Curiously Recurring Template Pattern
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.
#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) |
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(C++11) | allows an object to create a shared_ptr referring to itself (class template) |
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(C++20) | helper class template for defining a view, using the curiously recurring template pattern (class template) |
External links
| 1. | Replace CRTP with concepts? — Sandor Drago's blog |
| 2. | The Curiously Recurring Template Pattern (CRTP) — Sandor Drago's blog |
| 3. | The Curiously Recurring Template Pattern (CRTP) - 1 — Fluent{C++} |
| 4. | What the CRTP can bring to your code - 2 — Fluent{C++} |
| 5. | An implementation helper for the CRTP - 3 — Fluent{C++} |
| 6. | What is the Curiously Recurring Template Pattern (CRTP) — SO |
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