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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. [1] More generally it is known as F-bound polymorphism, and it is a form of F-bounded quantification.
The C++ examples in this section demonstrate the principle of using composition and interfaces to achieve code reuse and polymorphism. Due to the C++ language not having a dedicated keyword to declare interfaces, the following C++ example uses inheritance from a pure abstract base class.
Polymorphism can be distinguished by when the implementation is selected: statically (at compile time) or dynamically (at run time, typically via a virtual function). This is known respectively as static dispatch and dynamic dispatch, and the corresponding forms of polymorphism are accordingly called static polymorphism and dynamic polymorphism.
Examples are templates in C++, and generic programming in Fortran and other languages, in conjunction with function overloading (including operator overloading). Code is said to be monomorphised , with specific data types deduced and traced through the call graph , in order to instantiate specific versions of generic functions , and select ...
For example, the use of the << operator in C++ a << b shifts the bits in the variable a left by b bits if a and b are of an integer type, but if a is an output stream then the above code will attempt to write a b to the stream.
Roberts (p. 171) gives a related example in Java, using a Class to represent a stack frame. The example given is a solution to the Tower of Hanoi problem wherein a stack simulates polymorphic recursion with a beginning, temporary and ending nested stack substitution structure. [5]
C++ also allows a single instance of the multiple class to be created via the virtual inheritance mechanism (i.e. Worker::Human and Musician::Human will reference the same object). Common Lisp CLOS attempts to provide both reasonable default behavior and the ability to override it.
The problem is that more than one degree of polymorphism exist: one for dispatching the display_on method to an object and another for selecting the right code (or method) for displaying. A much cleaner and more maintainable solution is then to do a second dispatch, this time for selecting the right method for displaying the object on the medium: