Re: What's polymorphic in CRTP

From:

=?ISO-8859-1?Q?=D6=F6_Tiib?= <ootiib@hot.ee>

Newsgroups:

comp.lang.c++

Date:

Fri, 15 Feb 2013 08:33:01 -0800 (PST)

Message-ID:

<b52682e9-1a69-4a18-8091-bcd8a6ba8774@googlegroups.com>

On Friday, 15 February 2013 18:18:47 UTC+2, Cholo Lennon wrote:

How about this?

It is example that works but does not achieve much else but inefficiency.
;-)

#include <iostream>

template<class T>
class Base {

Note that T is meant to be derived class.
So you can not use Base<Derived>* as dynamically polymorphic interface
pointer for several different "Derived"s.

public:
   void callDynamic()
   {
     onDynamic();
   }

   void callStatic()
   {
     T* pThis = static_cast<T*>(this);
     pThis->onStatic();
   }

   virtual void onDynamic()
   {
     std::cout << "Base::onDynamic" << std::endl;
   }

   void onStatic()
   {
     std::cout << "Base::onStatic" << std::endl;
   }

};

class Derived: public Base<Derived> {

public:

   virtual void onDynamic()
   {
     std::cout << "Derived::onDynamic" << std::endl;
   }

   // "pseudo virtual" function :-)
   void onStatic()
   {
     std::cout << "Derived::onStatic" << std::endl;
   }
};

int main(int, char**)
{
   Derived derived;
   derived.callDynamic();
   derived.callStatic();
}

Also, try commenting out functions in Derived class to see what happens

Just try making it with Derived1, Derived2 and Derived3 and you see where you
went wrong. Do not mix CRTP with dynamic polymorphism. Use typical
pure abstract interface base and multiple inheritance side by side with
CRTP if you need to mix them.