The STL container class set is used for the storage and retrieval of data from a collection in which the values of the elements contained are unique and serve as the key values according to which the data is automatically ordered. The value of an element in a set may not be changed directly. Instead, you must delete old values and insert elements with new values.
For a list of all members of this type, see.
The element data type to be stored in the set.
The type that provides a function object that can compare two element values as sort keys to determine their relative order in the set. 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 set's allocation and deallocation of memory. This argument is optional, and the default value is allocator<Key>.
An STL set is:
An associative container, which a variable size container that supports the efficient retrieval of element values based on an associated key value. Further, it is a simple associative container because its element values are its key values.
Reversible, because it provides a bidirectional iterator 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. Since set is also a simple associative container, its elements are also unique.
A set is also described as a template class because the functionality it provides is generic and independent of the specific type of data contained as elements. The data type to be used is, instead, specified as a parameter in the class template along with the comparison function and allocator.
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 set should be the associative container of choice when the conditions associating the values with their keys are satisfied by the application. The elements of a set are unique and serve as their own sort keys. A model for this type of structure is an ordered list of, say, words in which the words may occur only once. If multiple occurrences of the words were allowed, then a multiset would be the appropriate container structure. If unique definitions were attached as values to the list of key words, then a map would be an appropriate structure to contain this data. If instead the definitions were not unique, then a multimap would be the container of choice.
The set 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 to 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 nonequivalent 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.
The iterator provided by the set class is a bidirectional iterator, but the class member functions insert and set 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.