Template metaprogramming: Difference between revisions

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{{Short description|Metaprogramming technique}}

{{More footnotes needed|date=June 2010}}

{{Programming paradigms}}

'''Template metaprogramming''' ('''TMP''') is a [[metaprogramming]] technique in which [[Generic programming|templates]] are used by a [[compiler]] to generate temporary [[source code]], which is merged by the compiler with the rest of the source code and then compiled. The output of these templates can include [[compile time|compile-time]] [[constant (programming)|constant]]s, [[data structure]]s, and complete [[function (computer science)|function]]s. The use of templates can be thought of as [[Compile-time function execution|compile-time polymorphism]]. The technique is used by a number of languages, the best-known being [[C++]], but also [[Curl programming language|Curl]], [[D programming language|D]], [[Nim (programming language)|Nim]], and [[XL Programming Language|XL]].

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==Compile-time code optimization==

{{see also|Compile-time function execution}}

The factorial example above is one example of compile-time code optimization in that all factorials used by the program are pre-compiled and injected as numeric constants at compilation, saving both run-time overhead and [[memory footprint]]. It is, however, a relatively minor optimization.

As another, more significant, example of compile-time [[loop unrolling]], template metaprogramming can be used to create length-''n'' vector classes (where ''n'' is known at compile time). The benefit over a more traditional length-''n'' vector is that the loops can be unrolled, resulting in very optimized code. As an example, consider the addition operator. A length-''n'' vector addition might be written as

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constexpr int OFFSET = 12;

template<typename VALUETYPE, VALUETYPEint OFFSET>

constexpr std::array<VALUETYPE, TABLE_SIZE> table = [] { // OR: constexpr auto table

std::array<VALUETYPE, TABLE_SIZE> A = {};

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==Benefits and drawbacks of template metaprogramming==

; Compile-time versus execution-time tradeofftradeoffs get :visible Ifif a great deal of template metaprogramming is used.

;* [[Generic programming]] : Template metaprogramming allows the programmer to focus on architecture and delegate to the compiler the generation of any implementation required by client code. Thus, template metaprogramming can accomplish truly [[Generic programming|generic code]], facilitating code minimization and better maintainability{{Citation needed|date=June 2014}}.

; Readability :* With respect to C++ prior to version ''C++11'', the syntax and idioms of template metaprogramming were esoteric compared to conventional C++ programming, and template metaprograms could be very difficult to understand.<ref>{{cite documentweb

| first1 = K.

| last1 = Czarnecki

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|quote=''C++ Template Metaprogramming suffers from a number of limitations, including portability problems due to compiler limitations (although this has significantly improved in the last few years), lack of debugging support or IO during template instantiation, long compilation times, long and incomprehensible errors, poor readability of the code, and poor error reporting.''

| ref = Czarnecki, O’Donnell, Striegnitz, Taha - DSL implementation in metaocaml, template haskell, and C++

}}</ref><ref>{{cite documentweb

| first1 = Tim

| last1 = Sheard

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* {{cite book | authorlink1 = David Abrahams (computer programmer) | first1 = David | last1 = Abrahams | authorlink2 = Aleksey Gurtovoy | first2 = Aleksey | last2 = Gurtovoy | title = C++ Template Metaprogramming: Concepts, Tools, and Techniques from Boost and Beyond | publisher = Addison-Wesley | isbn = 0-321-22725-5 | date = January 2005 }}

* {{cite book | authorlink1 = David Vandevoorde | first1 = David | last1 = Vandevoorde | authorlink2 = Nicolai M. Josuttis | first2 = Nicolai M. | last2 = Josuttis | title = C++ Templates: The Complete Guide | publisher = Addison-Wesley | isbn = 0-201-73484-2 | year = 2003 }}

* {{cite book | authorlink = Manuel Clavel | first = Manuel | last = Clavel | title = Reflection in Rewriting Logic: Metalogical Foundations and Metaprogramming Applications | isbn = 1-57586-238-7 | date = 2000-10-16 | publisher = Cambridge University Press }}

==External links==

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* {{cite web | url = https://wiki.haskell.org/Template_Haskell | title = Template Haskell }} (type-safe metaprogramming in Haskell)

* {{cite web | authorlink = Walter Bright | first = Walter | last = Bright | url = http://www.digitalmars.com/d/templates-revisited.html | title = Templates Revisited }} (template metaprogramming in the [[D programming language]])

* {{cite web | url = http://staff.ustc.edu.cn/~xyfeng/teaching/FOPL/lectureNotes/MetaprogrammingCpp.pdf | title = Metaprogramming in C++ | authorlink = Johannes Koskinen | first = Johannes | last = Koskinen | access-date = 2014-06-20 | archive-date = 2014-08-28 | archive-url = https://web.archive.org/web/20140828190250/http://staff.ustc.edu.cn/~xyfeng/teaching/FOPL/lectureNotes/MetaprogrammingCpp.pdf | url-status = dead }}

* {{cite web | url = http://lcgapp.cern.ch/project/architecture/ReflectionPaper.pdf | title = Reflection support by means of template metaprogramming | first1 = Giuseppe | last1 = Attardi | first2 = Antonio | last2 = Cisternino | access-date = 2008-10-24 | archive-date = 2016-03-03 | archive-url = https://web.archive.org/web/20160303183212/http://lcgapp.cern.ch/project/architecture/ReflectionPaper.pdf | url-status = dead }}

* {{cite CiteSeerX | citeseerx = 10.1.1.14.5881 | title = Static data structures | first1 = Michael C. | last1 = Burton | first2 = William G. | last2 = Griswold | first3 = Andrew D. | last3 = McCulloch | first4 = Gary A. | last4 = Huber | year = 2002 }}

* {{cite web | url = http://www.codeproject.com/Articles/19989/Template-Meta-Programming-and-Number-Theory | title = Template Meta Programming and Number Theory | first = Zeeshan | last = Amjad | date = 13 August 2007 }}

* {{cite web | url = http://www.codeproject.com/Articles/20180/Template-Meta-Programming-and-Number-Theory-Part | title = Template Meta Programming and Number Theory: Part 2 | first = Zeeshan | last = Amjad | date = 24 August 2007 }}

* {{cite web | url = http://www.intelib.org/intro.html | title = A library for LISP-style programming in C++ }}

{{Programming paradigms navbox}}

[[Category:Metaprogramming]]