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{{Short description|Software design pattern}}
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]].▼
▲The '''curiously recurring template pattern''' ('''CRTP''')
==History==
The technique was formalized in 1989 as "''F''-bounded quantification."<ref>{{cite web|url=http://
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
== General form ==
<
// The Curiously Recurring Template Pattern (CRTP)
template <class T>
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// ...
};
</syntaxhighlight>{{clarify|reason=members within Base<T> can use what template and what would that look like?|date=June 2025}}
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=978-0-470-68420-7 | year=2010}}</ref>
In addition, CRTP is used by the C++ standard library to implement the <code>std::enable_shared_from_this</code> functionality.<ref>{{cite web |title=std::enable_shared_from_this |url=https://en.cppreference.com/w/cpp/memory/enable_shared_from_this |access-date=22 November 2022}}</ref>
== Static polymorphism ==
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Typically, the base class template will take advantage of the fact that member function bodies (definitions) are not instantiated until long after their declarations, and will use members of the derived class within its own member functions, via the use of a [[Type conversion|cast]]; e.g.:
<
template <class T>
struct Base
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};
struct Derived : public Base<Derived>
{
void implementation();
static void static_sub_func();
};
</syntaxhighlight>
In the above example,
This technique achieves a similar effect to the use of [[virtual function]]s, without the costs (and some flexibility) of [[dynamic polymorphism]]. This particular use of the CRTP has been called "simulated dynamic binding" by some.<ref>{{cite web | url=http://www.pnotepad.org/devlog/archives/000083.html | title=Simulated Dynamic Binding | date=7 May 2003 | accessdate=13 January 2012 | url-status=dead | archiveurl=https://web.archive.org/web/20120209045146/http://www.pnotepad.org/devlog/archives/000083.html | archivedate=9 February 2012
To elaborate on the above example, consider a base class with '''no virtual functions'''. Whenever the base class calls another member function, it will always call its own base class functions. When we derive a class from this base class, we inherit all the member variables and member functions that
However, if base class member functions use CRTP for all member function calls, the overridden functions in the derived class will be selected at compile time. This effectively emulates the virtual function call system at compile time without the costs in size or function call overhead ([[Virtual method table|VTBL]] structures, and method lookups, multiple-inheritance VTBL machinery) at the disadvantage of not being able to make this choice at runtime.
== Object counter ==
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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 | date=April 1998 | url=http://www.drdobbs.com/cpp/counting-objects-in-c/184403484}}</ref> This can be easily solved using CRTP:
<
template <typename T>
struct counter
{
static inline int objects_created = 0;
static inline int objects_alive = 0;
counter()
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}
};
class X : counter<X>
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// ...
};
</syntaxhighlight>
Each time an object of class <code>X</code> is created, the constructor of <code>counter<X></code> is called, incrementing both the created and alive count. Each time an object of class <code>X</code> is destroyed, the alive count is decremented. It is important to note that <code>counter<X></code> and <code>counter<Y></code> are two separate classes and this is why they will keep separate counts of <code>X</code>
== Polymorphic chaining ==
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When the named parameter object pattern is applied to an object hierarchy, things can go wrong. Suppose we have such a base class:
<
class Printer
{
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ostream& m_stream;
};
</syntaxhighlight>
Prints can be easily chained:
<
Printer
</syntaxhighlight>
However, if we define the following derived class:
<
class CoutPrinter : public Printer
{
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}
};
</syntaxhighlight>
we "lose" the concrete class as soon as we invoke a function of the base:
<
// v----- we have a 'Printer' here, not a 'CoutPrinter'
CoutPrinter().print("Hello ").SetConsoleColor(Color.red).println("Printer!"); // compile error
</syntaxhighlight>
This happens because 'print' is a function of the base
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>
<
// Base class
template <typename ConcretePrinter>
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// usage
CoutPrinter().print("Hello ").SetConsoleColor(Color.red).println("Printer!");
</syntaxhighlight>
==Polymorphic copy construction==
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When using polymorphism, one sometimes needs to create copies of objects by the base class pointer. A commonly used idiom for this is adding a virtual clone function that is defined in every derived class. The CRTP can be used to avoid having to duplicate that function or other similar functions in every derived class.
<
// Base class has a pure virtual function for cloning
class AbstractShape {
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Shape() = default;
Shape(const Shape&) = default;
Shape(Shape&&) = default;
};
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class Circle : public Shape<Circle>{};
</syntaxhighlight>
This allows obtaining copies of squares, circles or any other shapes by <code>shapePtr->clone()</code>.
===Pitfalls===
One issue with static polymorphism is that without using a general base class like
==Deducing this==
The use of CRTP can be simplified using the [[C++23]] feature ''deducing this''.<ref>{{Cite web|url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0847r7.html|title=Deducing this|date=2021-07-12|author1=Gašper Ažman|author2=Sy Brand|author3=Ben Deane|author4=Barry Revzin
}}</ref><ref>{{Cite web|title=Explicit object parameter|url=https://en.cppreference.com/w/cpp/language/member_functions#Explicit_object_parameter|access-date=27 December 2023}}</ref> For the function <code>signature_dish</code> to call a derived member function <code>cook_signature_dish</code>, <code>ChefBase</code> needs to be a templated type and <code>CafeChef</code> needs to inherit from <code>ChefBase</code>, passing its type as the template parameter.
<syntaxhighlight lang="cpp">
template <typename T>
struct ChefBase
{
void signature_dish()
{
static_cast<T*>(this)->cook_signature_dish();
}
};
struct CafeChef : ChefBase<CafeChef>
{
void cook_signature_dish() {}
};
</syntaxhighlight>
If explicit object parameter is used, <code>ChefBase</code> does not need to be templated and <code>CafeChef</code> can derive from <code>ChefBase</code> plainly. Since the <code>self</code> parameter is automatically deduced as the correct derived type, no casting is required.
<syntaxhighlight lang="cpp">
struct ChefBase
{
template <typename Self>
void signature_dish(this Self&& self)
{
self.cook_signature_dish();
}
};
struct CafeChef : ChefBase
{
void cook_signature_dish() {}
};
</syntaxhighlight>
==See also==
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{{reflist}}
{{C++ programming language}}
▲{{use dmy dates|date=January 2012}}
[[Category:Software design patterns]]
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