12 Classes [class]

12.2 Class members [class.mem]

12.2.1 Member functions [class.mfct]

A member function may be defined in its class definition, in which case it is an inline member function, or it may be defined outside of its class definition if it has already been declared but not defined in its class definition.
A member function definition that appears outside of the class definition shall appear in a namespace scope enclosing the class definition.
Except for member function definitions that appear outside of a class definition, and except for explicit specializations of member functions of class templates and member function templates ([temp.spec]) appearing outside of the class definition, a member function shall not be redeclared.
An inline member function (whether static or non-static) may also be defined outside of its class definition provided either its declaration in the class definition or its definition outside of the class definition declares the function as inline or constexpr.
[Note
:
Member functions of a class in namespace scope have the linkage of that class.
Member functions of a local class have no linkage.
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[Note
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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.
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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
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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].
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[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.
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[Note
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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.
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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].
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]