Standard Template Library
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The Standard Template Library (STL) is a software library. It is part of the C++ Standard Library describing containers, iterators, and algorithms.
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Overview
The STL has been a major boon for C++ programmers: it gives programmers a ready-made set of common classes, 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.
The STL achieves this result through the heavy use of templates. While this approach is very powerful, the resulting complicated code was (and sometimes still is) a problem for many compilers, which sometimes failed to compile valid constructs, produced invalid code, or required the programmer to put in extra effort to get things to work.
The C++ Standard Library is defined by ISO/IEC 14882.
The Standard Template Library was created as the first library of generic algorithms and data structures, with four ideas in mind: generic programming, abstractness without loss of efficiency, the Von Neumann computation model, and value semantics.
History
The architecture of STL is largely the creation of one person, Alexander Stepanov. In 1979 he began working out his initial ideas of generic programming and exploring their potential for revolutionizing software development. Although Dave Musser had developed and advocated some aspects of generic programming as early as 1971, it was limited to a rather specialized area of software development (computer algebra).
Stepanov recognized the full potential for generic programming and persuaded his then-colleagues at General Electric Research and Development (including, primarily, Dave Musser and Deepak Kapur) that generic programming should be pursued as a comprehensive basis for software development. At the time there was no real support in any programming language for generic programming.
The first major language to provide such support was Ada, with its generic units feature. By 1987 Stepanov and Musser had developed and published an Ada library for list processing that embodied the results of much of their research on generic programming. However, Ada had not achieved much acceptance outside the defense industry and C++ seemed more likely to become widely used and provide good support for generic programming even though the language was relatively immature (it did not even have templates, added only later). Another reason for turning to C++, which Stepanov recognized early on, was that the C/C++ model of computation which allows very flexible access to storage via pointers is crucial to achieving generality without losing efficiency.
Much research and experimentation were needed, not just to develop individual components, but to develop an overall architecture for a component library based on generic programming. First at AT&T Bell Laboratories and later at Hewlett-Packard Research Labs, Stepanov experimented with many architectural and algorithm formulations, first in C and later in C++. Musser collaborated in this research and in 1992 Meng Lee joined Stepanov's project at HP and became a major contributor.
This work undoubtedly would have continued for some time as just a research project or at best would have resulted in an HP proprietary library if Andrew Koenig of Bell Labs had not become aware of the work and asked Stepanov to present the main ideas at a November 1993 meeting of the ANSI/ISO committee for C++ standardization. The committee's response was overwhelmingly favorable and led to a request from Koenig for a formal proposal in time for the March 1994 meeting. Despite the tremendous time pressure, Alex and Meng were able to produce a draft proposal that received preliminary approval at that meeting.
The committee had several requests for changes and extensions (some of them major), and a small group of committee members met with Stepanov and 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 to Musser. It would have been quite easy for the whole enterprise to spin out of control at this point, but again Stepanov and Lee met the challenge and produced a proposal that received final approval at the July 1994 ANSI/ISO committee meeting. (Additional details of this history can be found in an interview Alexander Stepanov gave in the March 1995 issue of Dr. Dobb's Journal.)
Subsequently, the Stepanov and Lee document 17 was incorporated into the ANSI/ISO C++ draft standard (1, parts of clauses 17 through 27). It also influenced other parts of the C++ Standard Library, such as the string facilities, and some of the previously adopted standards in those areas were revised accordingly.
In spite of STL's success with the committee, there remained the question of how STL would make its way into actual availability and use. With the STL requirements part of the publicly available draft standard, compiler vendors and independent software library vendors could have course develop their own implementations and market them as separate products or as selling points for their other wares. One of the first edition's authors, Atul Saini, was among the first to recognize the commercial potential and began exploring it as a line of business for his company, Modena Software Incorporated, even before STL had been fully accepted by the committee.
The prospects for early widespread dissemination of STL were considerably improved with Hewlett-Packard's decision to make its implementation freely available on the Internet in August 1994. This implementation, developed by Stepanov, Lee, and Musser during the standardization process, became the basis of all implementations offered by compiler and library vendors today.
Contents
Containers
The STL contains sequence containers and associative containers. The standard sequence containers include vector, string and deque. The standard associative containers are set, multiset, map and multimap.
vector - is a C-like array (i.e. capable of random access) with the ability to automatically resize itself when inserting or erasing an object. Inserting and removing an element to/from back of the vector at the end takes constant time. Inserting and erasing at the beginning or in the middle is linear in time.
deque (double ended queue) - a vector with insertion/erase at the beginning in amortized constant time, however lacking some guarantees on iterator validity after altering the deque.
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.
Libraries implementing STL often include hashed variants: hash_set, hash_multiset, hash_map and hash_multimap, however this extension is not part of standard and are defined in various namespaces among implementations as a result.
Iterators
The STL implements five different types of iterators. These are input iterators, output iterators, forward iterators, bidirectional iterators and random access iterators.
Functors
The STL includes classes that overload the function operator (operator()). Classes that do this are called functor classes or function classes. They are useful for keeping and retrieving state information in functions passed into other functions.
References
- Scott Meyers, Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library ISBN 0201749629
External links
- Collection of Links for the STL (http://dmoz.org/Computers/Programming/Languages/C%2b%2b/Class_Libraries/STL/)
- SGI Introduction to the STL (http://www.sgi.com/tech/stl/stl_introduction.html)de:Standard_Template_Library
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