12 Classes [class]

A member function may be defined in its class definition, in which case it is an inline member function, or it may be defined outside of its class definition if it has already been declared but not defined in its class definition. A member function definition that appears outside of the class definition shall appear in a namespace scope enclosing the class definition. Except for member function definitions that appear outside of a class definition, and except for explicit specializations of member functions of class templates and member function templates ([temp.spec]) appearing outside of the class definition, a member function shall not be redeclared.

An inline member function (whether static or non-static) may also be defined outside of its class definition provided either its declaration in the class definition or its definition outside of the class definition declares the function as inline or constexpr. [ Note: Member functions of a class in namespace scope have the linkage of that class. Member functions of a local class have no linkage. See [basic.link].  — end note ]

[ Note: There can be at most one definition of a non-inline member function in a program. There may be more than one inline member function definition in a program. See [basic.def.odr] and [dcl.inline].  — end note ]

If the definition of a member function is lexically outside its class definition, the member function name shall be qualified by its class name using the ​::​ operator. [ Note: A name used in a member function definition (that is, in the parameter-declaration-clause including the default arguments or in the member function body) is looked up as described in [basic.lookup].  — end note ] [ Example:

struct X {
  typedef int T;
  static T count;
  void f(T);
};
void X::f(T t = count) { }

The member function f of class X is defined in global scope; the notation X​::​f specifies that the function f is a member of class X and in the scope of class X. In the function definition, the parameter type T refers to the typedef member T declared in class X and the default argument count refers to the static data member count declared in class X.  — end example ]

[ Note: A static local variable or local type in a member function always refers to the same entity, whether or not the member function is inline.  — end note ]

Previously declared member functions may be mentioned in friend declarations.

Member functions of a local class shall be defined inline in their class definition, if they are defined at all.

[ Note: A member function can be declared (but not defined) using a typedef for a function type. The resulting member function has exactly the same type as it would have if the function declarator were provided explicitly, see [dcl.fct]. For example,

typedef void fv();
typedef void fvc() const;
struct S {
  fv memfunc1;      // equivalent to: void memfunc1();
  void memfunc2();
  fvc memfunc3;     // equivalent to: void memfunc3() const;
};
fv  S::* pmfv1 = &S::memfunc1;
fv  S::* pmfv2 = &S::memfunc2;
fvc S::* pmfv3 = &S::memfunc3;

Also see [temp.arg].  — end note ]

A non-static member function may be called for an object of its class type, or for an object of a class derived from its class type, using the class member access syntax ([over.match.call]). A non-static member function may also be called directly using the function call syntax ([expr.call], [over.match.call]) from within the body of a member function of its class or of a class derived from its class.

If a non-static member function of a class X is called for an object that is not of type X, or of a type derived from X, the behavior is undefined.

When an id-expression that is not part of a class member access syntax and not used to form a pointer to member ([expr.unary.op]) is used in a member of class X in a context where this can be used, if name lookup resolves the name in the id-expression to a non-static non-type member of some class C, and if either the id-expression is potentially evaluated or C is X or a base class of X, the id-expression is transformed into a class member access expression using (*this) as the postfix-expression to the left of the . operator. [ Note: If C is not X or a base class of X, the class member access expression is ill-formed.  — end note ] Similarly during name lookup, when an unqualified-id used in the definition of a member function for class X resolves to a static member, an enumerator or a nested type of class X or of a base class of X, the unqualified-id is transformed into a qualified-id in which the nested-name-specifier names the class of the member function. These transformations do not apply in the template definition context ([temp.dep.type]). [ Example:

struct tnode {
  char tword[20];
  int count;
  tnode* left;
  tnode* right;
  void set(const char*, tnode* l, tnode* r);
};

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

void f(tnode n1, tnode n2) {
  n1.set("abc",&n2,0);
  n2.set("def",0,0);
}

In the body of the member function tnode​::​set, the member names tword, count, left, and right refer to members of the object for which the function is called. Thus, in the call n1.set("abc",&n2,0), tword refers to n1.tword, and in the call n2.set("def",0,0), it refers to n2.tword. The functions strlen, perror, and strcpy are not members of the class tnode and should be declared elsewhere.110  — end example ]

A non-static member function may be declared const, volatile, or const volatile. These cv-qualifiers affect the type of the this pointer. They also affect the function type of the member function; a member function declared const is a const member function, a member function declared volatile is a volatile member function and a member function declared const volatile is a const volatile member function. [ Example:

struct X {
  void g() const;
  void h() const volatile;
};

X​::​g is a const member function and X​::​h is a const volatile member function.  — end example ]

A non-static member function may be declared with a ref-qualifier ([dcl.fct]); see [over.match.funcs].

A non-static member function may be declared virtual ([class.virtual]) or pure virtual ([class.abstract]).

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 to refer to a static member. A static member may be referred to using the class member access syntax, in which case the object expression is evaluated. [ Example:

struct process {
  static void reschedule();
};
process& g();

void f() {
  process::reschedule();        // OK: no object necessary
  g().reschedule();             // g() is called
}

 — end example ]

A static member may be referred to directly in the scope of its class or in the scope of a class derived from its class; in this case, the static member is referred to as if a qualified-id expression was used, with the nested-name-specifier of the qualified-id naming the class scope from which the static member is referenced. [ Example:

int g();
struct X {
  static int g();
};
struct Y : X {
  static int i;
};
int Y::i = g();                 // equivalent to Y​::​g();

 — end example ]

If an unqualified-id is used in the definition of a static member following the member's declarator-id, and name lookup finds that the unqualified-id refers to a static member, enumerator, or nested type of the member's class (or of a base class of the member's class), the unqualified-id is transformed into a qualified-id expression in which the nested-name-specifier names the class scope from which the member is referenced. [ Note: See [expr.prim] for restrictions on the use of non-static data members and non-static member functions.  — end note ]

Static members obey the usual class member access rules. When used in the declaration of a class member, the static specifier shall only be used in the member declarations that appear within the member-specification of the class definition. [ Note: It cannot be specified in member declarations that appear in namespace scope.  — end note ]

A member-declarator of the form

identifier${}_{opt}$  attribute-specifier-seq${}_{opt}$ : constant-expression

specifies a bit-field; its length is set off from the bit-field name by a colon. The optional attribute-specifier-seq appertains to the entity being declared. The bit-field attribute is not part of the type of the class member. The constant-expression shall be an integral constant expression with a value greater than or equal to zero. The value of the integral constant expression may be larger than the number of bits in the object representation of the bit-field's type; in such cases the extra bits are used as padding bits and do not participate in the value representation of the bit-field. Allocation of bit-fields within a class object is implementation-defined. Alignment of bit-fields is implementation-defined. Bit-fields are packed into some addressable allocation unit. [ Note: Bit-fields straddle allocation units on some machines and not on others. Bit-fields are assigned right-to-left on some machines, left-to-right on others.  — end note ]

A declaration for a bit-field that omits the identifier declares an unnamed bit-field. Unnamed bit-fields are not members and cannot be initialized. [ Note: An unnamed bit-field is useful for padding to conform to externally-imposed layouts.  — end note ] As a special case, an unnamed bit-field with a width of zero specifies alignment of the next bit-field at an allocation unit boundary. Only when declaring an unnamed bit-field may the value of the constant-expression be equal to zero.

A bit-field shall not be a static member. A bit-field shall have integral or enumeration type ([basic.fundamental]). A bool value can successfully be stored in a bit-field of any nonzero size. The address-of operator & shall not be applied to a bit-field, so there are no pointers to bit-fields. A non-const reference shall not be bound to a bit-field ([dcl.init.ref]). [ Note: If the initializer for a reference of type const T& is an lvalue that refers to a bit-field, the reference is bound to a temporary initialized to hold the value of the bit-field; the reference is not bound to the bit-field directly. See [dcl.init.ref].  — end note ]

If the value true or false is stored into a bit-field of type bool of any size (including a one bit bit-field), the original bool value and the value of the bit-field shall compare equal. If the value of an enumerator is stored into a bit-field of the same enumeration type and the number of bits in the bit-field is large enough to hold all the values of that enumeration type, the original enumerator value and the value of the bit-field shall compare equal. [ Example:

enum BOOL { FALSE=0, TRUE=1 };
struct A {
  BOOL b:1;
};
A a;
void f() {
  a.b = TRUE;
  if (a.b == TRUE)              // yields true
    { /* ... */ }
}

 — end example ]

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

[ Example:

int x;
int y;

struct enclose {
  int x;
  static int s;

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

inner* p = 0;                   // error: inner not in scope

 — end example ]

Member functions and static data members of a nested class can be defined in a namespace scope enclosing the definition of their class. [ Example:

struct enclose {
  struct inner {
    static int x;
    void f(int i);
  };
};

int enclose::inner::x = 1;

void enclose::inner::f(int i) { /* ... */ }

 — end example ]

If class X is defined in a namespace scope, a nested class Y may be declared in class X and later defined in the definition of class X or be later defined in a namespace scope enclosing the definition of class X. [ Example:

class E {
  class I1;                     // forward declaration of nested class
  class I2;
  class I1 { };                 // definition of nested class
};
class E::I2 { };                // definition of nested class

 — end example ]

Like a member function, a friend function defined within a nested class is in the lexical scope of that class; it obeys the same rules for name binding as a static member function of that class, but it has no special access rights to members of an enclosing class.

Type names obey exactly the same scope rules as other names. In particular, type names defined within a class definition cannot be used outside their class without qualification. [ Example:

struct X {
  typedef int I;
  class Y { /* ... */ };
  I a;
};

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

 — end example ]