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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]].
Template metaprogramming was, in a sense, discovered accidentally.<ref name="Meyers2005">{{cite book|author=Scott Meyers|title=Effective C++: 55 Specific Ways to Improve Your Programs and Designs|url=https://books.google.com/books?id=Qx5oyB49poYC&q=%22Template+metaprogramming%22|date=12 May 2005|publisher=Pearson Education|isbn=978-0-13-270206-5}}</ref><ref>See [[wikibooks:C++ Programming/Templates/Template Meta-Programming#History of TMP|History of TMP]] on Wikibooks</ref>
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==Components of template metaprogramming==
The use of templates as a metaprogramming technique requires two distinct operations: a template must be defined, and a defined template must be [[Instance (computer science)|instantiated]]. The
Template metaprogramming is [[Turing-complete]], meaning that any computation expressible by a computer program can be computed, in some form, by a template metaprogram.<ref name=Veldhuizen2003>{{cite
Templates are different from ''[[Macro (computer science)#Programming macros|macros]]''. A macro is a piece of code that executes at compile time and either performs textual manipulation of code to-be compiled (e.g. [[C++]] macros) or manipulates the [[abstract syntax tree]] being produced by the compiler (e.g. [[Rust (programming language)|Rust]] or [[Lisp (programming language)|Lisp]] macros). Textual macros are notably more independent of the syntax of the language being manipulated, as they merely change the in-memory text of the source code right before compilation.
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What exactly "programming at compile-time" means can be illustrated with an example of a factorial function, which in non-template C++ can be written using recursion as follows:
<syntaxhighlight lang=cpp>
unsigned
return n == 0 ? 1 : n * factorial(n - 1);
}
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By using template metaprogramming and template specialization to provide the ending condition for the recursion, the factorials used in the program—ignoring any factorial not used—can be calculated at compile time by this code:
<syntaxhighlight lang=cpp>
template <unsigned
struct factorial {
};
template <>
struct factorial<0> {
};
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To be able to use templates in this manner, the compiler must know the value of its parameters at compile time, which has the natural precondition that factorial<X>::value can only be used if X is known at compile time. In other words, X must be a constant literal or a constant expression.
In [[C++11]] and [[C++20]], [[constexpr]] and consteval were introduced to let the compiler execute code. Using constexpr and consteval, one can use the usual recursive factorial definition with the non-templated syntax.<ref>
==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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};
</syntaxhighlight>
Here the base class template will take advantage of the fact that member function bodies are not instantiated until after their declarations, and it will use members of the derived class within its own member functions, via the use of a <code>static_cast</code>, thus at compilation generating an object composition with polymorphic characteristics. As an example of real-world usage, the CRTP is used in the [[Boost library|Boost]] [[iterator]] library.<ref>{{Cite web|url=http://www.boost.org/libs/iterator/doc/iterator_facade.html|title = Iterator Facade - 1.79.0}}</ref>
Another similar use is the "[[Barton–Nackman trick]]", sometimes referred to as "restricted template expansion", where common functionality can be placed in a base class that is used not as a contract but as a necessary component to enforce conformant behaviour while minimising code redundancy.
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int main() {
for (int i=0; i < TABLE_SIZE; i++) {
std::cout << table[i] << std::endl; // run time use
}
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int main() {
for (int i=0; i < TABLE_SIZE; i++) {
std::cout << table[i] << std::endl; // run time use
}
std::cout << "FOUR: " << FOUR << std::endl;
}
</syntaxhighlight>
To show a more sophisticated example the code in the following listing has been extended to have a helper for value calculation (in preparation for more complicated computations), a table specific offset and a template argument for the type of the table values (e.g. uint8_t, uint16_t, ...).
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int main() {
for (int i = 0; i < TABLE_SIZE; i++) {
std::cout << table[i] << std::endl;
}
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constexpr int OFFSET = 12;
template<typename VALUETYPE,
constexpr std::array<VALUETYPE, TABLE_SIZE> table = [] { // OR: constexpr auto table
std::array<VALUETYPE, TABLE_SIZE> A = {};
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int main() {
for (int i = 0; i < TABLE_SIZE; i++) {
std::cout << table<uint16_t, OFFSET>[i] << std::endl;
}
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==Concepts==
The C++20 standard brought C++ programmers a new tool for meta template programming, concepts.<ref>{{Cite web|url=https://en.cppreference.com/w/cpp/language/constraints|title=Constraints and concepts (since C++20) - cppreference.com|website=en.cppreference.com}}</ref>
[[Concepts (C++)|Concepts]] allow programmers to specify requirements for the type, to make instantiation of template possible. The compiler looks for a template with the concept that has the highest requirements.
Here is an example of the famous [[Fizz buzz]] problem solved with Template Meta Programming.
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/**
* By specializing `res` structure, with concepts requirements, proper
*/
template<typename X> struct res;
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/**
* Predeclaration of
*/
template <size_t cnt, typename... Args>
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==Benefits and drawbacks of template metaprogramming==
| 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
| first1 = Tim
| last1 = Sheard
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| quote = ''Robinson’s provocative paper identifies C++ templates as a major, albeit accidental, success of the C++ language design. Despite the extremely baroque nature of template meta-programming, templates are used in fascinating ways that extend beyond the wildest dreams of the language designers. Perhaps surprisingly, in view of the fact that templates are functional programs, functional programmers have been slow to capitalize on C++’s success''
| ref = Sheard, S.P.Jones - Template Meta-programming for Haskell
}}</ref> But from C++11 onward the syntax for value computation metaprogramming becomes more and more akin to "normal" C++, with less and less readability penalty.
==See also==
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* [[Parametric polymorphism]]
* [[Expression templates]]
* [[Variadic
* [[Compile-time function execution]]
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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
* {{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]]
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