C++ for Scientists - Technische Universität Dresden

4.10. CURSORS AND PROPERTY MAPS 123
As illustrated above, iterators are usually used in pairs, where one is used for the actual iteration
and the second serves to mark the end of the collection. The iterators are created by the
corresponding container class using standard methods such as begin() and end(). The iterator
returned by begin() points to the first element. The iterator returned by end() points past the
end of elements to mark the end. All algorithms are implemented with right-open intervals
[b, e) operating on the value referred by b until b = e. Therefore intervals of the form [x, x) are
regarded empty.
A more general (and more useful) algorithm is the linear search on an arbitrary sequence. This
is provided by the STL function find in the following fashion:
template
InputIterator find(InputIterator first, InputIterator last, const T& value) {
while (first != last && ∗first != value)
++first;
return first;
}
find takes three arguments: two iterators that define the right-open interval of the search space,
and a value to search for in that range. Each entry referred by ‘first’ is compared with ‘value’.
When a match is found, the iterator pointing to it is returned. If the value is not contained
in the sequence, an iterator equal to ‘last’ is returned. Thus, the caller can test whether the
search was successful by comparing its result with ‘last’. In fact, one must perform this test
because after a failed search the returned iterator cannot dereferred correctly (it points outside
the given range and might cause segmentation violations or corrupt data).
This section only scratched the surface of STL and was primarily intended to introduce the
iterator concept that we will generalize in the following section.
4.10 Cursors and Property Maps
The essential idea of iterators is to represent a position and a referred value. A further generalization
of this idea is to decouple the the notion of position and value. Dietmar Kühl
proposed this mechanism in his master thesis (Diplomarbeit) [?] for the generic treatment of
grahps. The Boost Graph Library [?] provides the notion of property maps in the form that
properties are available for vertices and edges and all properties can be accessed independently
from each other and from the traversal of the graph.
As case study we implement a simple sparse matrix class with cursors and property maps. The
minimalistic implementation of the sparse matrix is:
#include
#include
#include
#include
template
class coo matrix
{
typedef Value value type; // better in trait
public:
coo matrix(int nr, int nc) : nr(nr), nc(nc) {}