <=> x is a valid expression of type std​::​strong_­ordering or std​::​strong_­equality and either does not invoke a three-way comparison operator or invokes a structural comparison operator.

10.10.2 Three-way comparison [class.spaceship]

The direct base class subobjects of C, in the order of their declaration in the base-specifier-list of C, followed by the non-static data members of C, in the order of their declaration in the member-specification of C, form a list of subobjects.
In that list, any subobject of array type is recursively expanded to the sequence of its elements, in the order of increasing subscript.
Let x be an lvalue denoting the element in the expanded list of subobjects for an object x (of length n), where x is formed by a sequence of derived-to-base conversions ([over.best.ics]), class member access expressions ([expr.ref]), and array subscript expressions ([expr.sub]) applied to x.
The type of the expression x <=> x is denoted by R.
It is unspecified whether virtual base class subobjects are compared more than once.
If the declared return type of a defaulted three-way comparison operator function is auto, then the return type is deduced as the common comparison type (see below) of R, R, , R.
[Note
:
Otherwise, the program will be ill-formed if the expression x <=> x is not implicitly convertible to the declared return type for any i.
end note
]
If the return type is deduced as void, the operator function is defined as deleted.
The return value V of type R of the defaulted three-way comparison operator function with parameters x and y of the same type is determined by comparing corresponding elements x and y in the expanded lists of subobjects for x and y until the first index i where x <=> y yields a result value v where v, contextually converted to bool, yields true; V is v converted to R.
If no such index exists, V is std​::​strong_­ordering​::​equal converted to R.
The common comparison type U of a possibly-empty list of n types T, T, , T is defined as follows:
  • If any T is not a comparison category type ([cmp.categories]), U is void.
  • Otherwise, if at least one T is std​::​weak_­equality, or at least one T is std​::​strong_­equality and at least one T is std​::​partial_­ordering or std​::​weak_­ordering, U is std​::​weak_­equality ([cmp.weakeq]).
  • Otherwise, if at least one T is std​::​strong_­equality, U is std​::​strong_­equality ([cmp.strongeq]).
  • Otherwise, if at least one T is std​::​partial_­ordering, U is std​::​partial_­ordering ([cmp.partialord]).
  • Otherwise, if at least one T is std​::​weak_­ordering, U is std​::​weak_­ordering ([cmp.weakord]).
  • Otherwise, U is std​::​strong_­ordering ([cmp.strongord]).
    [Note
    :
    In particular, this is the result when n is 0.
    end note
    ]

10.10.3 Other comparison operators [class.rel.eq]

A defaulted relational ([expr.rel]) or equality ([expr.eq]) operator function for some operator @ shall have a declared return type bool.
The operator function with parameters x and y is defined as deleted if
  • overload resolution ([over.match]), as applied to x <=> y (also considering synthesized candidates with reversed order of parameters ([over.match.oper])), results in an ambiguity or a function that is deleted or inaccessible from the operator function, or
  • the operator @ cannot be applied to the return type of x <=> y or y <=> x.
Otherwise, the operator function yields x <=> y @ 0 if an operator<=> with the original order of parameters was selected, or 0 @ y <=> x otherwise.
[Example
:
struct C {
  friend std::strong_equality operator<=>(const C&, const C&);
  friend bool operator==(const C& x, const C& y) = default; // OK, returns x <=> y == 0
  bool operator<(const C&) = default;                       // OK, function is deleted
};
end example
]

10.11 Free store [class.free]

Any allocation function for a class T is a static member (even if not explicitly declared static).
[Example
:
class Arena;
struct B {
  void* operator new(std::size_t, Arena*);
};
struct D1 : B {
};

Arena*  ap;
void foo(int i) {
  new (ap) D1;      // calls B​::​operator new(std​::​size_­t, Arena*)
  new D1[i];        // calls ​::​operator new[](std​::​size_­t)
  new D1;           // ill-formed: ​::​operator new(std​::​size_­t) hidden
}
end example
]
When an object is deleted with a delete-expression, 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]).
Class-specific deallocation function lookup is a part of general deallocation function lookup ([expr.delete]) and occurs as follows.
If the delete-expression is used to deallocate a class object whose static type has a virtual destructor, the deallocation function is the one selected at the point of definition of the dynamic type's virtual destructor ([class.dtor]).120
Otherwise, if the delete-expression is used to deallocate an object of class T or array thereof, the deallocation function's name is looked up in the scope of T.
If this lookup fails to find the name, general deallocation function lookup ([expr.delete]) continues.
If the result of the lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function, the program is ill-formed.
Any 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*, std::size_t);
};

class Y {
  void operator delete(void*, std::size_t);
  void operator delete[](void*);
};
end example
]
Since 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 deallocation function actually called is looked up in the scope of the class that is the dynamic type of the object if the destructor is virtual, the effect is the same in that case.
For example,
struct B {
  virtual ~B();
  void operator delete(void*, std::size_t);
};

struct D : B {
  void operator delete(void*);
};

struct E : B {
  void log_deletion();
  void operator delete(E *p, std::destroying_delete_t) {
    p->log_deletion();
    p->~E();
    ::operator delete(p);
  }
};

void f() {
  B* bp = new D;
  delete bp;        // 1: uses D​::​operator delete(void*)
  bp = new E;
  delete bp;        // 2: uses E​::​operator delete(E*, std​::​destroying_­delete_­t)
}
Here, storage for the object of class D is deallocated by D​::​operator delete(), and the object of class E is destroyed and its storage is deallocated by E​::​operator delete(), due to the virtual destructor.
end note
]
[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*, std::size_t);
};

struct D : B {
  void operator delete[](void*, std::size_t);
};

void f(int i) {
  D* dp = new D[i];
  delete [] dp;     // uses D​::​operator delete[](void*, std​::​size_­t)
  B* bp = new D[i];
  delete[] bp;      // undefined behavior
}
end note
]
Access 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 private, the delete expression would have been ill-formed.
end example
]
[Note
:
If a deallocation function has no explicit noexcept-specifier, it has a non-throwing exception specification ([except.spec]).
end note
]
A similar provision is not needed for the array version of operator delete because [expr.delete] requires that in this situation, the static type of the object to be deleted be the same as its dynamic type.