Problem copying objects.

[esuvs81]

Hi, imagine I have a simple hierarchy with three classes - 'Base',
'Derived1', and Derived2. Base is an absrtact class and the two Derived
classes are concrete. Imagine at some point in my code I have a pointer
p1 of type 'Base' which actually points to an instance of one of the
derived objects. I would like to create a new pointer p2, also of type
'Base', which points to a copy of whatever the first one pointed to.
That is, I would like to copy the object at p1 without knowing it's
exact type. Is that possible?

My first attempt was something like:

Base* p2 = new Base(*p1);

But this didn't make sense because i was trying to instanciate an
abstract class. Maybe i actually need to know the exact type? Another
approach I thought of was to give each class a virtual function called
'copy' which would duplicate the object and return a pointer to the
duplicate. Does this make sense or is there a better way?

Thanks for any help,

David

 

[Jakob Bieling]

Hi, imagine I have a simple hierarchy with three classes - 'Base',
'Derived1', and Derived2. Base is an absrtact class and the two
Derived classes are concrete. Imagine at some point in my code I have
a pointer p1 of type 'Base' which actually points to an instance of
one of the derived objects. I would like to create a new pointer p2,
also of type 'Base', which points to a copy of whatever the first one
pointed to. That is, I would like to copy the object at p1 without
knowing it's exact type. Is that possible?

approach I thought of was to give each class a virtual function called
'copy' which would duplicate the object and return a pointer to the
duplicate. Does this make sense or is there a better way?

Exactly, this is known as a 'virtual constructor'. Personally, I
find the name a bit misleading, as it does not have anything to do with
making the _constructor_ of the object virtual (because this is
impossible). Instead, you use a regular virtual function, as you
described, which will return a copy of the appropriate type. This is how
it works:

class base
{
public:
virtual base* clone () const = 0;
};

class derived1
{
public
virtual derived1* clone () const { return new derived1; }
};

class derived2
{
public
virtual derived2* clone () const { return new derived2; }
};

int main ()
{
base* b1 = new derived1;
base* b2 = b1->clone ();
};

hth

 

[esuvs81]

Exactly, this is known as a 'virtual constructor'. Personally, I
find the name a bit misleading, as it does not have anything to do with
making the _constructor_ of the object virtual (because this is
impossible). Instead, you use a regular virtual function, as you
described, which will return a copy of the appropriate type. This is how
it works:

class base
{
public:
virtual base* clone () const = 0;
};

class derived1
{
public
virtual derived1* clone () const { return new derived1; }
};

class derived2
{
public
virtual derived2* clone () const { return new derived2; }
};

int main ()
{
base* b1 = new derived1;
base* b2 = b1->clone ();
};

hth

Great, thanks. I'll try that approach. I figured it would be a fairly
common problem and there would be some kind of design pattern for it
but didn't know what it was called. Thanks again.

David

 

[Thomas J. Gritzan]

class base
{
public:
virtual base* clone () const = 0;
};

class derived1
{
public
virtual derived1* clone () const { return new derived1; }

virtual derived1* clone () const { return new derived1(*this); }

You want to clone here, don't you?

};

class derived2
{
public
virtual derived2* clone () const { return new derived2; }

virtual derived2* clone () const { return new derived2(*this); }

 

[Jakob Bieling]

Jakob Bieling schrieb:

virtual derived1* clone () const { return new derived1(*this); }

You want to clone here, don't you?

Right, but I take it it's a matter of preference if you want to
create a new object by cloning the type (which I usually do) or by
cloning the actual object (which is what you are doing).

regards