In addition to defining the copy-control members, classes that manage resources often also define a function named
swap (§ 9.2.5, p. 339). Defining
swap is particularly important for classes that we plan to use with algorithms that reorder elements (§ 10.2.3, p. 383). Such algorithms call
swap whenever they need to exchange two elements.
If a class defines its own
swap, then the algorithm uses that class-specific version. Otherwise, it uses the
swap function defined by the library. Although, as usual, we don’t know how
swap is implemented, conceptually it’s easy to see that swapping two objects involves a copy and two assignments. For example, code to swap two objects of our valuelike
HasPtr class (§ 13.2.1, p. 511) might look something ...