The STL map class is used for the storage and retrieval of data from a collection in which the each element is a pair that has both a data value and a sort key. The value of the key is unique and is used to order the data automatically. The value of an element in a map, but not its associated key value, may be changed directly. Instead, key values associated with old elements must be deleted and new key values associated with new elements inserted.
template < class Key, class Type, class Traits = less<Key>, class Allocator=allocator<pair <const Key, Type> > > class map
- The key data type to be stored in the map.
- The element data type to be stored in the map.
- The type that provides a function object that can compare two element values as sort keys to determine their relative order in the map. This argument is optional and the binary predicate less<Key> is the default value.
- The type that represents the stored allocator object that encapsulates details about the map's allocation and deallocation of memory. This argument is optional and the default value is allocator<pair <const Key, Type> >.
The STL map class is:
- An associative container, which a variable size container that supports the efficient retrieval of element values based on an associated key value.
- Reversible, because it provides bidirectional iterators to access its elements.
- Sorted, because its elements are ordered by key values within the container in accordance with a specified comparison function.
- Unique in the sense that each of its elements must have a unique key.
- A pair associative container, because its element data values are distinct from its key values.
- A template class, because the functionality it provides is generic and so independent of the specific type of data contained as elements or keys. The data types to be used for elements and keys are, instead, specified as parameters in the class template along with the comparison function and allocator.
The iterator provided by the map class is a bidirectional iterator, but the class member functions insert and map have versions that take as template parameters a weaker input iterator, whose functionality requirements are more minimal than those guaranteed by the class of bidirectional iterators. The different iterator concepts form a family related by refinements in their functionality. Each iterator concept has its own set of requirements and the algorithms that work with them must limit their assumptions to the requirements provided by that type of iterator. It may be assumed that an input iterator may be dereferenced to refer to some object and that it may be incremented to the next iterator in the sequence. This is a minimal set of functionality, but it is enough to be able to talk meaningfully about a range of iterators [_First, _Last) in the context of the class's member functions.
The choice of container type should be based in general on the type of searching and inserting required by the application. Associative containers are optimized for the operations of lookup, insertion and removal. The member functions that explicitly support these operations are efficient, performing them in a time that is on average proportional to the logarithm of the number of elements in the container. Inserting elements invalidates no iterators, and removing elements invalidates only those iterators that had specifically pointed at the removed elements.
The map should be the associative container of choice when the conditions associating the values with their keys are satisfied by the application. A model for this type of structure is an ordered list of uniquely occurring key words with associated string values providing, say, definitions. If, instead, the words had more than one correct definition, so that keys were not unique, then a multimap would be the container of choice. If, on the other hand, just the list of words were being stored, then a set would be the correct container. If multiple occurrences of the words were allowed, then a multiset would be the appropriate container structure.
The map orders the sequence it controls by calling a stored function object of type key_compare. This stored object is a comparison function that may be accessed by calling the member function key_comp. In general, the elements need be merely less than comparable to establish this order: so that, given any two elements, it may be determined either that they are equivalent (in the sense that neither is less than the other) or that one is less than the other. This results in an ordering between the non-equivalent elements. On a more technical note, the comparison function is a binary predicate that induces a strict weak ordering in the standard mathematical sense. A binary predicate f(x,y) is a function object that has two argument objects x and y and a return value of true or false. An ordering imposed on a set is a strict weak ordering if the binary predicate is irreflexive, antisymmetric, and transitive and if equivalence is transitive, where two objects x and y are defined to be equivalent when both f(x,y) and f(y,x) are false. If the stronger condition of equality between keys replaces that of equivalence, then the ordering becomes total (in the sense that all the elements are ordered with respect to each other) and the keys matched will be indiscernible from each other.