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

From:

Sana <sanatakos@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 20 Feb 2009 06:47:44 -0800 (PST)

Message-ID:

<91f11357-79e7-46b7-ab13-fb8b3a0597b9@n2g2000vba.googlegroups.com>

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.

virtual Base* clone() const = 0;

--
sana