Standard Template Library

Not to be confused withC++ Standard Library.

For other uses, seeSTL.

TheStandard Template Library(STL) is asoftware libraryoriginally designed byAlexander Stepanovfor theC++programming language that influenced many parts of theC++ Standard Library.It provides four components calledalgorithms,containers,functions,anditerators.

The STL provides a set of commonclassesfor C++, such as containers andassociative 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 oftemplates.This approach providescompile-time polymorphismthat is often more efficient than traditionalrun-time polymorphism.Modern C++compilersare tuned to minimize abstraction penalties 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,abstractnesswithout loss of efficiency, theVon Neumann computation model,andvalue semantics.

The STL and theC++ Standard Libraryare two distinct entities.

History

Main article:History of the Standard Template Library

In November 1993Alexander Stepanovpresented a library based on generic programming to theANSI/ISO committeefor C++ standardization. The committee's response was overwhelmingly favorable and led to a request fromAndrew Koenigfor a formal proposal in time for the March 1994 meeting. The committee had several requests for changes and extensions and the committee members met with Stepanov and Meng Lee to help work out the details. The requirements for the most significant extension (associative containers) had to be shown to be consistent by fully implementing them, a task Stepanov delegated toDavid Musser.A proposal received final approval at the July 1994 ANSI/ISO committee meeting. Subsequently, the Stepanov and Lee document 17 was incorporated into the ANSI/ISO C++ draft standard (1, parts of clauses 17 through 27).

The prospects for early widespread dissemination of the STL were considerably improved with Hewlett-Packard's decision to make its implementation freely available on theInternetin August 1994. This implementation, developed by Stepanov, Lee, and Musser during the standardization process, became the basis of many implementations offered by compiler and library vendors today.

Composition

Containers

The STL contains sequencecontainersand associative containers. The containers are objects that store data. The standardsequence containersincludevector,deque,andlist.The standardassociative containersareset,multiset,map,multimap,hash_set,hash_map,hash_multisetandhash_multimap.There are alsocontainer adaptorsqueue,priority_queue,andstack,that are containers with specific interface, using other containers as implementation.

Container	Description
Simple containers
pair	The pair container is a simple associative container consisting of a 2-tupleof data elements or objects, called 'first' and 'second', in that fixed order. The STL 'pair' can be assigned, copied and compared. The array of objects allocated in a map or hash_map (described below) are of type 'pair' by default, where all the 'first' elements act as the unique keys, each associated with their 'second' value objects.
Sequences(arrays/linked lists): ordered collections
vector	adynamic array,likeCarray (i.e., capable ofrandom access) with the ability to resize itself automatically when inserting or erasing an object. Inserting an element to the back of the vector at the end takesamortized constant time.Removing the last element takes only constant time, because no resizing happens. Inserting and erasing at the beginning or in the middle is linear in time. A specialization for typeboolexists, which optimizes for space by storingboolvalues as bits.
list	adoubly 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).
slist	asingly 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). It has slightly more efficient insertion and deletion, and uses less memory than a doubly linked list, but can only be iterated forwards. It is implemented in the C++ standard library asforward_list.
deque(double-endedqueue)	a vector with insertion/erase at the beginning or end inamortized constant time,however lacking some guarantees on iterator validity after altering the deque.
Container adaptors
queue	ProvidesFIFOqueueinterface in terms ofpush/pop/front/backoperations. Any sequence supporting operationsfront(),back(),push_back(),andpop_front()can be used to instantiate queue (e.g.listanddeque).
priority queue	Providespriority queueinterface in terms ofpush/pop/topoperations (the element with the highest priority is on top). Anyrandom-accesssequence supporting operationsfront(),push_back(),andpop_back()can be used to instantiate priority_queue (e.g.vectoranddeque). It is implemented using aheap. Elements should additionally support comparison (to determine which element has a higher priority and should be popped first).
stack	ProvidesLIFOstackinterface in terms ofpush/pop/topoperations (the last-inserted element is on top). Any sequence supporting operationsback(),push_back(),andpop_back()can be used to instantiate stack (e.g.vector,list,anddeque).
Associative containers:unordered collections
set	a mathematicalset;inserting/erasing elements in a set does not invalidate iterators pointing in the set. Provides set operationsunion,intersection,difference,symmetric differenceand test of inclusion. Type of data must implement comparison operator<or custom comparator function must be specified; such comparison operator or comparator function must guaranteestrict weak ordering,otherwise behavior is undefined. Typically implemented using aself-balancing binary search tree.
multiset	same as a set, but allows duplicate elements (mathematicalmultiset).
map	anassociative 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; such comparison operator or comparator function must guaranteestrict weak ordering,otherwise behavior is undefined. Typically 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 ahash table;keys are not ordered, but ahash functionmust exist for the key type. These types were left out of the C++ standard; similar containers were standardized inC++11,but with different names (unordered_setandunordered_map).
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. Provides random access.
valarray	Another array data type, intended for numerical use (especially to representvectorsandmatrices); the C++ standard allows specific optimizations for this intended purpose. According to Josuttis,valarraywas badly designed, by people "who left the [C++ standard] committee a long time before the standard was finished", andexpression templatelibraries are to be preferred.A proposed rewrite of thevalarraypart of the standard in this vein was rejected, instead becoming a permission to implement it using expression template.

Iterators

The STL implements five different types ofiterators.These areinput iterators(that can only be used to read a sequence of values),output iterators(that can only be used to write a sequence of values),forward iterators(that can be read, written to, and move forward),bidirectional iterators(that are like forward iterators, but can also move backwards) andrandom-access iterators(that can move freely any number of steps in one operation).

A fundamental STL concept is arangewhich is a pair of iterators that designate the beginning and end of the computation, and most of the library's algorithmic templates that operate on data structures have interfaces that use ranges.

It is possible to have bidirectional iterators act like random-access iterators, so 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 that 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 anddeques.User-createdcontainersonly have to provide an iterator that implements one of the five 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 anassociative containersuch 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 activities 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 that provides an interface by iterators). Searching algorithms likebinary_searchandlower_boundusebinary searchand like sorting algorithms require that the type of data must implement comparison operator<or custom comparator function must be specified; such comparison operator or comparator function must guaranteestrict weak ordering.Apart from these, algorithms are provided for making heap from a range of elements, generating lexicographically ordered permutations of a range of elements, merge sorted ranges and performunion,intersection,differenceof sorted ranges.

Functors

The STL includes classes thatoverloadthe function call operator (operator()). Instances of such classes are called functors orfunction objects.Functors allow the behavior of the associated function to be parameterized (e.g. through arguments passed to the functor'sconstructor) and can be used to keep associated per-functor state information along with the function. Since both functors and function pointers can be invoked using the syntax of a function call, they are interchangeable as arguments to templates when the corresponding parameter only appears in function call contexts.

A particularly common type of functor is thepredicate.For example, algorithms likefind_iftake aunarypredicate that operates on the elements of a sequence. Algorithms like sort, partial_sort, nth_element and all sortedcontainersuse abinarypredicate that must provide astrict weak ordering,that is, it must behave like amembershiptest on a transitive, non-reflexive and asymmetricbinary relation.If none is supplied, these algorithms and containers uselessby default, which in turn calls the less-than-operator <.

Criticisms

Quality of implementation of C++ compilers

The Quality of Implementation (QoI) of the C++ compiler has a large impact on usability of the STL (and templated code in general):

• Error messages involving templates tend to be very long and difficult to decipher. This problem has been considered so severe that a number of tools have been written that simplify andprettyprintSTL-related error messages to make them more comprehensible.
• Careless use of templates can lead tocode bloat.This has been countered with special techniques within STL implementations (e.g. using void* containers internally and other "diet template" techniques) and improving compilers' optimization techniques. However, this symptom is similar to naively manually copying a set of functions to work with a different type, in that both can be avoided with care and good technique.
• Template instantiation can increase compilation time and memory usage, in exchange for typically reducing runtime decision-making (e.g. via virtual functions). Until the compiler technology improves enough, this problem can be only partially eliminated by careful coding, avoiding certain idioms, and simply not using templates where they are not appropriate or where compile-time performance is prioritized.

Other issues

• Initialization of STLcontainerswith constants within the source code is not as easy as data structures inherited from C (addressed inC++11withinitializer lists).
• STL containers are not intended to be used as base classes (their destructors are deliberately non-virtual); deriving from a container is a common mistake.
• Theconceptof iterators as implemented by the STL can be difficult to understand at first: for example, if a value pointed to by the iterator is deleted, the iterator itself is then no longer valid. This is a common source of errors. Most implementations of the STL provide a debug mode that is slower, but can locate such errors if used. A similar problem exists in other languages, for exampleJava.Rangeshave been proposed as a safer, more flexible alternative to iterators.
• Certain iteration patterns such as callback enumeration APIs cannot be made to fit the STL model without the use ofcoroutines,which were outside the C++ standard until C++20.
• Compiler compliance does not guarantee thatAllocatorobjects, used for memory management forcontainers,will work with state-dependent behavior. For example, a portable library can not define an allocator type that will pull memory from different pools using different allocator objects of that type. (Meyers, p. 50) (addressed inC++11).
• The set of algorithms is not complete: for example, thecopy_ifalgorithm was left out,though it has been added inC++11.

Implementations

• Original STL implementation by Stepanov and Lee. 1994,Hewlett-Packard.No longer maintained.
  • SGI STL, based on original implementation by Stepanov & Lee. 1997,Silicon Graphics.No longer maintained.
    • STLPort,based on SGI STL
    • Rogue Wave Standard Library(HP, SGI,SunSoft,Siemens-Nixdorf)
    • Apache C++ Standard Library(The starting point for this library was the 2005 version of the Rogue Wave standard library)
    • Libstdc++uses code derived from SGI STL for the algorithms and containers defined inC++03.
• Dinkum STL library byP.J. Plauger
• TheMicrosoft STLwhich ships with Visual C++ is a licensed derivative of Dinkum's STL.Source is available on Github.
• EASTL,developed by Paul Pedriana at Electronic Arts and published as part ofEA Open Source.

See also

• List of C++ template libraries
• C++11
• Boost C++ Libraries

References

• Alexander Stepanovand Meng Lee,The Standard Template Library. HP Laboratories Technical Report 95-11(R.1), 14 November 1995. (Revised version of A. A. Stepanov and M. Lee: The Standard Template Library, Technical Report X3J16/94-0095, WG21/N0482, ISO Programming Language C++ Project, May 1994.)
• Alexander Stepanov (2007).Notes on Programming(PDF).Stepanov reflects about the design of the STL.
• Nicolai M. Josuttis(2000).The C++ Standard Library: A Tutorial and Reference.Addison-Wesley.ISBN0-201-37926-0.
• Scott Meyers(2001).Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library.Addison-Wesley.ISBN0-201-74962-9.
• Al Stevens(March 1995)."Al Stevens Interviews Alex Stepanov".Dr. Dobb's Journal.Retrieved18 July2007.
• David VandevoordeandNicolai M. Josuttis(2002).C++ Templates: The Complete Guide.Addison-Wesley Professional.ISBN0-201-73484-2.
• Atul SainiandDavid R. Musser.STL Tutorial and Reference Guide: C+ + Programming with the Standard Template Library. Foreword by Alexander Stepanov; Copyright Modena Software Inc.Addison-Wesley.ISBN0-201-63398-1.

External links

• C++ reference
• C++ STL reference,includes C++11 features
• STL programmer's guidefromSGI.Originally at[1](retired content).
• Apache (formerly Rogue Wave) C++ Standard Library Class Reference
• Apache (formerly Rogue Wave) C++ Standard Library User Guide
• Bjarne Stroustrup on The emergence of the STL(Page 5, Section 3.1)
• C++ Standard Specification