Curiously Recurring Template Pattern

 

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The Curiously Recurring Template Pattern is an idiom in which a class X derives from a class template Y, taking a template parameter Z, where Y is instantiated with Z = X. For example,

template<class Z>
class Y {};
 
class X : public Y<X> {};

Example

CRTP may be used to implement "compile-time polymorphism", when a base class exposes an interface, and derived classes implement such interface.

Run this code

#include <cstdio>
 
#ifndef __cpp_explicit_this_parameter // Traditional syntax
 
template <class Derived>
struct Base
{
    void name() { static_cast<Derived*>(this)->impl(); }
protected:
    Base() = default; // prohibits the creation of Base objects, which is UB
};
struct D1 : public Base<D1> { void impl() { std::puts("D1::impl()"); } };
struct D2 : public Base<D2> { void impl() { std::puts("D2::impl()"); } };
 
#else // C++23 deducing-this syntax
 
struct Base { void name(this auto&& self) { self.impl(); } };
struct D1 : public Base { void impl() { std::puts("D1::impl()"); } };
struct D2 : public Base { void impl() { std::puts("D2::impl()"); } };
 
#endif
 
int main()
{
    D1 d1; d1.name();
    D2 d2; d2.name();
}

Output:

D1::impl()
D2::impl()

See also

Explicit object member functions (deducing this) (C++23)
enable_shared_from_this (C++11)	allows an object to create a shared_ptr referring to itself (class template)
ranges::view_interface (C++20)	helper class template for defining a view, using the curiously recurring template pattern (class template)