11 Classes [class]

11.1 Preamble [class.pre]

1
#
A class is a type.
Its name becomes a class-name ([class.name]) within its scope.
An object of a class consists of a (possibly empty) sequence of members and base class objects.
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.spec.partial]).
[Note 1: 
An unnamed class thus can't be final.
— end note]
Otherwise, the class-name is an identifier; it is not looked up, and the class-specifier introduces it.
2
#
The component name of the class-name is also bound in the scope of the class (template) 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.
3
#
If a class-head-name contains a nested-name-specifier, the class-specifier shall not inhabit a class scope.
If its class-name is an identifier, the class-specifier shall correspond to one or more declarations nominable in the class, class template, or namespace to which the nested-name-specifier refers; they shall all have the same target scope, and the target scope of the class-specifier is that scope.
[Example 1: namespace N { template<class> struct A { struct B; }; } using N::A; template<class T> struct A<T>::B {}; // OK template<> struct A<void> {}; // OK — end example]
4
#
[Note 2: 
The class-key determines whether the class is a union ([class.union]) and whether access is public or private by default ([class.access]).
A union holds the value of at most one data member at a time.
— end note]
5
#
If a class is marked with the class-virt-specifier final and it appears as a class-or-decltype in a base-clause ([class.derived]), 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 2: 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]
6
#
[Note 3: 
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]
7
#
[Note 4: 
Class objects can be assigned ([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]

11.2 Properties of classes [class.prop]

1
#
A trivially copyable class is a class:
  • (1.1)
    that has at least one eligible copy constructor, move constructor, copy assignment operator, or move assignment operator ([special], [class.copy.ctor], [class.copy.assign]),
  • (1.2)
    where each eligible copy constructor, move constructor, copy assignment operator, and move assignment operator is trivial, and
  • (1.3)
    that has a trivial, non-deleted destructor ([class.dtor]).
2
#
A class S is a standard-layout class if it:
  • (2.1)
    has no non-static data members of type non-standard-layout class (or array of such types) or reference,
  • (2.2)
    has no virtual functions and no virtual base classes,
  • (2.3)
    has the same access control for all non-static data members,
  • (2.4)
    has no non-standard-layout base classes,
  • (2.5)
    has at most one base class subobject of any given type,
  • (2.6)
    has all non-static data members and bit-fields in the class and its base classes first declared in the same class, and
  • (2.7)
    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.88
    [Note 1: 
    M(X) is the set of the types of all non-base-class subobjects that can be at a zero offset in X.
    — end note]
    • (2.7.1)
      If X is a non-union class type with no non-static data members, the set M(X) is empty.
    • (2.7.2)
      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 .
    • (2.7.3)
      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.
    • (2.7.4)
      If X is an array type with element type , the set M(X) consists of and the elements of .
    • (2.7.5)
      If X is a non-class, non-array type, the set M(X) is empty.
3
#
[Example 1: 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]
4
#
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.
5
#
[Note 2: 
Standard-layout classes are useful for communicating with code written in other programming languages.
Their layout is specified in [class.mem].
— end note]
6
#
[Example 2: struct N { // neither trivially copyable nor standard-layout int i; int j; virtual ~N(); }; struct T { // trivially copyable but not standard-layout int i; private: int j; }; struct SL { // standard-layout but not trivially copyable int i; int j; ~SL(); }; struct POD { // both trivially copyable and standard-layout int i; int j; }; — end example]
7
#
[Note 3: 
Aggregates of class type are described in [dcl.init.aggr].
— end note]
8
#
A class S is an implicit-lifetime class if
  • (8.1)
    it is an aggregate whose destructor is not user-provided or
  • (8.2)
    it has at least one trivial eligible constructor and a trivial, non-deleted destructor.
88)88)
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]).

11.3 Class names [class.name]

1
#
A class definition introduces a new type.
[Example 1: 
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 overloads ([over]) named 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]
2
#
[Note 1: 
It can be necessary to use an elaborated-type-specifier to refer to a class that belongs to a scope in which its name is also bound to a variable, function, or enumerator ([basic.lookup.elab]).
[Example 2: struct stat { // ... }; stat gstat; // use plain stat to define variable int stat(struct stat*); // stat now also names a function void f() { struct stat* ps; // struct prefix needed to name struct stat stat(ps); // call stat function } — end example]
An elaborated-type-specifier can also be used to declare an identifier as a class-name.
[Example 3: 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]
Such declarations allow definition of classes that refer to each other.
[Example 4: 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]
3
#
[Note 2: 
An elaborated-type-specifier ([dcl.type.elab]) can also be used as a type-specifier as part of a declaration.
It differs from a class declaration in that it can refer to an existing class of the given name.
— end note]
[Example 5: struct s { int a; }; void g(int s) { struct s* p = new struct s; // global s p->a = s; // parameter s } — end example]
4
#
[Note 3: 
The declaration of a class name takes effect immediately after the identifier is seen in the class definition or elaborated-type-specifier.
[Example 6: 
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 example]
— end note]
5
#
A simple-template-id is only a class-name if its template-name names a class template.

11.4 Class members [class.mem]

11.4.1 General [class.mem.general]

virt-specifier:
override
final
1
#
In the absence of a virt-specifier-seq, the token sequence = 0 is treated as a pure-specifier if the type of the declarator-id ([dcl.meaning.general]) is a function type, and is otherwise treated as a brace-or-equal-initializer.
[Note 1: 
If the member declaration acquires a function type through template instantiation, the program is ill-formed; see [temp.spec.general].
— end note]
2
#
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 members ([class.union.anon]) and direct members thereof.
Members of a class are data members, member functions ([class.mfct]), nested types, enumerators, and member templates ([temp.mem]) and specializations thereof.
[Note 2: 
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]
3
#
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.
4
#
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 ([dcl.enum]) declared in the class and arbitrary types declared as members by use of a typedef declaration ([dcl.typedef]) or alias-declaration.
The enumerators of an unscoped enumeration defined in the class are members of the class.
5
#
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 3: 
A non-static data member of non-reference type is a member subobject of a class object.
— end note]
6
#
A member shall not be declared twice in the member-specification, except that
[Note 4: 
A single name can denote several member functions provided their types are sufficiently different ([basic.scope.scope]).
— end note]
7
#
A redeclaration of a class member outside its class definition shall be a definition, an explicit specialization, or an explicit instantiation ([temp.expl.spec], [temp.explicit]).
The member shall not be a non-static data member.
8
#
A complete-class context of a class (template) is a within the member-specification of the class or class template.
[Note 5: 
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]
9
#
A class C is complete at a program point P if the definition of C is reachable from P ([module.reach]) or if P is in a complete-class context of C.
Otherwise, C is incomplete at P.
10
#
If a member-declaration matches the syntactic requirements of friend-type-declaration, it is a friend-type-declaration.
11
#
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 1: struct S { using T = void(); T * p = 0; // OK, brace-or-equal-initializer virtual T f = 0; // OK, pure-specifier }; — end example]
12
#
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 2: 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]
13
#
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.
An immediate invocation ([expr.const]) that is a potentially-evaluated subexpression ([intro.execution]) of a default member initializer is neither evaluated nor checked for whether it is a constant expression at the point where the subexpression appears.
14
#
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.
15
#
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.
16
#
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.
17
#
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]).
18
#
The type of a non-static data member shall not be an incomplete type ([basic.types.general]), an abstract class type ([class.abstract]), or a (possibly multidimensional) array thereof.
[Note 6: 
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]
19
#
[Note 7: 
See [expr.prim.id] for restrictions on the use of non-static data members and non-static member functions.
— end note]
20
#
[Note 8: 
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]
21
#
[Example 3: 
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]
22
#
[Note 9: 
Non-variant non-static data members of non-zero size ([intro.object]) are allocated so that later members have higher addresses within a class object ([expr.rel]).
Implementation alignment requirements can cause two adjacent members not to be allocated immediately after each other; so can requirements for space for managing virtual functions ([class.virtual]) and virtual base classes ([class.mi]).
— end note]
23
#
If T is the name of a class, then each of the following shall have a name different from T:
  • (23.1)
    every static data member of class T;
  • (23.2)
    every member function of class T;
    [Note 10: 
    This restriction does not apply to constructors, which do not have names ([class.ctor]).
    — end note]
  • (23.3)
    every member of class T that is itself a type;
  • (23.4)
    every member template of class T;
  • (23.5)
    every enumerator of every member of class T that is an unscoped enumeration type; and
  • (23.6)
    every member of every anonymous union that is a member of class T.
24
#
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.
25
#
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
[Example 4: 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]
26
#
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]).
27
#
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 ([basic.types.general]).
28
#
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 5: 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 11: 
Reading a volatile object through a glvalue of non-volatile type has undefined behavior ([dcl.type.cv]).
— end note]
29
#
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 12: 
There can 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 13: 
The object and its first subobject are pointer-interconvertible ([basic.compound], [expr.static.cast]).
— end note]

11.4.2 Member functions [class.mfct]

1
#
If a member function is attached to the global module and is defined ([dcl.fct.def]) in its class definition, it is inline ([dcl.inline]).
[Note 1: 
A member function is also inline if it is declared inline, constexpr, or consteval.
— end note]
2
#
[Example 1: struct X { typedef int T; static T count; void f(T); }; void X::f(T t = count) { }
The definition of the member function f of class X inhabits the global scope; the notation X​::​f indicates 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]
3
#
Member functions of a local class shall be defined inline in their class definition, if they are defined at all.
4
#
[Note 2: 
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] and [temp.arg].
[Example 2: 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; — end example]
— end note]

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

1
#
A non-static member function may be called for an object of its class type, or for an object of a class derived ([class.derived]) from its class type, using the class member access syntax ([expr.ref], [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 its class or a class derived from its class, or a member thereof, as described below.
2
#
[Note 1: 
An implicit object member function can be declared with cv-qualifiers, which affect the type of the this pointer ([expr.prim.this]), and/or a ref-qualifier ([dcl.fct]); both affect overload resolution ([over.match.funcs]).
— end note]
3
#
An implicit object member function may be declared virtual ([class.virtual]) or pure virtual ([class.abstract]).

11.4.4 Special member functions [special]

1
#
Default constructors ([class.default.ctor]), copy constructors, move constructors ([class.copy.ctor]), copy assignment operators, move assignment operators ([class.copy.assign]), and prospective destructors ([class.dtor]) are special member functions.
[Note 1: 
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 as needed ([dcl.fct.def.default]).
— 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.
2
#
Programs may explicitly refer to implicitly-declared special member functions.
[Example 1: 
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]
3
#
[Note 2: 
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]
4
#
Special member functions obey the usual access rules ([class.access]).
[Example 2: 
Declaring a constructor protected ensures that only derived classes and friends can create objects using it.
— end example]
5
#
Two special member functions are of the same kind if
  • (5.1)
    they are both default constructors,
  • (5.2)
    they are both copy or move constructors with the same first parameter type, or
  • (5.3)
    they are both copy or move assignment operators with the same first parameter type and the same cv-qualifiers and ref-qualifier, if any.
6
#
An eligible special member function is a special member function for which:
  • (6.1)
    the function is not deleted,
  • (6.2)
    the associated constraints ([temp.constr]), if any, are satisfied, and
  • (6.3)
    no special member function of the same kind whose associated constraints, if any, are satisfied is more constrained ([temp.constr.order]).
7
#
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.

11.4.5 Constructors [class.ctor]

11.4.5.1 General [class.ctor.general]

1
#
A declarator declares a constructor if it 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:
Constructors do not have names.
In a constructor declaration, each decl-specifier in the optional decl-specifier-seq shall be friend, inline, constexpr, consteval, or an explicit-specifier.
[Example 1: struct S { S(); // declares the constructor }; S::S() { } // defines the constructor — end example]
2
#
A constructor is used to initialize objects of its class type.
[Note 1: 
Because constructors do not have names, they are never found during unqualified 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.
The syntax looks like an explicit call of the constructor.
— end note]
[Example 2: complex zz = complex(1,2.3); cprint( complex(7.8,1.2) ); — end example]
[Note 2: 
For initialization of objects of class type see [class.init].
— end note]
3
#
An object created in this way is unnamed.
[Note 3: 
[class.temporary] describes the lifetime of temporary objects.
— end note]
[Note 4: 
Explicit constructor calls do not yield lvalues, see [basic.lval].
— end note]
4
#
[Note 5: 
Some language constructs have special semantics when used during construction; see [class.base.init] and [class.cdtor].
— end note]
5
#
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.
6
#
The address of a constructor shall not be taken.
[Note 6: 
A return statement in the body of a constructor cannot specify a return value ([stmt.return]).
— end note]
7
#
A constructor shall not be a coroutine.
8
#
A constructor shall not have an explicit object parameter ([dcl.fct]).

11.4.5.2 Default constructors [class.default.ctor]

1
#
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 or constructor template 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.
2
#
A defaulted default constructor for class X is defined as deleted if
  • (2.1)
    any non-static data member with no default member initializer ([class.mem]) is of reference type,
  • (2.2)
    any non-variant non-static data member of const-qualified type (or possibly multidimensional array thereof) with no brace-or-equal-initializer is not const-default-constructible ([dcl.init]),
  • (2.3)
    X is a union and all of its variant members are of const-qualified type (or possibly multidimensional array thereof),
  • (2.4)
    X is a non-union class and all members of any anonymous union member are of const-qualified type (or possibly multidimensional array thereof),
  • (2.5)
    any potentially constructed subobject, except for a non-static data member with a brace-or-equal-initializer or a variant member of a union where another non-static data member has a brace-or-equal-initializer, has class type M (or possibly multidimensional array thereof) and overload resolution ([over.match]) as applied to find M's corresponding constructor either does not result in a usable candidate ([over.match.general]) or, in the case of a variant member, selects a non-trivial function, or
  • (2.6)
    any potentially constructed subobject has class type M (or possibly multidimensional array thereof) and M has a destructor that is deleted or inaccessible from the defaulted default constructor.
3
#
A default constructor is trivial if it is not user-provided and if
  • (3.1)
    its class has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • (3.2)
    no non-static data member of its class has a default member initializer ([class.mem]), and
  • (3.3)
    all the direct base classes of its class have trivial default constructors, and
  • (3.4)
    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.
4
#
An implicitly-defined ([dcl.fct.def.default]) 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 ([class.base.init]) 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 be constexpr-suitable ([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 are implicitly defined.
[Note 1: 
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]
5
#
[Note 2: 
A default constructor is implicitly invoked to initialize a class object when no initializer is specified ([dcl.init.general]).
Such a default constructor needs to be accessible ([class.access]).
— end note]
6
#
[Note 3: 
[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]

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

1
#
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 1: 
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]
2
#
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 2: 
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]
3
#
[Note 1: 
All forms of copy/move constructor can be declared for a class.
[Example 3: struct X { X(const X&); X(X&); // OK X(X&&); X(const X&&); // OK, but possibly not sensible }; — end example]
— end note]
4
#
[Note 2: 
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 cv X.
[Example 4: 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]
5
#
A declaration of a constructor for a class X is ill-formed if its first parameter is of type cv 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 5: 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]
6
#
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 defaulted ([dcl.fct.def]).
The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor ([depr.impldec]).
7
#
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&.89
Otherwise, the implicitly-declared copy constructor will have the form X::X(X&)
8
#
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
  • (8.1)
    X does not have a user-declared copy constructor,
  • (8.2)
    X does not have a user-declared copy assignment operator,
  • (8.3)
    X does not have a user-declared move assignment operator, and
  • (8.4)
    X does not have a user-declared destructor.
[Note 3: 
When the move constructor is not implicitly declared or explicitly supplied, expressions that otherwise would have invoked the move constructor might instead invoke a copy constructor.
— end note]
9
#
The implicitly-declared move constructor for class X will have the form X::X(X&&)
10
#
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:
  • (10.1)
    a potentially constructed subobject of type M (or possibly multidimensional array thereof) for which overload resolution ([over.match]), as applied to find M's corresponding constructor, either does not result in a usable candidate ([over.match.general]) or, in the case of a variant member, selects a non-trivial function,
  • (10.2)
    any potentially constructed subobject of class type M (or possibly multidimensional array thereof) where M has a destructor that is deleted or inaccessible from the defaulted constructor, or,
  • (10.3)
    for the copy constructor, a non-static data member of rvalue reference type.
[Note 4: 
A defaulted move constructor that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).
Such a constructor would otherwise interfere with initialization from an rvalue which can use the copy constructor instead.
— end note]
11
#
A copy/move constructor for class X is trivial if it is not user-provided and if
  • (11.1)
    class X has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • (11.2)
    the constructor selected to copy/move each direct base class subobject is trivial, and
  • (11.3)
    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.
12
#
[Note 5: 
The copy/move constructor is implicitly defined even if the implementation elided its odr-use ([basic.def.odr], [class.temporary]).
— end note]
If an implicitly-defined ([dcl.fct.def.default]) constructor would be constexpr-suitable ([dcl.constexpr]), the implicitly-defined constructor is constexpr.
13
#
Before the defaulted copy/move constructor for a class is implicitly defined, all non-user-provided copy/move constructors for its potentially constructed subobjects are implicitly defined.
[Note 6: 
An implicitly-declared copy/move constructor has an implied exception specification ([except.spec]).
— end note]
14
#
The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its bases and members.
[Note 7: 
Default member initializers of non-static data members are ignored.
— 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:
  • (14.1)
    if the member is an array, each element is direct-initialized with the corresponding subobject of x;
  • (14.2)
    if a member m has rvalue reference type T&&, it is direct-initialized with static_cast<T&&>(x.m);
  • (14.3)
    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]).
15
#
The implicitly-defined copy/move constructor for a union X copies the object representation ([basic.types.general]) of X.
For each object nested within ([intro.object]) the object that is the source of the copy, a corresponding object o nested within the destination is identified (if the object is a subobject) or created (otherwise), and the lifetime of o begins before the copy is performed.
89)89)
This implies that the reference parameter of the implicitly-declared copy constructor cannot bind to a volatile lvalue; see [diff.class].

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

1
#
A user-declared copy assignment operator X​::​operator= is a non-static non-template member function of class X with exactly one non-object parameter of type X, X&, const X&, volatile X&, or const volatile X&.90
[Note 1: 
More than one form of copy assignment operator can be declared for a class.
— end note]
[Note 2: 
If a class X only has a copy assignment operator with a non-object parameter of type X&, an expression of type const X cannot be assigned to an object of type X.
[Example 1: 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]
2
#
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 defaulted ([dcl.fct.def]).
The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor ([depr.impldec]).
The implicitly-declared copy assignment operator for a class X will have the form X& X::operator=(const X&) if
  • (2.1)
    each direct base class B of X has a copy assignment operator whose non-object parameter is of type const B&, const volatile B&, or B, and
  • (2.2)
    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 non-object parameter is of type const M&, const volatile M&, or M.91
Otherwise, the implicitly-declared copy assignment operator will have the form X& X::operator=(X&)
3
#
A user-declared move assignment operator X​::​operator= is a non-static non-template member function of class X with exactly one non-object parameter of type X&&, const X&&, volatile X&&, or const volatile X&&.
[Note 3: 
More than one form of move assignment operator can be declared for a class.
— end note]
4
#
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
  • (4.1)
    X does not have a user-declared copy constructor,
  • (4.2)
    X does not have a user-declared move constructor,
  • (4.3)
    X does not have a user-declared copy assignment operator, and
  • (4.4)
    X does not have a user-declared destructor.
[Example 2: 
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]
5
#
The implicitly-declared move assignment operator for a class X will have the form X& X::operator=(X&&)
6
#
The implicitly-declared copy/move assignment operator for class X has the return type X&.
An implicitly-declared copy/move assignment operator is an inline public member of its class.
7
#
A defaulted copy/move assignment operator for class X is defined as deleted if X has:
  • (7.1)
    a non-static data member of const non-class type (or possibly multidimensional array thereof), or
  • (7.2)
    a non-static data member of reference type, or
  • (7.3)
    a direct non-static data member of class type M (or possibly multidimensional 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, either does not result in a usable candidate ([over.match.general]) or, in the case of a variant member, selects a non-trivial function.
[Note 4: 
A defaulted move assignment operator that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).
— end note]
8
#
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]).
[Note 5: 
A using-declaration in a derived class C that names an assignment operator from a base class never suppresses the implicit declaration of an assignment operator of C, even if the base class assignment operator would be a copy or move assignment operator if declared as a member of C.
— end note]
9
#
A copy/move assignment operator for class X is trivial if it is not user-provided and if
  • (9.1)
    class X has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
  • (9.2)
    the assignment operator selected to copy/move each direct base class subobject is trivial, and
  • (9.3)
    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.
10
#
An implicitly-defined ([dcl.fct.def.default]) copy/move assignment operator is constexpr.
11
#
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 are implicitly defined.
[Note 6: 
An implicitly-declared copy/move assignment operator has an implied exception specification ([except.spec]).
— end note]
12
#
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:
  • (12.1)
    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);
  • (12.2)
    if the subobject is an array, each element is assigned, in the manner appropriate to the element type;
  • (12.3)
    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 3: 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]
13
#
The implicitly-defined copy/move assignment operator for a union X copies the object representation ([basic.types.general]) of X.
If the source and destination of the assignment are not the same object, then for each object nested within ([intro.object]) the object that is the source of the copy, a corresponding object o nested within the destination is created, and the lifetime of o begins before the copy is performed.
14
#
The implicitly-defined copy/move assignment operator for a class returns the object for which the assignment operator is invoked, that is, the object assigned to.
90)90)
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.
91)91)
This implies that the reference parameter of the implicitly-declared copy assignment operator cannot bind to a volatile lvalue; see [diff.class].

11.4.7 Destructors [class.dtor]

1
#
A declaration whose declarator-id has an unqualified-id that begins with a ~ declares a prospective destructor; its declarator shall be 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:
A prospective destructor shall take no arguments ([dcl.fct]).
Each decl-specifier of the decl-specifier-seq of a prospective destructor declaration (if any) shall be friend, inline, virtual, or constexpr.
2
#
If a class has no user-declared prospective destructor, a prospective destructor is implicitly declared as defaulted ([dcl.fct.def]).
An implicitly-declared prospective destructor is an inline public member of its class.
3
#
An implicitly-declared prospective destructor for a class X will have the form ~X()
4
#
At the end of the definition of a class, overload resolution is performed among the prospective destructors declared in that class with an empty argument list to select the destructor for the class, also known as the selected destructor.
The program is ill-formed if overload resolution fails.
Destructor selection does not constitute a reference to, or odr-use ([basic.def.odr]) of, the selected destructor, and in particular, the selected destructor may be deleted ([dcl.fct.def.delete]).
5
#
The address of a destructor shall not be taken.
[Note 1: 
A return statement in the body of a destructor cannot specify a return value ([stmt.return]).
— end note]
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.
6
#
[Note 2: 
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]
7
#
A defaulted destructor for a class X is defined as deleted if
  • (7.1)
    any potentially constructed subobject has class type M (or possibly multidimensional array thereof) and M has a destructor that is deleted or is inaccessible from the defaulted destructor or, in the case of a variant member, is non-trivial,
  • (7.2)
    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.
8
#
A destructor is trivial if it is not user-provided and if
  • (8.1)
    the destructor is not virtual,
  • (8.2)
    all of the direct base classes of its class have trivial destructors, and
  • (8.3)
    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.
9
#
A defaulted destructor is a constexpr destructor if it is constexpr-suitable ([dcl.constexpr]).
10
#
Before a defaulted destructor for a class is implicitly defined, all the non-user-provided destructors for its base classes and its non-static data members are implicitly defined.
11
#
A prospective destructor can be declared virtual ([class.virtual]) and with a pure-specifier ([class.abstract]).
If the destructor of a class is virtual and any objects of that class or any derived class are created in the program, the destructor shall be defined.
12
#
[Note 3: 
Some language constructs have special semantics when used during destruction; see [class.cdtor].
— end note]
13
#
After executing the body of the destructor and destroying any objects with automatic storage duration allocated within the body, a destructor for class X calls the destructors for X's direct non-variant non-static data members other than anonymous unions, the destructors for X's non-virtual direct base classes and, if X is 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]).
[Note 4: 
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.
— end note]
Destructors for elements of an array are called in reverse order of their construction (see [class.init]).
14
#
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 ([expr.delete]) for a constructed object allocated by a new-expression ([expr.new]); the context of the invocation is the delete-expression.
[Note 5: 
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], [stmt.return], [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.
15
#
At the point of definition of a virtual destructor (including an implicit definition), 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 6: 
This assures that a deallocation function corresponding to the dynamic type of an object is available for the delete-expression ([class.free]).
— end note]
16
#
In an explicit destructor call, the destructor is specified by a ~ followed by a type-name or computed-type-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 7: 
Invoking delete on a null pointer does not call the destructor; see [expr.delete].
— end note]
[Example 1: 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 8: 
An explicit destructor call must always be written using a member access operator ([expr.ref]) or a qualified-id ([expr.prim.id.qual]); in particular, the unary-expression ~X() in a member function is not an explicit destructor call ([expr.unary.op]).
— end note]
17
#
[Note 9: 
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.
[Example 2: 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 example]
— end note]
18
#
Once a destructor is invoked for an object, the object's lifetime ends; the behavior is undefined if the destructor is invoked for an object whose lifetime has ended ([basic.life]).
[Example 3: 
If the destructor for an object with automatic storage duration 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]
19
#
[Note 10: 
The notation for explicit call of a destructor can be used for any scalar type name ([expr.prim.id.dtor]).
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]
20
#
A destructor shall not be a coroutine.

11.4.8 Conversions [class.conv]

11.4.8.1 General [class.conv.general]

1
#
Type conversions of class objects can be specified by constructors and by conversion functions.
These conversions are called user-defined conversions and are used for implicit type conversions ([conv]), for initialization ([dcl.init]), and for explicit type conversions ([expr.type.conv], [expr.cast], [expr.static.cast]).
2
#
User-defined conversions are applied only where they are unambiguous ([class.member.lookup], [class.conv.fct]).
Conversions obey the access control rules ([class.access]).
Access control is applied after ambiguity resolution ([basic.lookup]).
3
#
[Note 1: 
See [over.match] for a discussion of the use of conversions in function calls.
— end note]
4
#
At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.
[Example 1: struct X { operator int(); }; struct Y { operator X(); }; Y a; int b = a; // error: no viable conversion (a.operator X().operator int() not considered) int c = X(a); // OK, a.operator X().operator int() — end example]

11.4.8.2 Conversion by constructor [class.conv.ctor]

1
#
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 1: 
🔗
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]
2
#
[Note 1: 
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 can be an explicit constructor; such a constructor will be used to perform default-initialization or value-initialization ([dcl.init]).
[Example 2: 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]
3
#
[Note 2: 
A non-explicit copy/move constructor ([class.copy.ctor]), including one implicitly declared, is a converting constructor.
— end note]

11.4.8.3 Conversion functions [class.conv.fct]

1
#
A declaration whose declarator-id has an unqualified-id that is a conversion-function-id declares a conversion function; its declarator shall be a function declarator ([dcl.fct]) of the form
noptr-declarator parameters-and-qualifiers
where the noptr-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:
2
#
A conversion function shall have no non-object parameters and shall be a non-static member function of a class or class template X; its declared return type is the conversion-type-id and it specifies a conversion from X to the type specified by the conversion-type-id, interpreted as a type-id ([dcl.name]).
A decl-specifier in the decl-specifier-seq of a conversion function (if any) shall not be a defining-type-specifier.
3
#
[Note 1: 
A conversion function is never invoked for implicit or explicit conversions of an object to the same object type (or a reference to it), to a base class of that type (or a reference to it), or to cv void.
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.
— end note]
[Example 1: 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]
4
#
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 2: class Y { }; struct Z { explicit operator Y() const; }; void h(Z z) { Y y1(z); // OK, direct-initialization Y y2 = z; // error: no conversion function candidate for 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]
5
#
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 2: 
This prevents ambiguities between the declarator operator * and its expression counterparts.
[Example 3: &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 4: operator int [[noreturn]] (); // error: noreturn attribute applied to a type — end example]
— end note]
6
#
[Note 3: 
A conversion function in a derived class hides only conversion functions in base classes that convert to the same type.
A conversion function template with a dependent return type hides only templates in base classes that correspond to it ([class.member.lookup]); otherwise, it hides and is hidden as a non-template function.
Function overload resolution ([over.match.best]) selects the best conversion function to perform the conversion.
[Example 5: struct X { operator int(); }; struct Y : X { operator char(); }; void f(Y& a) { if (a) { // error: ambiguous between X​::​operator int() and Y​::​operator char() } } — end example]
— end note]
7
#
Conversion functions can be virtual.
8
#
A conversion function template shall not have a deduced return type ([dcl.spec.auto]).
[Example 6: struct S { operator auto() const { return 10; } // OK template<class T> operator auto() const { return 1.2; } // error: conversion function template }; — end example]

11.4.9 Static members [class.static]

11.4.9.1 General [class.static.general]

1
#
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 1: struct process { static void reschedule(); }; process& g(); void f() { process::reschedule(); // OK, no object necessary g().reschedule(); // g() is called } — end example]
2
#
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 1: 
It cannot be specified in member declarations that appear in namespace scope.
— end note]

11.4.9.2 Static member functions [class.static.mfct]

1
#
[Note 1: 
The rules described in [class.mfct] apply to static member functions.
— end note]
2
#
[Note 2: 
A static member function does not have a this pointer ([expr.prim.this]).
A static member function cannot be qualified with const, volatile, or virtual ([dcl.fct]).
— end note]

11.4.9.3 Static data members [class.static.data]

1
#
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.
2
#
A static data member shall not be mutable ([dcl.stc]).
A static data member shall not be a direct member ([class.mem]) of an unnamed ([class.pre]) or local ([class.local]) class or of a (possibly indirectly) nested class ([class.nest]) thereof.
3
#
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.
[Note 1: 
The initializer in the definition of a static data member is in the scope of its class ([basic.scope.class]).
— end note]
[Example 1: class process { static process* run_chain; static process* running; }; process* process::running = get_main(); process* process::run_chain = running;
The definition of the static data member run_chain of class process inhabits the global scope; the notation process​::​run_chain indicates 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 2: 
Once the static data member has been defined, it exists even if no objects of its class have been created.
[Example 2: 
In the example above, run_chain and running exist even if no objects of class process are created by the program.
— end example]
The initialization and destruction of static data members is described in [basic.start.static], [basic.start.dynamic], and [basic.start.term].
— end note]
4
#
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.
The declaration of an inline static data member (which is a definition) 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.
5
#
[Note 3: 
There is exactly one definition of a static data member that is odr-used ([basic.def.odr]) in a valid program.
— end note]
6
#
[Note 4: 
Static data members of a class in namespace scope have the linkage of the name of the class ([basic.link]).
— end note]

11.4.10 Bit-fields [class.bit]

1
#
The optional attribute-specifier-seq appertains to the entity being declared.
A bit-field shall not be a static member.
A bit-field shall have integral or (possibly cv-qualified) enumeration type; the bit-field semantic property 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 and is called the width of the bit-field.
If the width of a bit-field is larger than the width of the bit-field's type (or, in case of an enumeration type, of its underlying type), the extra bits are padding bits ([basic.types.general]).
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 1: 
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]
2
#
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 2: 
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 width be zero.
3
#
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 bind to a bit-field ([dcl.init.ref]).
[Note 3: 
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]
4
#
If a value of integral type (other than bool) is stored into a bit-field of width N and the value would be representable in a hypothetical signed or unsigned integer type with width N and the same signedness as the bit-field's type, the original value and the value of the bit-field compare equal.
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 compare equal.
If a value of an enumeration type is stored into a bit-field of the same type and the width is large enough to hold all the values of that enumeration type ([dcl.enum]), the original value and the value of the bit-field compare equal.
[Example 1: 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]

11.4.11 Allocation and deallocation functions [class.free]

1
#
Any allocation function for a class T is a static member (even if not explicitly declared static).
2
#
[Example 1: 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; // error: ​::​operator new(std​::​size_t) hidden } — end example]
3
#
Any deallocation function for a class X is a static member (even if not explicitly declared static).
[Example 2: 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]
4
#
Since member allocation and deallocation functions are static they cannot be virtual.
[Note 1: 
However, when the cast-expression of a delete-expression refers to an object of class type with a virtual destructor, because the deallocation function is chosen by the destructor of the dynamic type of the object, the effect is the same in that case.
[Example 3: 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 example]
— end note]
[Note 2: 
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.
[Example 4: 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 example]
— end note]
5
#
Access to the deallocation function is checked statically, even if a different one is actually executed.
[Example 5: 
For the call on line “// 1” above, if B​::​operator delete() had been private, the delete expression would have been ill-formed.
— end example]
6
#
[Note 3: 
If a deallocation function has no explicit noexcept-specifier, it has a non-throwing exception specification ([except.spec]).
— end note]

11.4.12 Nested class declarations [class.nest]

1
#
A class can be declared within another class.
A class declared within another is called a nested class.
[Note 1: 
See [expr.prim.id] for restrictions on the use of non-static data members and non-static member functions.
— end note]
[Example 1: 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 found — end example]
2
#
[Note 2: 
Nested classes can be defined either in the enclosing class or in an enclosing namespace; member functions and static data members of a nested class can be defined either in the nested class or in an enclosing namespace scope.
[Example 2: struct enclose { struct inner { static int x; void f(int i); }; }; int enclose::inner::x = 1; void enclose::inner::f(int i) { /* ... */ } 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]
— end note]
3
#
A friend function ([class.friend]) defined within a nested class has no special access rights to members of an enclosing class.

11.5 Unions [class.union]

11.5.1 General [class.union.general]

1
#
A union is a class defined with the class-key union.
2
#
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 1: 
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, the common initial sequence of any of the standard-layout struct members can be inspected; see [class.mem].
— end note]
3
#
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 non-union class.
[Note 2: 
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]
4
#
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 3: 
Absent default member initializers ([class.mem]), if any non-static data member of a union has a non-trivial default constructor ([class.default.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.
[Example 1: 
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]
— end note]
5
#
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:
  • (5.1)
    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.
  • (5.2)
    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.
  • (5.3)
    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) and each anonymous union member X ([class.union.anon]) that is a member of a union and has such an element as an immediate subobject (recursively), 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 4: 
This ends the lifetime of the previously-active member of the union, if any ([basic.life]).
— end note]
[Example 2: 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]
6
#
[Note 5: 
In cases where the above rule does not apply, the active member of a union can only be changed by the use of a placement new-expression.
— end note]
[Example 3: 
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]

11.5.2 Anonymous unions [class.union.anon]

1
#
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 member if it is a non-static data member or an anonymous union variable otherwise.
Each member-declaration in the member-specification of an anonymous union shall either define one or more public non-static data members or be a static_assert-declaration.
Nested types, anonymous unions, and functions shall not be declared within an anonymous union.
The names of the members of an anonymous union are bound in the scope inhabited by the union declaration.
[Example 1: 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]
2
#
An anonymous union declared in the scope of a namespace with external linkage shall use the storage-class-specifier static.
Anonymous unions declared at block scope shall not use a storage-class-specifier that is not permitted in the declaration of a block variable.
An anonymous union declaration at class scope shall not have a storage-class-specifier.
3
#
[Note 1: 
A union for which objects, pointers, or references are declared is not an anonymous union.
[Example 2: 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]
— end note]
[Note 2: 
Initialization of unions with no user-declared constructors is described in [dcl.init.aggr].
— end note]
4
#
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 3: union U { int x = 0; union { int k; }; union { int z; int y = 1; // error: initialization for second variant member of U }; }; — end example]

11.6 Local class declarations [class.local]

1
#
A class can be declared within a function definition; such a class is called a local class.
[Note 1: 
A declaration in a local class cannot odr-use ([basic.def.odr]) a local entity from an enclosing scope.
— end note]
[Example 1: 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 found — end example]
2
#
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.
3
#
A class nested within a local class is a local class.
A member of a local class X shall be declared only in the definition of X or, if the member is a nested class, in the nearest enclosing block scope of X.
4
#
[Note 2: 
A local class cannot have static data members ([class.static.data]).
— end note]

11.7 Derived classes [class.derived]

11.7.1 General [class.derived.general]

2
#
The component names of a class-or-decltype are those of its nested-name-specifier, type-name, and/or simple-template-id.
A class-or-decltype shall denote a (possibly cv-qualified) class type that is not an incompletely defined class ([class.mem]); any cv-qualifiers are ignored.
The class denoted by the class-or-decltype of a base-specifier is called a direct base class for the class being defined.
The lookup for the component name of the type-name or simple-template-id is type-only ([basic.lookup]).
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 1: 
See [class.access] for the meaning of access-specifier.
— end note]
Members of a base class are also members of the derived class.
[Note 2: 
Constructors of a base class can be explicitly inherited ([namespace.udecl]).
Base class 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]).
The scope resolution operator ​::​ ([expr.prim.id.qual]) can be used to refer to a direct or indirect base member explicitly, even if it is hidden 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]
3
#
The base-specifier-list specifies the type of the base class subobjects contained in an object of the derived class type.
[Example 1: 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]
4
#
A base-specifier followed by an ellipsis is a pack expansion ([temp.variadic]).
5
#
The order in which the base class subobjects are allocated in the most derived object ([intro.object]) is unspecified.
[Note 3: 
A derived class and its base class subobjects can be represented by a directed acyclic graph (DAG) where an arrow means “directly derived from” (see Figure 3).
An arrow need not have a physical representation in memory.
A DAG of subobjects is often referred to as a “subobject lattice”.
— end note]
dag Base Base Derived1 Derived1 Derived1->Base Derived2 Derived2 Derived2->Derived1
Figure 3: Directed acyclic graph  [fig:class.dag]
6
#
[Note 4: 
Initialization of objects representing base classes can be specified in constructors; see [class.base.init].
— end note]
7
#
[Note 5: 
A base class subobject can have a layout different from the layout of a most derived object of the same type.
A base class subobject can have a polymorphic behavior ([class.cdtor]) different from the polymorphic behavior of a most derived object of the same type.
A base class subobject can be of zero size; however, two subobjects that have the same class type and that belong to the same most derived object cannot be allocated at the same address ([intro.object]).
— end note]

11.7.2 Multiple base classes [class.mi]

1
#
A class can be derived from any number of base classes.
[Note 1: 
The use of more than one direct base class is often called multiple inheritance.
— end note]
[Example 1: class A { /* ... */ }; class B { /* ... */ }; class C { /* ... */ }; class D : public A, public B, public C { /* ... */ }; — end example]
2
#
[Note 2: 
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]
3
#
A class shall not be specified as a direct base class of a derived class more than once.
[Note 3: 
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; lookup that finds its non-static data members and member functions in the scope of the derived class will be ambiguous.
However, the static members, enumerations and types can be unambiguously referred to.
— end note]
[Example 2: class X { /* ... */ }; class Y : public X, public X { /* ... */ }; // error 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]
4
#
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.
5
#
[Note 4: 
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 4.
nonvirt L1 L L2 L A A A->L1 B B B->L2 C C C->A C->B
Figure 4: Non-virtual base  [fig:class.nonvirt]
In such lattices, explicit qualification can be used to specify which subobject is meant.
The body of function C​::​f can 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]
6
#
[Note 5: 
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 5: Virtual base  [fig:class.virt]
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 5.
— end note]
7
#
[Note 6: 
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 6.
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 6: Virtual and non-virtual base  [fig:class.virtnonvirt]
— end note]

11.7.3 Virtual functions [class.virtual]

1
#
A non-static member function is a virtual function if it is first declared with the keyword virtual or if it overrides a virtual member function declared in a base class (see below).92
[Note 1: 
Virtual functions support dynamic binding and object-oriented programming.
— end note]
A class with a virtual member function is called a polymorphic class.93
2
#
If a virtual member function F is declared in a class B, and, in a class D derived (directly or indirectly) from B, a declaration of a member function G corresponds ([basic.scope.scope]) to a declaration of F, ignoring trailing requires-clauses, then G overrides94 F.
For convenience, we say that any virtual function overrides itself.
A virtual member function V 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) has another member function that overrides V.
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 1: 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 2: 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]
3
#
[Note 2: 
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]
4
#
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 3: struct B { virtual void f() const final; }; struct D : B { void f() const; // error: D​::​f attempts to override final B​::​f }; — end example]
5
#
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 4: 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]
6
#
A virtual function shall not have a trailing requires-clause ([dcl.decl]).
[Example 5: template<typename T> struct A { virtual void f() requires true; // error: virtual function cannot be constrained ([temp.constr.decl]) }; — end example]
7
#
The ref-qualifier, or lack thereof, of an overriding function shall be the same as that of the overridden function.
8
#
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:
  • (8.1)
    both are pointers to classes, both are lvalue references to classes, or both are rvalue references to classes95
  • (8.2)
    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
  • (8.3)
    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.
9
#
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 locus ([basic.scope.pdecl]) of the overriding declaration 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 6: 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(); // error: argument mismatch } — end example]
10
#
[Note 3: 
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]
11
#
[Note 4: 
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]
12
#
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]).
13
#
[Example 7: 
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(); // error: 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]
14
#
[Example 8: 
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 { // error }; 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]
15
#
[Example 9: 
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]
16
#
Explicit qualification with the scope operator ([expr.prim.id.qual]) suppresses the virtual call mechanism.
[Example 10: 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]
17
#
A deleted function ([dcl.fct.def]) shall not override a function that is not deleted.
Likewise, a function that is not deleted shall not override a deleted function.
18
#
A consteval virtual function shall not override a virtual function that is not consteval.
A consteval virtual function shall not be overridden by a virtual function that is not consteval.
92)92)
The use of the virtual specifier in the declaration of an overriding function is valid but redundant (has empty semantics).
93)93)
If all virtual functions are immediate functions, the class is still polymorphic even if its internal representation does not otherwise require any additions for that polymorphic behavior.
94)94)
A function with the same name but a different parameter list ([over]) as a virtual function is not necessarily virtual and does not override.
Access control ([class.access]) is not considered in determining overriding.
95)95)
Multi-level pointers to classes or references to multi-level pointers to classes are not allowed.

11.7.4 Abstract classes [class.abstract]

1
#
[Note 1: 
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]
2
#
A virtual function is specified as a pure virtual function by using a pure-specifier ([class.mem]) in the function declaration in the class definition.
[Note 2: 
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 3: 
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 1: 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 4: 
A function declaration cannot provide both a pure-specifier and a definition.
— end note]
[Example 2: struct C { virtual void f() = 0 { }; // error }; — end example]
3
#
[Note 5: 
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]
4
#
A class is abstract if it has at least one pure virtual function for which the final overrider is pure virtual.
[Example 3: 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]
5
#
[Note 6: 
An abstract class can be derived from a class that is not abstract, and a pure virtual function can override a virtual function which is not pure.
— end note]
6
#
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.

11.8 Member access control [class.access]

11.8.1 General [class.access.general]

1
#
A member of a class can be
  • (1.1)
    private, that is, it can be named only by members and friends of the class in which it is declared;
  • (1.2)
    protected, that is, it can be named 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]); or
  • (1.3)
    public, that is, it can be named anywhere without access restriction.
[Note 1: 
A constructor or destructor can be named by an expression ([basic.def.odr]) even though it has no name.
— end note]
2
#
A member of a class can also access all the members to which the class has access.
A local class of a member function may access the same members that the member function itself may access.96
3
#
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 1: class X { int a; // X​::​a is private by default }; struct S { int a; // S​::​a is public by default }; — end example]
4
#
Access control is applied uniformly to declarations and expressions.
[Note 2: 
Access control applies to members nominated by friend declarations ([class.friend]) and using-declarations ([namespace.udecl]).
— end note]
When a using-declarator is named, access control is applied to it, not to the declarations that replace it.
For an overload set, access control is applied only to the function selected by overload resolution.
[Example 2: struct S { void f(int); private: void f(double); }; void g(S* sp) { sp->f(2); // OK, access control applied after overload resolution } — end example]
[Note 3: 
Because access control applies to the declarations named, if access control is applied to a typedef-name, only the accessibility of the typedef or alias declaration itself is considered.
The accessibility of the entity referred to by the typedef-name is not considered.
[Example 3: class A { class B { }; public: typedef B BB; }; void f() { A::BB x; // OK, typedef A​::​BB is public A::B y; // access error, A​::​B is private } — end example]
— end note]
5
#
[Note 4: 
Access control does not prevent members from being found by name lookup or implicit conversions to base classes from being considered.
— end note]
The interpretation of a given construct is established without regard to access control.
If the interpretation established makes use of inaccessible members or base classes, the construct is ill-formed.
6
#
All access controls in [class.access] affect the ability to name a class member 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 5: 
This access also applies to implicit references to constructors, conversion functions, and destructors.
— end note]
7
#
[Example 4: 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]
8
#
Access is checked for a default argument ([dcl.fct.default]) at the point of declaration, 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].
9
#
Access for a default template-argument ([temp.param]) is checked in the context in which it appears rather than at any points of use of it.
[Example 5: 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]
96)96)
Access permissions are thus transitive and cumulative to nested and local classes.

11.8.2 Access specifiers [class.access.spec]

1
#
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 1: 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]
2
#
Any number of access specifiers is allowed and no particular order is required.
[Example 2: 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]
3
#
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 3: struct S { class A; enum E : int; private: class A { }; // error: cannot change access enum E: int { e0 }; // error: cannot change access }; — end example]
4
#
[Note 1: 
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 4: class A { }; class B : private A { }; class C : public B { A* p; // error: injected-class-name A is inaccessible ::A* q; // OK }; — end example]

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

1
#
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.97
2
#
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 1: 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]
3
#
[Note 1: 
A member of a private base class can be inaccessible as inherited, but accessible directly.
Because of the rules on pointer conversions ([conv.ptr]) and explicit casts ([expr.type.conv], [expr.static.cast], [expr.cast]), a conversion from a pointer to a derived class to a pointer to an inaccessible base class can be ill-formed if an implicit conversion is used, but well-formed if an explicit cast is used.
[Example 2: 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 example]
— end note]
4
#
A base class B of N is accessible at R, if
  • (4.1)
    an invented public member of B would be a public member of N, or
  • (4.2)
    R occurs in a direct member or friend of class N, and an invented public member of B would be a private or protected member of N, or
  • (4.3)
    R occurs in a direct 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
  • (4.4)
    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 3: 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]
5
#
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 2: 
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 whose scope name lookup performed a search that found the member.
[Note 3: 
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
  • (5.1)
    m as a member of N is public, or
  • (5.2)
    m as a member of N is private, and R occurs in a direct member or friend of class N, or
  • (5.3)
    m as a member of N is protected, and R occurs in a direct 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
  • (5.4)
    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 4: 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]
6
#
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 4: 
This requirement is in addition to the requirement that the member be accessible as named.
— end note]
97)97)
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.

11.8.4 Friends [class.friend]

1
#
A friend of a class is a function or class that is given permission to name the private and protected members of 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 1: 
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]
2
#
Declaring a class to be a friend implies that private and protected members of the class granting friendship can be named in the base-specifiers and member declarations of the befriended class.
[Example 2: 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 3: 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]
3
#
A friend declaration that does not declare a function shall be a friend-type-declaration.
[Note 1: 
A friend declaration can be the declaration in a template-declaration ([temp.pre], [temp.friend]).
— end note]
If a friend-type-specifier in a friend declaration designates a (possibly cv-qualified) class type, that class is declared as a friend; otherwise, the friend-type-specifier is ignored.
[Example 4: class C; typedef C Ct; class E; class X1 { friend C; // OK, class C is a friend }; class X2 { friend Ct; // OK, class C is a friend friend D; // error: D not found friend class D; // OK, elaborated-type-specifier declares new class }; template <typename ... Ts> class R { friend Ts...; }; template <class... Ts, class... Us> class R<R<Ts...>, R<Us...>> { friend Ts::Nested..., Us...; }; R<C> rc; // class C is a friend of R<C> R<C, E> rce; // classes C and E are friends of R<C, E> R<int> Ri; // OK, “friend int;'' is ignored struct E { struct Nested; }; R<R<E>, R<C, int>> rr; // E​::​Nested and C are friends of R<R<E>, R<C, int>> — end example]
4
#
[Note 2: 
A friend declaration refers to an entity, not (all overloads of) a name.
A member function of a class X can be a friend of a class Y.
[Example 5: class Y { friend char* X::foo(int); friend X::X(char); // constructors can be friends friend X::~X(); // destructors can be friends }; — end example]
— end note]
5
#
A function may be defined in a friend declaration of a class if and only if the class is a non-local class ([class.local]) and the function name is unqualified.
[Example 6: class M { friend void f() { } // definition of global f, a friend of M, // not the definition of a member function }; — end example]
6
#
Such a function is implicitly an inline ([dcl.inline]) function if it is attached to the global module.
[Note 3: 
If a friend function is defined outside a class, it is not in the scope of the class.
— end note]
7
#
No storage-class-specifier shall appear in the decl-specifier-seq of a friend declaration.
8
#
A member nominated by a friend declaration shall be accessible in 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.
9
#
Friendship is neither inherited nor transitive.
[Example 7: 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]
10
#
[Note 4: 
A friend declaration never binds any names ([dcl.meaning], [dcl.type.elab]).
— end note]
[Example 8: // Assume f and g have not yet been declared. void h(int); template <class T> void f2(T); namespace A { class X { friend void f(X); // A​::​f(X) is a friend class Y { friend void g(); // A​::​g is a friend friend void h(int); // A​::​h is a friend // ​::​h not considered friend void f2<>(int); // ​::​f2<>(int) is a friend }; }; // A​::​f, A​::​g and A​::​h are not visible here X x; void g() { f(x); } // definition of A​::​g void f(X) { /* ... */ } // definition of A​::​f void h(int) { /* ... */ } // definition of A​::​h // A​::​f, A​::​g and A​::​h are visible here and known to be friends } using A::x; void h() { A::f(x); A::X::f(x); // error: f is not a member of A​::​X A::X::Y::g(); // error: g is not a member of A​::​X​::​Y } — end example]
[Example 9: 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]

11.8.5 Protected member access [class.protected]

1
#
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]).98
As described earlier, access to a protected member is granted because the reference occurs in a friend or direct 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 1: 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; // error p1->i = 2; // error 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; // error int B::* pmi_B2 = &D2::i; // OK (type of &D2​::​i is int B​::​*) B::j = 5; // error: 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; // error p1->i = 2; // error i = 3; // OK (access through this) B::i = 4; // OK (access through this, qualification ignored) int B::* pmi_B = &B::i; // error 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; // error p1->i = 2; // error p2->i = 3; // error } — end example]
98)98)
This additional check does not apply to other members, e.g., static data members or enumerator member constants.

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

1
#
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 1: 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]
2
#
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.

11.8.7 Multiple access [class.paths]

1
#
If a declaration can be reached by several paths through a multiple inheritance graph, the access is that of the path that gives most access.
[Example 1: 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]

11.8.8 Nested classes [class.access.nest]

1
#
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 1: 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]

11.9 Initialization [class.init]

11.9.1 General [class.init.general]

1
#
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].
2
#
An object of class type (or array thereof) can be explicitly initialized; see [class.expl.init] and [class.base.init].
3
#
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 1: 
Destructors for the array elements are called in reverse order of their construction.
— end note]

11.9.2 Explicit initialization [class.expl.init]

1
#
An object of class type can be initialized with a parenthesized expression-list, where the expression-list is construed as an argument list for a constructor that is called to initialize the object.
Alternatively, a single assignment-expression can be specified as an initializer using the = form of initialization.
Either direct-initialization semantics or copy-initialization semantics apply; see [dcl.init].
[Example 1: 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 1: 
Overloading of the assignment operator ([over.ass]) has no effect on initialization.
— end note]
2
#
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 2: 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 of 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 2: 
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]
3
#
[Note 3: 
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]
4
#
[Note 4: 
The order in which objects with static or thread storage duration are initialized is described in [basic.start.dynamic] and [stmt.dcl].
— end note]

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

1
#
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
2
#
Lookup for an unqualified name in a mem-initializer-id ignores the constructor's function parameter scope.
[Note 1: 
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 can 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.
3
#
A mem-initializer-list can initialize a base class using any class-or-decltype that denotes that base class type.
[Example 1: 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]
4
#
If a mem-initializer-id is ambiguous because it designates both a direct non-virtual base class and an indirect virtual base class, the mem-initializer is ill-formed.
[Example 2: struct A { A(); }; struct B: public virtual A { }; struct C: public A, public B { C(); }; C::C(): A() { } // error: which A? — end example]
5
#
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.
6
#
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 3: 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]
7
#
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 4: 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 2: 
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.
8
#
A temporary expression bound to a reference member in a mem-initializer is ill-formed.
[Example 5: struct A { A() : v(42) { } // error const int& v; }; — end example]
9
#
In a non-delegating constructor other than an implicitly-defined copy/move constructor ([class.copy.ctor]), 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
  • (9.1)
    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];
  • (9.2)
    otherwise, if the entity is an anonymous union or a variant member ([class.union.anon]), no initialization is performed;
  • (9.3)
    otherwise, the entity is default-initialized ([dcl.init]).
[Note 3: 
An abstract class ([class.abstract]) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers can be omitted.
— end note]
An attempt to initialize more than one non-static data member of a union renders the program ill-formed.
[Note 4: 
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 or erroneous value ([basic.indet]).
— end note]
[Example 6: 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 or erroneous value int j = 5; // OK, j has the value 5 }; — end example]
10
#
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 7: 
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]
11
#
A temporary expression bound to a reference member from a default member initializer is ill-formed.
[Example 8: 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]
12
#
In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked ([class.dtor]).
[Note 5: 
This provision ensures that destructors can be called for fully-constructed subobjects in case an exception is thrown ([except.ctor]).
— end note]
13
#
In a non-delegating constructor, initialization proceeds in the following order:
  • (13.1)
    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.
  • (13.2)
    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).
  • (13.3)
    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).
  • (13.4)
    Finally, the compound-statement of the constructor body is executed.
[Note 6: 
The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization.
— end note]
14
#
[Example 9: 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]
15
#
[Note 7: 
The expression-list or braced-init-list of a mem-initializer is in the function parameter scope of the constructor and can use this to refer to the object being initialized.
— end note]
[Example 10: 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]
16
#
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 11: class A { public: A(int); }; class B : public A { int j; public: int f(); B() : A(f()), // undefined behavior: 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 behavior: calls member function but base C not yet initialized i(f()) { } // well-defined: bases are all initialized }; — end example]
17
#
[Note 8: 
[class.cdtor] describes the results 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]
18
#
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 12: template<class... Mixins> class X : public Mixins... { public: X(const Mixins&... mixins) : Mixins(mixins)... { } }; — end example]

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

1
#
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 inherited constructor if the base class subobject were to be initialized as part of the D object ([class.base.init]).
The invocation of the inherited constructor, including the evaluation of any arguments, is omitted if the B subobject is not to be initialized as part of the D object.
The complete initialization is considered to be a single function call; in particular, unless omitted, the initialization of the inherited constructor's parameters is sequenced before the initialization of any part of the D object.
[Example 1: 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 no default constructor } struct D2 : B2 { using B2::B2; B1 b; }; D2 f(1.0); // error: B1 has no 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]
[Example 2: struct V { V() = default; V(int); }; struct Q { Q(); }; struct A : virtual V, Q { using V::V; A() = delete; }; int bar() { return 42; } struct B : A { B() : A(bar()) {} // OK }; struct C : B {}; void foo() { C c; } // bar is not invoked, because the V subobject is not initialized as part of B — end example]
2
#
If the constructor was inherited from multiple base class subobjects of type B, the program is ill-formed.
[Example 3: 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); // error: 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]
3
#
When an object is initialized by an inherited constructor, initialization of the object is complete when the initialization of all subobjects is complete.

11.9.5 Construction and destruction [class.cdtor]

1
#
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 1: 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 behavior: refers to base class member int* p2 = &bobj.y.i; // undefined behavior: refers to member's member A* pa = &bobj; // undefined behavior: upcast to a base class type B bobj; // definition of bobj extern X xobj; int* p3 = &xobj.i; // OK, all constructors of X are trivial X xobj;
For another example, struct W { int j; }; struct X : public virtual W { }; struct Y { int* p; X x; Y() : p(&x.j) { // undefined, x is not yet constructed } };
— end example]
2
#
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 2: 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]
3
#
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 3: 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 behavior: 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]
4
#
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 4: 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]
5
#
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.
6
#
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 5: 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]

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

1
#
When certain criteria are met, an implementation is allowed to omit the creation of a class object from a source object of the same type (ignoring cv-qualification), even if the selected constructor and/or the destructor for the object have side effects.
In such cases, the implementation treats the source and target of the omitted initialization 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.
[Note 1: 
Because only one object is destroyed instead of two, and the creation of one object is omitted, there is still one object destroyed for each one constructed.
— end note]
This elision of object creation, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):
  • (1.1)
    in a return statement ([stmt.return]) in a function with a class return type, when the expression is the name of a non-volatile object o with automatic storage duration (other than a function parameter or a variable introduced by the exception-declaration of a handler ([except.handle])), the copy-initialization of the result object can be omitted by constructing o directly into the function call's result object;
  • (1.2)
    in a throw-expression ([expr.throw]), when the operand is the name of a non-volatile object o with automatic storage duration (other than a function parameter or a variable introduced by the exception-declaration of a handler) that belongs to a scope that does not contain the innermost enclosing compound-statement associated with a try-block (if there is one), the copy-initialization of the exception object can be omitted by constructing o directly into the exception object;
  • (1.3)
    in a coroutine, a copy of a coroutine parameter can be omitted and references to that copy replaced with references to the corresponding parameter if the meaning of the program will be unchanged except for the execution of a constructor and destructor for the parameter copy object;
  • (1.4)
    when the exception-declaration of a handler ([except.handle]) declares an object o, the copy-initialization of o 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 2: 
    There cannot be a move from the exception object because it is always an lvalue.
    — end note]
Copy elision is not permitted where an expression is evaluated in a context requiring a constant expression ([expr.const]) and in constant initialization ([basic.start.static]).
[Note 3: 
It is possible that copy elision is performed if the same expression is evaluated in another context.
— end note]
2
#
[Example 1: 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 loc; return loc; } constexpr A a; // well-formed, a.p points to a constexpr A b = g(); // error: b.p would be dangling ([expr.const]) void h() { A c = g(); // well-formed, c.p can point to c or be dangling }
Here the criteria for elision can eliminate the copying of the object t with automatic storage duration into the result object for the function call f(), which is the non-local object t2.
Effectively, the construction of t can be viewed as directly initializing 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 object with automatic storage duration to t2 that is elided.
— end example]
3
#
[Example 2: 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(bool b) { static Weird w1; Weird w2; if (b) return w1; // OK, uses Weird(Weird&) else return w2; // error: w2 in this context is an xvalue } int& h(bool b, int i) { static int s; if (b) return s; // OK else return i; // error: i is an xvalue } decltype(auto) h2(Thing t) { return t; // OK, t is an xvalue and h2's return type is Thing } decltype(auto) h3(Thing t) { return (t); // OK, (t) is an xvalue and h3's return type is Thing&& } — end example]
4
#
[Example 3: template<class T> void g(const T&); template<class T> void f() { T x; try { T y; try { g(x); } catch (...) { if (/*...*/) throw x; // does not move throw y; // moves } g(y); } catch(...) { g(x); g(y); // error: y is not in scope } } — end example]

11.10 Comparisons [class.compare]

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

1
#
A defaulted comparison operator function ([over.binary]) shall be a non-template function that
  • (1.1)
    is a non-static member or friend of some class C,
  • (1.2)
    is defined as defaulted in C or in a context where C is complete, and
  • (1.3)
    either has two parameters of type const C& or two parameters of type C, where the implicit object parameter (if any) is considered to be the first parameter.
Such a comparison operator function is termed a defaulted comparison operator function for class C.
Name lookups and access checks in the implicit definition ([dcl.fct.def.default]) of a comparison operator function are performed from a context equivalent to its function-body.
A definition of a comparison operator as defaulted that appears in a class shall be the first declaration of that function.
[Example 1: struct S; bool operator==(S, S) = default; // error: S is not complete struct S { friend bool operator==(S, const S&) = default; // error: parameters of different types }; enum E { }; bool operator==(E, E) = default; // error: not a member or friend of a class — end example]
2
#
A defaulted <=> or == operator function for class C is defined as deleted if any non-static data member of C is of reference type or C has variant members ([class.union.anon]).
3
#
A binary operator expression a @ b is usable if either
  • (3.1)
    a or b is of class or enumeration type and overload resolution ([over.match]) as applied to a @ b results in a usable candidate, or
  • (3.2)
    neither a nor b is of class or enumeration type and a @ b is a valid expression.
4
#
If the member-specification does not explicitly declare any member or friend named operator==, an == operator function is declared implicitly for each three-way comparison operator function defined as defaulted in the member-specification, with the same access and function-definition and in the same class scope as the respective three-way comparison operator function, except that the return type is replaced with bool and the declarator-id is replaced with operator==.
[Note 1: 
Such an implicitly-declared == operator for a class X is defined as defaulted in the definition of X and has the same parameter-declaration-clause and trailing requires-clause as the respective three-way comparison operator.
It is declared with friend, virtual, constexpr, or consteval if the three-way comparison operator function is so declared.
If the three-way comparison operator function has no noexcept-specifier, the implicitly-declared == operator function has an implicit exception specification ([except.spec]) that can differ from the implicit exception specification of the three-way comparison operator function.
— end note]
[Example 2: template<typename T> struct X { friend constexpr std::partial_ordering operator<=>(X, X) requires (sizeof(T) != 1) = default; // implicitly declares: friend constexpr bool operator==(X, X) requires (sizeof(T) != 1) = default; [[nodiscard]] virtual std::strong_ordering operator<=>(const X&) const = default; // implicitly declares: [[nodiscard]] virtual bool operator==(const X&) const = default; }; — end example]
[Note 2: 
The == operator function is declared implicitly even if the defaulted three-way comparison operator function is defined as deleted.
— end note]
5
#
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 be an lvalue denoting the element in the expanded list of subobjects for an object x (of length n), where 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.

11.10.2 Equality operator [class.eq]

1
#
A defaulted equality operator function ([over.binary]) shall have a declared return type bool.
2
#
A defaulted == operator function for a class C is defined as deleted unless, for each in the expanded list of subobjects for an object x of type C, is usable ([class.compare.default]).
3
#
The return value of a defaulted == operator function with parameters x and y is determined by comparing corresponding elements and in the expanded lists of subobjects for x and y (in increasing index order) until the first index i where yields a result value which, when contextually converted to bool, yields false.
The return value is false if such an index exists and true otherwise.
4
#
[Example 1: struct D { int i; friend bool operator==(const D& x, const D& y) = default; // OK, returns x.i == y.i }; — end example]

11.10.3 Three-way comparison [class.spaceship]

1
#
The synthesized three-way comparison of type R ([cmp.categories]) of glvalues a and b of the same type is defined as follows:
  • (1.1)
    If a <=> b is usable ([class.compare.default]) and can be explicitly converted to R using static_cast, static_cast<R>(a <=> b).
  • (1.2)
    Otherwise, if overload resolution for a <=> b is performed and finds at least one viable candidate, the synthesized three-way comparison is not defined.
  • (1.3)
    Otherwise, if R is not a comparison category type, or either the expression a == b or the expression a < b is not usable, the synthesized three-way comparison is not defined.
  • (1.4)
    Otherwise, if R is strong_ordering, then a == b ? strong_ordering::equal : a < b ? strong_ordering::less : strong_ordering::greater
  • (1.5)
    Otherwise, if R is weak_ordering, then a == b ? weak_ordering::equivalent : a < b ? weak_ordering::less : weak_ordering::greater
  • (1.6)
    Otherwise (when R is partial_ordering), a == b ? partial_ordering::equivalent : a < b ? partial_ordering::less : b < a ? partial_ordering::greater : partial_ordering::unordered
[Note 1: 
A synthesized three-way comparison is ill-formed if overload resolution finds usable candidates that do not otherwise meet the requirements implied by the defined expression.
— end note]
2
#
Let R be the declared return type of a defaulted three-way comparison operator function, and let be the elements of the expanded list of subobjects for an object x of type C.
  • (2.1)
    If R is auto, then let be the type of the expression .
    The operator function is defined as deleted if that expression is not usable or if is not a comparison category type ([cmp.categories.pre]) for any i.
    The return type is deduced as the common comparison type (see below) of , , …, .
  • (2.2)
    Otherwise, R shall not contain a placeholder type.
    If the synthesized three-way comparison of type R between any objects and is not defined, the operator function is defined as deleted.
3
#
The return value 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 and in the expanded lists of subobjects for x and y (in increasing index order) until the first index i where the synthesized three-way comparison of type R between and yields a result value where , contextually converted to bool, yields true.
The return value is a copy of if such an index exists and static_cast<R>(std​::​strong_ordering​::​equal) otherwise.
4
#
The common comparison type U of a possibly-empty list of n comparison category types , , …, is defined as follows:
  • (4.1)
    If at least one is std​::​partial_ordering, U is std​::​partial_ordering ([cmp.partialord]).
  • (4.2)
    Otherwise, if at least one is std​::​weak_ordering, U is std​::​weak_ordering ([cmp.weakord]).
  • (4.3)
    Otherwise, U is std​::​strong_ordering ([cmp.strongord]).
    [Note 2: 
    In particular, this is the result when n is 0.
    — end note]

11.10.4 Secondary comparison operators [class.compare.secondary]

1
#
A secondary comparison operator is a relational operator ([expr.rel]) or the != operator.
A defaulted operator function ([over.binary]) for a secondary comparison operator @ shall have a declared return type bool.
2
#
The operator function with parameters x and y is defined as deleted if
  • (2.1)
    a first overload resolution ([over.match]), as applied to x @ y,
    • (2.1.1)
      does not result in a usable candidate, or
    • (2.1.2)
      the selected candidate is not a rewritten candidate, or
  • (2.2)
    a second overload resolution for the expression resulting from the interpretation of x @ y using the selected rewritten candidate ([over.match.oper]) does not result in a usable candidate (for example, that expression might be (x <=> y) @ 0), or
  • (2.3)
    x @ y cannot be implicitly converted to bool.
In any of the two overload resolutions above, the defaulted operator function is not considered as a candidate for the @ operator.
Otherwise, the operator function yields x @ y.
3
#
[Example 1: struct HasNoLessThan { }; struct C { friend HasNoLessThan operator<=>(const C&, const C&); bool operator<(const C&) const = default; // OK, function is deleted }; — end example]