Functions declared in the definition of a class, excluding those declared with a friend specifier ([class.friend]), are called member functions of that class. A member function may be declared static in which case it is a static member function of its class ([class.static]); otherwise it is a non-static member function of its class ([class.mfct.non-static], [class.this]).
A member function may be defined ([dcl.fct.def]) in its class definition, in which case it is an inline member function ([dcl.fct.spec]), 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. [ Note: Member functions of a class in namespace scope have external linkage. Member functions of a local class ([class.local]) have no linkage. See [basic.link]. — end note ]
There shall be at most one definition of a non-inline member function in a program; no diagnostic is required. There may be more than one inline member function definition in a program. See [basic.def.odr] and [dcl.fct.spec].
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 ([dcl.fct.default]) 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 ]
A static local variable in a member function always refers to the same object, whether or not the member function is inline.
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(void);
typedef void fvc(void) const;
struct S {
fv memfunc1; // equivalent to: void memfunc1(void);
void memfunc2();
fvc memfunc3; // equivalent to: void memfunc3(void) 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 (Clause [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 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 ([expr.prim]) that is not part of a class member access syntax ([expr.ref]) 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 ([expr.prim.general]), if name lookup ([basic.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 ([expr.ref]) using (*this) ([class.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 ([expr.prim]) 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 ([expr.prim]) in which the nested-name-specifier names the class of the member function. [ 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 ([class.this]). They also affect the function type ([dcl.fct]) of the member function; a member function declared const is a const member function, a member function declared volatile is a volatile member function and a member function declared const volatile is a const volatile member function. [ Example:
struct X {
void g() const;
void h() const volatile;
};
X::g is a const member function and X::h is a const volatile member function. — end example ]
A non-static member function may be declared with a ref-qualifier ([dcl.fct]); see [over.match.funcs].
A non-static member function may be declared virtual ([class.virtual]) or pure virtual ([class.abstract]).
See, for example, <cstring> ([c.strings]).
In the body of a non-static ([class.mfct]) member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*. If the member function is declared const, the type of this is const X*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*.
In a const member function, the object for which the function is called is accessed through a const access path; therefore, a const member function shall not modify the object and its non-static data members. [ Example:
struct s {
int a;
int f() const;
int g() { return a++; }
int h() const { return a++; } // error
};
int s::f() const { return a; }
The a++ in the body of s::h is ill-formed because it tries to modify (a part of) the object for which s::h() is called. This is not allowed in a const member function because this is a pointer to const; that is, *this has const type. — end example ]
Similarly, volatile semantics ([dcl.type.cv]) apply in volatile member functions when accessing the object and its non-static data members.
A cv-qualified member function can be called on an object-expression ([expr.ref]) only if the object-expression is as cv-qualified or less-cv-qualified than the member function. [ Example:
void k(s& x, const s& y) {
x.f();
x.g();
y.f();
y.g(); // error
}
The call y.g() is ill-formed because y is const and s::g() is a non-const member function, that is, s::g() is less-qualified than the object-expression y. — end example ]
Constructors ([class.ctor]) and destructors ([class.dtor]) shall not be declared const, volatile or const volatile. [ Note: However, these functions can be invoked to create and destroy objects with cv-qualified types, see ([class.ctor]) and ([class.dtor]). — end note ]