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> {};CRTP may be used to implement "compile-time polymorphism", when a base class exposes an interface, and derived classes implement such interface.
#include <cstdio>
#ifndef __cpp_explicit_this_parameter // Traditional syntax
template <class Derived>
struct Base { void name() { (static_cast<Derived*>(this))->impl(); } };
struct D1 : public Base<D1> { void impl() { std::puts("D1::impl()"); } };
struct D2 : public Base<D2> { void impl() { std::puts("D2::impl()"); } };
void test()
{
// Base<D1> b1; b1.name(); //undefined behavior
// Base<D2> b2; b2.name(); //undefined behavior
D1 d1; d1.name();
D2 d2; d2.name();
}
#else // C++23 alternative syntax; https://godbolt.org/z/s1o6qTMnP
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()"); } };
void test()
{
D1 d1; d1.name();
D2 d2; d2.name();
}
#endif
int main()
{
test();
}Output:
D1::impl() D2::impl()
|
(C++11) | allows an object to create a shared_ptr referring to itself (class template) |
|
(C++20) | helper class template for defining a view, using the curiously recurring template pattern (class template) |
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https://en.cppreference.com/w/cpp/language/crtp