C++ Primer, Fifth Edition

7.1.3. Defining Nonmember Class-Related Functions

Class authors often define auxiliary functions, such as our add, read, and print functions. Although such functions define operations that are conceptually part of the interface of the class, they are not part of the class itself.

We define nonmember functions as we would any other function. As with any other function, we normally separate the declaration of the function from its definition (§ 6.1.2, p. 206). Functions that are conceptually part of a class, but not defined inside the class, are typically declared (but not defined) in the same header as the class itself. That way users need to ...