Curiously recurring template pattern

Software design pattern

The curiously recurring template pattern (CRTP) is an idiom, originally in C++, in which a class X derives from a class template instantiation using X itself as a template argument. More generally it is known as F bound polymorphism, and it is a form of F bounded quantification.

The technique was formalized in 1989 as "F bounded quantification." The name "CRTP" was independently coined by Jim Coplien in 1995, 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. It is sometimes called "Upside Down Inheritance" 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 could not possibly compile in the Microsoft compiler available at the time. Following the revelation that it did indeed work, Christian based the entire ATL and Windows Template Library (WTL) design on this mistake.

One issue with static polymorphism is that without using a general base class like AbstractShape from the above example, derived classes cannot be stored homogeneously – that is, putting different types derived from the same base class in the same container. For example, a container defined as std::vector<Shape*> does not work because Shape is not a class, but a template needing specialization. A container defined as std::vector<Shape<Circle>*> can only store Circles, not Squares. This is because each of the classes derived from the CRTP base class Shape is a unique type. A common solution to this problem is to inherit from a shared base class with a virtual destructor, like the AbstractShape example above, allowing for the creation of a std::vector<AbstractShape*>.