Re: delete() and new() on elements of an array created by new[]()

From:

"[rob desbois]" <rob.desbois@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 15 Apr 2008 06:24:50 -0700 (PDT)

Message-ID:

<b06a7d11-3e79-4aed-9557-e9ac6bd4fa55@u36g2000prf.googlegroups.com>

On Apr 15, 1:50 pm, Michael DOUBEZ <michael.dou...@free.fr> wrote:

[rob desbois] a =E9crit :

I have an array created with:
FooClass* foo = new FooClass[numberOfFooObjects];

I want to be able to replace an element of that array at will with a
new FooClass.
Am I right in thinking that to delete foo[3], say, and replace with a
new FooClass, would require placement new?

If indeed you can destroy the object
(foo+3)->~FooClass();
And then
new(foo+3) FooClass();

Would this be wise?

Of course not.
It doesn't matter when you do it in raw memory but if you cannot
recreate the object (if it throws), then you have a big problem with how
to deal with your array when you want to destroy it.
(foo+3)->~FooClass();
try
{
new(foo+3) FooClass();}

catch(...)
{
//}

delete[] foo; //UB when destroying foo[3]

Ah yes, didn't think of that...

It seems that if I want to be able to replace elements at will with
newly constructed elements (rather than using the assignment
operator), that it would be better to do this:
FooClass** foo = new FooClass*[numberOfFooObjects];
Then create each object on the heap and store the pointers instead.
Replacing elements is easy and obvious then...

That or defining a void FooClass::swap(FooClass&)throw() method that
swap the two object.

Michael

Great, thanks very much Michael :-)