11 Classes [class]

11.4 Class members [class.mem]

11.4.8 Conversions [class.conv] General [class.conv.general]

Type conversions of class objects can be specified by constructors and by conversion functions.
These conversions are called user-defined conversions and are used for implicit type conversions ([conv]), for initialization ([dcl.init]), and for explicit type conversions ([expr.type.conv], [expr.cast], [expr.static.cast]).
User-defined conversions are applied only where they are unambiguous ([class.member.lookup], [class.conv.fct]).
Conversions obey the access control rules ([class.access]).
Access control is applied after ambiguity resolution ([basic.lookup]).
[Note 1:
See [over.match] for a discussion of the use of conversions in function calls as well as examples below.
— end note]
At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.
[Example 1: struct X { operator int(); }; struct Y { operator X(); }; Y a; int b = a; // error: no viable conversion (a.operator X().operator int() not considered) int c = X(a); // OK: a.operator X().operator int() — end example]
User-defined conversions are used implicitly only if they are unambiguous.
A conversion function in a derived class does not hide a conversion function in a base class unless the two functions convert to the same type.
Function overload resolution ([over.match.best]) selects the best conversion function to perform the conversion.
[Example 2: struct X { operator int(); }; struct Y : X { operator char(); }; void f(Y& a) { if (a) { // error: ambiguous between X​::​operator int() and Y​::​operator char() } } — end example]