C++ Primer, Fifth Edition

14.9.1. Conversion Operators

A conversion operator is a special kind of member function that converts a value of a class type to a value of some other type. A conversion function typically has the general form

operator type() const;

where type represents a type. Conversion operators can be defined for any type (other than void) that can be a function return type (§ 6.1, p. 204). Conversions to an array or a function type are not permitted. Conversions to pointer types—both data and function pointers—and to reference types are allowed.

Conversion operators have no explicitly stated return type and no parameters, and they must be defined as member functions. Conversion operations ordinarily should not change the object they are converting. As a ...