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

James Kanze <>
Sun, 1 Mar 2009 16:31:30 -0800 (PST)
On Feb 20, 3:25 pm, Juha Nieminen <nos...@thanks.invalid> wrote:

I'm sure this is a frequently asked question, but...

Suppose that I have a hierarchy of objects which are typically
allocated dynamically and handled with some smart pointer or
whatever. However, I want to be able to create copies of
those objects, while having only a pointer-to-base-class. The
solution is to write a clone() member function like this:

class Base
    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
    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.

Isn't there any way of automating this process? (And without
making the base class a template class, which would be rather
tedious if the base class is large, especially since we don't
have export templates in practice.)

I would imagine some development tools would have a means of
automatically generating the function. Otherwise, blargg and
Noah Roberts have proposed an interesting solution, albeit with
some runtime overhead (an additional dynamic
allocation---probably not significant for the types of things
which usually get cloned). But is the extra code really a
problem? It's not that much. The "classical" solution settles
for a runtime check:

    class Base
        Base* clone() const
            Base* result = doClone() ;
            assert( typeid( *result ) == typeid( *this ) ) ;
            return result ;

        virtual Base* doClone() const = 0 ;
    } ;

(Actually, the classical solution doesn't do anything special.
In practice, I've never found forgetting to implement the clone
function in a derived class to be a real problem. All the more
so in that hierarchies which support cloning rarely contain
classes which derive from a concrete type anyway, so making
clone() pure virtual in the base class suffices to ensure a
compiler error.)

James Kanze (GABI Software)
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