C++ for Scientists - Technische Universität Dresden

122 CHAPTER 4. GENERIC PROGRAMMING
Data Structures Algorithms
vector
set
map
queue
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Iterators
Figure 4.2: Central role of iterators in STL
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Evidently, not all algorithms can be implemented on every data structure. Which algorithm
works on a given data structure depends on the kind of iterator provided by the container.
Iterators can be distinguished by the form of access:
InputIterator: an iterator concept for reading the referred entries.
OutputIterator: an iterator concept for writing to the referred entries.
Note that the ability to write does not imply readability, e.g., an ostream iterator is an STL
interface used to write to output streams like files opened in write mode. Another differentiation
of iterators is the form of traversal:
ForwardIterator: a concept for iterators that can pass from one element to the next, i.e. types
that provide an operator++. It is a refinement of InputIterator and OutputIterator. In contrast
to those, ForwardIterator they allows for traversing multiple times.
BidirectionalIterator: a concept for iterators with step-wise forward and backward traversal,
i.e. types with operator++ and operator−−. It refines ForwardIterator.
RandomAccessIterator: a concept for iterators that can increment their position by an arbitrary
integer, i.e. types that also provide operator[]. It refines BidirectionalIterator.
Data structures that provide more refined iterators (e.g. modeling RandomAccessIterator) can be
used in more algorithms. Dually, algorithm implementations that require less refined iterators
(like InputIterator) can be applied to more data structures. The interfaces are designed with
backward compatibility in mind and old-style pointers can be used as iterators.
All standard container templates provide a rich and consistent set of iterator types. The
following very simple example shows a typical use of iterators:
std::list l ;
for (std::list::const iterator it = l.begin(); it != l.end(); ++it) {
std::cout ≪ ∗it ≪ std::endl;
}
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