[class.conv.fct] (original) (raw)

11 Classes [class]

11.4 Class members [class.mem]

11.4.8 Conversions [class.conv]

11.4.8.3 Conversion functions [class.conv.fct]

A member function of a class X having no parameters with a name of the form

conversion-function-id:
operator conversion-type-id

specifies a conversion fromXto the type specified by theconversion-type-id.

Such functions are called conversion functions.

The type of the conversion function ([dcl.fct]) is “function taking no parameter returningconversion-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 cv void.110

[Example 1: struct X { operator int();operator auto() -> short; };void f(X a) { int i = int(a); i = (int)a; i = a;}

In all three cases the value assigned will be converted byX​::​operator int().

— _end example_]

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 2: class Y { };struct Z { explicit operator Y() const;};void h(Z z) { Y y1(z); Y y2 = z; Y y3 = (Y)z; } void g(X a, X b) { int i = (a) ? 1+a : 0;int j = (a&&b) ? a+b : i;if (a) { } } — _end example_]

Theconversion-type-idshall not represent a function type nor an array type.

Theconversion-type-idin aconversion-function-idis the longest sequence of tokens that could possibly form a conversion-type-id.

[Note 1:

This prevents ambiguities between the declarator operator * and its expression counterparts.

[Example 3: &ac.operator int*i;

The * is the pointer declarator and not the multiplication operator.

— _end example_]

This rule also prevents ambiguities for attributes.

[Example 4: operator int [[noreturn]] (); — _end example_]

— _end note_]

Conversion functions are inherited.

Conversion functions can be virtual.

A conversion function template shall not have a deduced return type ([dcl.spec.auto]).

[Example 5: struct S { operator auto() const { return 10; } template<class T> operator auto() const { return 1.2; } }; — _end example_]