Curiously recurring template pattern: Difference between revisions

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The '''curiously recurring template pattern''' ('''CRTP''') is an idiom in [[C++]] in which a class <code>X</code> derives from a class [[Template (C++)|template]] instantiation using <code>X</code> itself as template argument.<ref>{{cite book | first1=David | last1=Abrahams | authorlink1=David Abrahams (computer programmer) | 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 }}</ref> More generally it is known as '''F-bound polymorphism''', and it is a form of [[F-bounded quantification|''F''-bounded quantification]].

==History==

The technique was formalized in 1989 as "''F''-bounded quantification."<ref>{{cite web|url=http://staff.ustc.edu.cn/~xyfeng/teaching/FOPL/lectureNotes/CookFBound89.pdf|title=F-Bounded Polymorphism for Object-Oriented Programming|author=William Cook|date=1989|display-authors=etal}}</ref> The name "CRTP" was independently coined by [[Jim Coplien]] in 1995,<ref>{{cite journal | author=Coplien, James O. | title=Curiously Recurring Template Patterns | journal=C++ Report | yeardate=February 1995 | pages=24–27 | url=http://sites.google.com/a/gertrudandcope.com/info/Publications/InheritedTemplate.pdf}}</ref> who had observed it in some of the earliest [[C++]] template code

as well as in code examples that [[Timothy Budd]] created in his multiparadigm language Leda.<ref>{{cite book | first=Timothy | last=Budd | authorlink=Timothy Budd | title=[[Multiparadigm programming in Leda]] | publisher=Addison-Wesley | isbn=0-201-82080-3 | year=1994| title-link=Multiparadigm programming in Leda }}</ref> It is sometimes called "Upside-Down Inheritance"<ref>{{Cite web|url=http://www.apostate.com/programming/atlupsidedown.html |title=Apostate Café: ATL and Upside-Down Inheritance |date=2006-03-15 |access-date=2016-10-09 |url-status=bot: unknown |archiveurl=https://web.archive.org/web/20060315072824/http://www.apostate.com/programming/atlupsidedown.html |archivedate=15 March 2006 |df=dmy }}</ref><ref>{{Cite web|url=http://archive.devx.com/free/mgznarch/vcdj/1999/julmag99/atlinherit1.asp |title=ATL and Upside-Down Inheritance |date=2003-06-04 |access-date=2016-10-09 |url-status=bot: unknown |archiveurl=https://web.archive.org/web/20030604104137/http://archive.devx.com/free/mgznarch/vcdj/1999/julmag99/atlinherit1.asp |archivedate=4 June 2003 |df=dmy }}</ref> due to the way it allows class hierarchies to be extended by substituting different base classes.

The Microsoft Implementation of CRTP in [[Active Template Library]] (ATL) was independently discovered, also in 1995 by Jan Falkin who accidentally derived a base class from a derived class. Christian Beaumont first saw Jan's code and initially thought it couldn't possibly compile in the Microsoft compiler available at the time. Following this revelation that it did indeed work, Christian based the entire ATL and [[Windows Template Library]] (WTL) design on this mistake.{{Citation needed|date=August 2018}}

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Some use cases for this pattern are [[Template metaprogramming#Static polymorphism|static polymorphism]] and other metaprogramming techniques such as those described by [[Andrei Alexandrescu]] in ''[[Modern C++ Design]]''.<ref>{{cite book | first=Andrei | last=Alexandrescu | authorlink=Andrei Alexandrescu | title=[[Modern C++ Design]]: Generic Programming and Design Patterns Applied | publisher=Addison-Wesley | isbn=0-201-70431-5 | year=2001}}</ref>

It also figures prominently in the C++ implementation of the [[Data, Context, and Interaction]] paradigm.<ref>{{cite book | first1=James | last1=Coplien | authorlink1=James Coplien | first2=Gertrud | last2=Bjørnvig | title=[[Lean Architecture]]: for agile software development | publisher=Wiley | isbn=978-0-470-68420-87 | year=2010}}</ref>

== Static polymorphism ==

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== Object counter ==

The main purpose of an object counter is retrieving statistics of object creation and destruction for a given class.<ref>{{cite journal | author=Meyers, Scott | title=Counting Objects in C++ | journal=C/C++ Users Journal | yeardate=April 1998 | url=http://www.drdobbs.com/cpp/counting-objects-in-c/184403484}}</ref> This can be easily solved using CRTP:

<source lang="cpp">

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This happens because 'print' is a function of the base - 'Printer' - and then it returns a 'Printer' instance.

The CRTP can be used to avoid such problem and to implement "Polymorphic chaining":<ref>{{cite web|last1=Arena|first1=Marco|title=Use CRTP for polymorphic chaining|url=https://marcoarena.wordpress.com/2012/04/29/use-crtp-for-polymorphic-chaining/|accessdate=15 March 2017|date=29 April 2012}}</ref>

<source lang="cpp>