What is the curiously recurring template pattern (CRTP)?

Question

Without referring to a book, can anyone please provide a good explanation for CRTP with a code example?

Answer 1

CRTP is a technique to implement compile-time polymorphism. Here's a very simple example. In the below example, ProcessFoo() is working with Base class interface and Base::Foo invokes the derived object's foo() method, which is what you aim to do with virtual methods.

http://coliru.stacked-crooked.com/a/2d27f1e09d567d0e

template 
struct Base {
  void foo() {
    (static_cast(this))->foo();
  }
};

struct Derived : public Base {
  void foo() {
    cout << "derived foo" << endl;
  }
};

struct AnotherDerived : public Base {
  void foo() {
    cout << "AnotherDerived foo" << endl;
  }
};

template
void ProcessFoo(Base* b) {
  b->foo();
}


int main()
{
    Derived d1;
    AnotherDerived d2;
    ProcessFoo(&d1);
    ProcessFoo(&d2);
    return 0;
}

Output:

derived foo
AnotherDerived foo