Automatically create a clone() function for derived classes - comp.lang.c++

Message from discussion Automatically create a clone() function for derived classes

Victor Bazarov

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More options Feb 20 2009, 3:09 pm

Newsgroups: comp.lang.c++

From: Victor Bazarov <v.Abaza...@comAcast.net>

Date: Fri, 20 Feb 2009 10:09:50 -0500

Local: Fri, Feb 20 2009 3:09 pm

Subject: Re: Automatically create a clone() function for derived classes

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Sana wrote:
> On Feb 20, 9:25 am, Juha Nieminen <nos...@thanks.invalid> wrote:
>> class Base
>> {
>> public:
>> virtual Base* clone() const { return new Base(*this); }

>> // other stuff here

>> };

>> The problem is that this clone() function must be replicated in all
>> the derived classes as well, and their implementation is always the
>> same, except for the type which is being allocated. For example:

>> class Derived: public Base
>> {
>> public:
>> virtual Base* clone() const { return new Derived(*this); }

>> };

>> It's not only tedious to have to write this line in each of the
>> derived classes, but it's also error-prone: If you ever forget to add
>> that line to some derived class, no compiler error will happen, and the
>> program will even run, but when cloning a derived object using a base
>> type pointer, you get a crippled copy, so the whole thing malfunctions,
>> and it might malfunction in a very non-obvious way which is hard to
>> track and debug.

> Make the Base::clone() method pure and don't provide a body. this way,
> if the derived class doesn't implement the clone() method, the linker
> will complain.

Then the problem would be with a derived class of a derived class (a
"grandchild" of Base). Once the derived has implemented the clone
function, any of its own derived classes won't be forced to do that.

And that's actually reflected in the original inquiry since Base::clone
is *not* pure. IOW there is no simple way to force (or provide) the
implementation of such a "clone" function in the derived class *in case*
the base class' member is not pure.

In certain circles, a macro is used

class Derived : public Base {
DECLARE_BASE(Base);
DECLARE_CLONABLE(Derived);

which expands to declare the 'clone' member (with some possible
compile-time checks etc.) It's a bit better than declaring/defining the
whole separate member, but the programmer still needs to use it in order
to accomplish the goal at hand.

V
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