Re: Correct vesion (Re: Clone an object with an abstract base class)

From:

JiiPee <no@notvalid.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 12 Oct 2014 01:37:29 +0100

Message-ID:

<i3k_v.625607$m41.496202@fx34.am4>

On 12/10/2014 01:31, Ian Collins wrote:

JiiPee wrote:

On 12/10/2014 01:20, Mr Flibble wrote:

On 12/10/2014 01:18, Mr Flibble wrote:

On 12/10/2014 00:55, JiiPee wrote:

On 12/10/2014 00:26, JiiPee wrote:

How about checking the type of the *b object first and creating that
type of object:

void f2(B *b) {
     B *b1 = nullptr;
     if( typeid(*b).name() == typeid(D1).name() )
         b1 = new D1(*b1); // *b is type D1
     else if( typeid(*b).name() == typeid(D2).name() )
         b1 = new D2(*b1); // *b is type D2
}

and maybe also *b1 must be converted to its type like:
new D1( *((D1*)b1) )
?

this is better and compiles:

void f2(B *b) {
      B *b1 = nullptr;
      if( typeid(*b).name() == typeid(D1).name() )
          b1 = new D1(*(reinterpret_cast<D1*>(b1)));
      else if( typeid(*b).name() == typeid(D2).name() )
          b1 = new D2(*(reinterpret_cast<D2*>(b1)));
}

You should use static_cast not reinterpret_cast when casting from base
to derived.

Of course your solution doesn't scale: what if b points to an object
derived from D1?

Much better solution is to have a pure virtual clone() method.

/Flibble

Then that new derived class should be added, so one if- more. But this
is also the case if more classes are derived from from B; they all need
to have their own if-block.

Why is that better than a clone method?

he asked: "Is there a way to do this in C++98 without the need to add a
new function like clone() in each derived class?"

I guess I was just trying answer his this question: in my solution there
is no need to create clone().
But yes, the clone might be better.