10.5.2. Algorithm Parameter Patterns

Superimposed on any other classification of the algorithms is a set of parameter conventions. Understanding these parameter conventions can aid in learning new algorithms—by knowing what the parameters mean, you can concentrate on understanding the operation the algorithm performs. Most of the algorithms have one of the following four forms:

alg(beg, end, other args);
alg(beg, end, dest, other args);
alg(beg, end, beg2, other args);
alg(beg, end, beg2, end2, other args);

where alg is the name of the algorithm, and beg and end denote the input range on which the algorithm operates. Although nearly all algorithms take an input range, the presence of the other parameters depends on the work being performed. The common ones listed here—dest, beg2, and end2—are all iterators. When used, these iterators fill similar roles. In addition to these iterator parameters, some algorithms take additional, noniterator parameters that are algorithm specific.

Algorithms with a Single Destination Iterator

A dest parameter is an iterator that denotes a destination in which the algorithm can write its output. Algorithms assume that it is safe to write as many elements as needed.

If dest is an iterator that refers directly to a container, then the algorithm writes its output to existing elements within the container. More commonly, dest is bound to an insert iterator (§ 10.4.1, p. 401) or an ostream_iterator (§ 10.4.2, p. 403). An insert iterator adds new elements to the container, thereby ensuring that there is enough space. An ostream_iterator writes to an output stream, again presenting no problem regardless of how many elements are written.

Algorithms with a Second Input Sequence

Algorithms that take either beg2 alone or beg2 and end2 use those iterators to denote a second input range. These algorithms typically use the elements from the second range in combination with the input range to perform a computation.

When an algorithm takes both beg2 and end2, these iterators denote a second range. Such algorithms take two completely specified ranges: the input range denoted by [beg, end), and a second input range denoted by [beg2, end2).

Algorithms that take only beg2 (and not end2) treat beg2 as the first element in a second input range. The end of this range is not specified. Instead, these algorithms assume that the range starting at beg2 is at least as large as the one denoted by beg, end.