10.12. Converting Between Types

Most programs process information of many types. Sometimes all the operations “stay within a type.” For example, adding an int to an int produces an int. It’s often necessary, however, to convert data of one type to data of another type. This can happen in assignments, in calculations, in passing values to functions and in returning values from functions. The compiler knows how to perform certain conversions among fundamental types. You can use cast operators to force conversions among fundamental types.

But what about user-defined types? The compiler cannot know in advance how to convert among user-defined types, and between user-defined types and fundamental types, so you must specify how to do this. Such conversions can be performed with conversion constructors—constructors that can be called with a single argument (we’ll refer to these as single-argument constructors). Such constructors can turn objects of other types (including fundamental types) into objects of a particular class.