Re: What's polymorphic in CRTP

From:
Jarek Blakarz <jumianek@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 12 Feb 2013 07:55:59 -0800 (PST)
Message-ID:
<0f9c9e20-d86b-471d-8bed-c737cc1e28d8@googlegroups.com>
On Monday, February 11, 2013 5:38:03 PM UTC+1, =D6=F6 Tiib wrote:

On Monday, 11 February 2013 17:40:04 UTC+2, Jarek Blakarz wrote:
 

Hi

 

 

 

I understand what is CRTP. What I don't understand is why CRTP is calle=

d a

 

static polymorphism or rather polymorphism in particular.

 
 
 
It is not called static polymorphism. It is common technique in C++ to ac=

hieve

 
static polymorphism.
 
 
 

I think that it is "static" since the function call is resolved at comp=

ile

 

time, right ?

 
 
 
Yes. You are correct. Exactly like there are arrays with static size
 
(std::array<T,N>) and dynamic size (std::vector<T>). Static size means lo=

st

 
flexibility (that is maybe not needed) and won efficiency (that is always
 
great to have).
 
 
 

Assuming that I'm right about "static", plese explain me based on the=

 

 

following example why it is "polymorphic".

 
 
 

template <class Derived> struct Base {

 

    void interface() { static_cast<Derived*>(this)->implementation(); }

 
 
 
This may or not be bare-bone interface. This is basic part of implementat=

ion

 
that calls possibly "statically overriden" member function from derived c=

lass.

 
You can even provide default implementation here:
 
 
 
      void implementation() { }
 
 
 

};

 

 

 

struct Derived1 : Base<Derived1> {

 

    void implementation() { }

 

};

 

 

 

struct Derived2 : Base<Derived2> {

 

    void implementation() { }

 

};

 
 
 
Lets take you have default and also such Derived3 here:
 
 
 
 struct Derived3 : Base<Derived3> {
 
 };
 
 
 
 
 

Derived1 *d1 = new Derived1;

 

Derived2 *d2 = new Derived2;

 

d1->interface();

 

d2->interface();

 
 
 
Your example leaks memory and feels like Java with its tons of news at
 
each corner. C++11 just forgot to add 'std::make_unique<T>()', if that
 
was in language then need to write 'new' or 'delete' in C++ code was
 
close to none.
 
 
 
 Derived1 d1;
 
 Derived2 d2;
 
 Derived3 d3;
 
 d1.interface();
 
 d2.interface();
 
 d3.interface();
 
 
 
It is polymorphism. All 3 calls may do different things. All 3 calls have
 
part (possibly everything) of what they do inherited from Base, and part
 
(possibly nothing) overriden. All 3 calls have exactly same common user
 
interface.
 
 
 
It is subtly less flexible and more efficient than virtual call ... so
 
better *you* explain ... what is your problem of calling it polymorphism?


Thanks for explanation to everyone.
My goal was just to understand why it is called polymorphism.
Earlier I only suspected why. Now I think I got it.

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