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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> {};

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();
    Base<D2> b2; b2.name();
    D1 d1; d1.name();
    D2 d2; d2.name();
}
 
#else // C++23 alternative syntax; https://godbolt.org/z/KbG8bq3oP
 
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();
}

Possible output:

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

[edit] See also

allows an object to create a shared_ptr referring to itself
(class template) [edit]
helper class template for defining a view, using the curiously recurring template pattern
(class template) [edit]