Re: Copying varied types

"Balog Pal" <>
Wed, 25 Nov 2009 01:11:58 +0100
"Pat" <>

Now, I want to write a function to walk that tree and
create an exact copy of it. It's ok to assume that the
tree contains no loops.

I could write a fairly simple recursive function that
1) allocates a new object, 2) copies the old node's
data, and 3) calls itself for each child in the old

But the trouble is in step 1, because I don't know what
type to allocate...

You do: same type as the one you copy.

My first thought was to use a > templated "clone" function

No fly.

So, then, I thought that each derived class should have
a virtual clone() function that returns the appropriate

Yeah. clone() certainly does all the job you describe above, and the copy is
invoked by the topmost node->clone();

clone() normally is nothing but {return new tSelf( *original);} and can be
injected by a macro along with other cool functions supporting hierarchy
build and serialization (where you normally need a static Create( Key )
function that you register with a factory, and virtual GetKey()... When
reading from file you really need to create the object from the thin air. )

The copy ctor of leafs can be left the factory one, if you implement the
child collection managenemt in the Root base class...

But then there's no way to call the correct
function at run-time. Each class's clone() function
would have a different return type.

Actually you can use covariant types -- each function returning tSelf *,
that doesn't spoil the virtual game -- but for your purpose it is no problem
if Root * is returned everywhere. You don't call anything on that pointer
anyway, just put in in the collection...

I could have some sort of switch statement, I guess,
but that's ugly...

I wouldn't suggest unless you have a fixed number of classes that will not
change in the lifetime of the program. (kinda rare case... ;)

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