| | Root class of all the maps, provides utilitites
for managing the buckets.
Maps are dynamically extended data structures where
data is quickly accessed with a key.
General properties of maps
- Map items may be (complex) non-unitary data; they
may be difficult to manage with an array. Moreover, the
map allows a data structure to be indexed by complex data.
- The size of a map is dynamically extended. So a map
may be first dimensioned for a little number of items.
Maps avoid the use of large and quasi-empty arrays.
- The access to a map item is much faster than the one
to a sequence, a list, a queue or a stack item.
- The access time to a map item may be compared with
the one to an array item. First of all, it depends on the
size of the map. It also depends on the quality of a user
redefinable function (the hashing function) to find
quickly where the item is.
- The exploration of a map may be of better performance
than the exploration of an array because the size of the
map is adapted to the number of inserted items.
These properties explain why maps are commonly used as
internal data structures for algorithms.
Definitions
- A map is a data structure for which data is addressed by keys.
- Once inserted in the map, a map item is referenced as an entry of the map.
- Each entry of the map is addressed by a key. Two
different keys address two different entries of the map.
- The position of an entry in the map is called a bucket.
- A map is dimensioned by its number of buckets, i.e. the
maximum number of entries in the map. The
performance of a map is conditioned by the number of buckets.
- The hashing function transforms a key into a bucket
index. The number of values that can be computed by
the hashing function is equal to the number of buckets of the map.
- Both the hashing function and the equality test
between two keys are provided by a hasher object.
- A map may be explored by a map iterator. This
exploration provides only inserted entries in the map
(i.e. non empty buckets).
Collections' generic maps
The Collections component provides numerous generic derived maps.
- These maps include automatic management of the
number of buckets: they are automatically resized when
the number of keys exceeds the number of buckets. If
you have a fair idea of the number of items in your map,
you can save on automatic resizing by specifying a
number of buckets at the time of construction, or by using
a resizing function. This may be considered for crucial optimization issues.
- Keys, items and hashers are parameters of these generic derived maps.
- TCollection_MapHasher class describes the
functions required by any hasher which is to be used
with a map instantiated from the Collections component.
- An iterator class is automatically instantiated at the
time of instantiation of a map provided by the
Collections component if this map is to be explored
with an iterator. Note that some provided generic maps
are not to be explored with an iterator but with indexes (indexed maps).
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1.6.3