15 Special member functions [special]

15.3 Conversions [class.conv]

15.3.2 Conversion functions [class.conv.fct]

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
]
A conversion function may be explicit, in which case it is only considered as a user-defined conversion for direct-initialization.
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
]
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
]
Conversion functions are inherited.
Conversion functions can be virtual.
A conversion function template shall not have a deduced return type.
[Example
:
struct S {
  operator auto() const { return 10; }      // OK
  template<class T>
  operator auto() const { return 1.2; }     // error: conversion function template
};
end example
]
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.
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.