15 Special member functions [special]

15.3 Conversions [class.conv]

15.3.2 Conversion functions [class.conv.fct]

1
#
A member function of a class X having no parameters with a name of the form
Such functions are called conversion functions.
A decl-specifier in the decl-specifier-seq of a conversion function (if any) shall be neither a defining-type-specifier nor static.
The type of the conversion function ([dcl.fct]) is “function taking no parameter returning conversion-type-id”.
A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void.117
[ Example
: 
struct X {
  operator int();
  operator auto() -> short;     // error: trailing return type
};

void f(X a) {
  int i = int(a);
  i = (int)a;
  i = a;
}
In all three cases the value assigned will be converted by X​::​operator int().
— end example
 ]
2
#
A conversion function may be explicit ([dcl.fct.spec]), in which case it is only considered as a user-defined conversion for direct-initialization ([dcl.init]).
Otherwise, user-defined conversions are not restricted to use in assignments and initializations.
[ Example
: 
class Y { };
struct Z {
  explicit operator Y() const;
};

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

void g(X a, X b) {
  int i = (a) ? 1+a : 0;
  int j = (a&&b) ? a+b : i;
  if (a) {
  }
}
— end example
 ]
3
#
The conversion-type-id shall not represent a function type nor an array type.
The conversion-type-id in a conversion-function-id is the longest sequence of tokens that could possibly form a conversion-type-id.
[ Note
:
This prevents ambiguities between the declarator operator * and its expression counterparts.
[ Example
: 
&ac.operator int*i; // syntax error:
                    // parsed as: &(ac.operator int *)i
                    // not as: &(ac.operator int)*i
The * is the pointer declarator and not the multiplication operator.
— end example
 ]
This rule also prevents ambiguities for attributes.
[ Example
: 
operator int [[noreturn]] ();   // error: noreturn attribute applied to a type
— end example
 ]
— end note
 ]
4
#
Conversion functions are inherited.
5
#
Conversion functions can be virtual.
6
#
A conversion function template shall not have a deduced return type ([dcl.spec.auto]).
[ Example
: 
struct S {
  operator auto() const { return 10; }      // OK
  template<class T>
  operator auto() const { return 1.2; }     // error: conversion function template
};
— end example
 ]
117)
These conversions are considered as standard conversions for the purposes of overload resolution ([over.best.ics], [over.ics.ref]) and therefore initialization ([dcl.init]) and explicit casts ([expr.static.cast]).
A conversion to void does not invoke any conversion function ([expr.static.cast]).
Even though never directly called to perform a conversion, such conversion functions can be declared and can potentially be reached through a call to a virtual conversion function in a base class.