subject: Standard Template Library [print this page] Overview Overview
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2007)
The STL provides a ready-made set of common classes for C++, such as containers and associative arrays, that can be used with any built-in type and with any user-defined type that supports some elementary operations (such as copying and assignment). STL algorithms are independent of containers, which significantly reduces the complexity of the library.
The STL achieves its results through the use of templates. This approach provides compile-time polymorphism that is often more efficient than traditional run-time polymorphism. Modern C++ compilers are tuned to minimize any abstraction penalty arising from heavy use of the STL.
The STL was created as the first library of generic algorithms and data structures for C++, with four ideas in mind: generic programming, abstractness without loss of efficiency, the Von Neumann computation model, and value semantics.
Contents
Containers
The STL contains sequence containers and associative containers. The standard sequence containers include vector, deque, and list. The standard associative containers are set, multiset, map, and multimap. There are also container adaptors queue, priority_queue, and stack, that are containers with specific interface, using other containers as implementation.
a dynamic array, like C array (i.e., capable of random access) with the ability to resize itself automatically when inserting or erasing an object. Inserting and removing an element to/from back of the vector at the end takes amortized constant time. Inserting and erasing at the beginning or in the middle is linear in time.
A specialization for type bool exists, which optimizes for space by storing bool values as bits.
list
a doubly-linked list; elements are not stored in contiguous memory. Opposite performance from a vector. Slow lookup and access (linear time), but once a position has been found, quick insertion and deletion (constant time).
deque (double ended queue)
a vector with insertion/erase at the beginning or end in amortized constant time, however lacking some guarantees on iterator validity after altering the deque.
Container adaptors
queue
Provides FIFO queue interface in terms of push/pop/front/back operations.
Any sequence supporting operations front(), back(), push_back(), and pop_front() can be used to instantiate queue (e.g. list and deque).
priority_queue
Provides priority queue interface in terms of push/pop/top operations (the element with the highest priority is on top).
Any random-access sequence supporting operations front(), push_back(), and pop_front() can be used to instantiate priority_queue (e.g. vector and deque).
Elements should additionally support comparison (to determine which element has a higher priority and should be popped first).
stack
Provides LIFO stack interface in terms of push/pop/top operations (the last-inserted element is on top).
Any sequence supporting operations back(), push_back(), and pop_back() can be used to instantiate stack (e.g. vector, list, and deque).
Associative containers - unordered collections
set
a mathematical set; inserting/erasing elements in a set does not invalidate iterators pointing in the set. Provides set operations union, intersection, difference, symmetric difference and test of inclusion. Type of data must implement comparison operator < or custom comparator function must be specified. Implemented using a self-balancing binary search tree.
multiset
same as a set, but allows duplicate elements.
map
an associative array; allows mapping from one data item (a key) to another (a value). Type of key must implement comparison operator < or custom comparator function must be specified. Implemented using a self-balancing binary search tree.
multimap
same as a map, but allows duplicate keys.
hash_set
hash_multiset
hash_map
hash_multimap
similar to a set, multiset, map, or multimap, respectively, but implemented using a hash table; keys are not ordered, but a hash function must exist for the key type. These containers are not part of the C++ Standard Library, but are included in SGI's STL extensions, and are included in common libraries such as the GNU C++ Library in the __gnu_cxx namespace. These are scheduled to be added to the C++ standard as part of TR1, with the slightly different names of unordered_set, unordered_multiset, unordered_map and unordered_multimap.
Other types of containers
bitset
stores series of bits similar to a fixed-sized vector of bools. Implements bitwise operations and lacks iterators. Not a Sequence.
valarray
another C-like array like vector, but is designed for high speed numerics at the expense of some programming ease and general purpose use. It has many features that make it ideally suited for use with vector processors in traditional vector supercomputers and SIMD units in consumer-level scalar processors, and also ease vector mathematics programming even in scalar computers.
Iterators
The STL implements five different types of iterators. These are input iterators (which can only be used to read a sequence of values), output iterators (which can only be used to write a sequence of values), forward iterators (which can be read, written to, and move forward), bidirectional iterators (which are like forward iterators but can also move backwards) and random access iterators (which can move freely any number of steps in one operation).
It is possible to have bidirectional iterators act like random access iterators, as moving forward ten steps could be done by simply moving forward a step at a time a total of ten times. However, having distinct random access iterators offers efficiency advantages. For example, a vector would have a random access iterator, but a list only a bidirectional iterator.
Iterators are the major feature which allow the generality of the STL. For example, an algorithm to reverse a sequence can be implemented using bidirectional iterators, and then the same implementation can be used on lists, vectors and deques. User-created containers only have to provide an iterator which implements one of the 5 standard iterator interfaces, and all the algorithms provided in the STL can be used on the container.
This generality also comes at a price at times. For example, performing a search on an associative container such as a map or set can be much slower using iterators than by calling member functions offered by the container itself. This is because an associative container's methods can take advantage of knowledge of the internal structure, which is opaque to algorithms using iterators.
Algorithms
A large number of algorithms to perform operations such as searching and sorting are provided in the STL, each implemented to require a certain level of iterator (and therefore will work on any container which provides an interface by iterators).
Functors
The STL includes classes that overload the function operator (operator()). Classes that do this are called functors or function objects. They are useful for keeping and retrieving state information in functions passed into other functions. Regular function pointers can also be used as functors.
A particularly common type of functor is the predicate. For example, algorithms like find_if take a unary predicate that operates on the elements of a sequence. Algorithms like sort, partial_sort, nth_element and all sorted containers use a binary predicate which must provide a strict weak ordering, that is, it must behave like a membership test on a transitive, irreflexive and antisymmetric binary relation. If none is supplied, these algorithms and containers use less by default, which in turn calls the less-than-operator
