C++ The Curiously Recurring Template Pattern (CRTP)


CRTP is a powerful, static alternative to virtual functions and traditional inheritance that can be used to give types properties at compile time. It works by having a base class template which takes, as one of its template parameters, the derived class. This permits it to legally perform a static_cast of its this pointer to the derived class.

Of course, this also means that a CRTP class must always be used as the base class of some other class. And the derived class must pass itself to the base class.


Let's say you have a set of containers that all support the functions begin() and end(). The standard library's requirements for containers require more functionality. We can design a CRTP base class that provides that functionality, based solely on begin() and end():

#include <iterator>
template <typename Sub>
class Container {
    // self() yields a reference to the derived type
    Sub& self() { return *static_cast<Sub*>(this); }
    Sub const& self() const { return *static_cast<Sub const*>(this); }

    decltype(auto) front() {
      return *self().begin();

    decltype(auto) back() {
      return *std::prev(self().end());

    decltype(auto) size() const {
      return std::distance(self().begin(), self().end());

    decltype(auto) operator[](std::size_t i) {
      return *std::next(self().begin(), i);

The above class provides the functions front(), back(), size(), and operator[] for any subclass which provides begin() and end(). An example subclass is a simple dynamically allocated array:

#include <memory>
// A dynamically allocated array
template <typename T>
class DynArray : public Container<DynArray<T>> {
    using Base = Container<DynArray<T>>;

    DynArray(std::size_t size)
      : size_{size},
    { }

    T* begin() { return data_.get(); }
    const T* begin() const { return data_.get(); }
    T* end() { return data_.get() + size_; }
    const T* end() const { return data_.get() + size_; }

    std::size_t size_;
    std::unique_ptr<T[]> data_;

Users of the DynArray class can use the interfaces provided by the CRTP base class easily as follows:

DynArray<int> arr(10);
arr.front() = 2;
arr[2] = 5;
assert(arr.size() == 10);

Usefulness: This pattern particularly avoids virtual function calls at run-time which occur to traverse down the inheritance hierarchy and simply relies on static casts:

DynArray<int> arr(10);
DynArray<int>::Base & base = arr;
base.begin(); // no virtual calls

The only static cast inside the function begin() in the base class Container<DynArray<int>> allows the compiler to drastically optimize the code and no virtual table look up happens at runtime.

Limitations: Because the base class is templated and different for two different DynArrays it is not possible to store pointers to their base classes in an type-homogenous array as one could generally do with normal inheritance where the base class is not dependent on the derived type:

class A {};
class B: public A{};

A* a = new B;