Re: CArray

"David Webber" <>
Mon, 5 Nov 2007 10:49:36 -0000
"Giovanni Dicanio" <> wrote in message

"David Webber" <> ha scritto nel messaggio

But I'd prefer

class CMyArray1 :: std::vector< CString >

Dave: when inheriting from a class, for proper cleanup, should the base
class have a *virtual* destructor?

I believe it is only necessary if you're using polymorphism:

class D : public class B

B *pB = new D;
delete pB;

When you delete the bass class pointer, the virtual destructor of B is
needed to ensure proper destruction of the derived class D.

In the case of:

class CMyArray1 :: std::vector< CString >

there is nothing to encourage one to do this. I just regard it as
convenient to be able to use

CMyArray1 a;
a.push_back( .... );

and so on, without a chain of contained objects. In this case it even fits
the purist's criterion CMyArray1 *is* the array.

I am *not* tempted to do anything like

std::vector< CString > *pArray = new CMyArray1;

as there would be no point. [A large part of the point is to avoid clumsy
phrases like "std::vector< CString >" in the source code!]

I'm not a C++ guru language-lawyer, but, as a rule to avoid uncorrect
cleanups and uncorrect destructor calls through an inheritance chain, I
define the base class destructor as virtual (and there is also the "rule
of 3", i.e.: if one defines a virtual destructor, he needs also copy ctor
and operator=).

As I say, I write copy constructors and = operators for just about every
class I produce. It's statements like

B *pB = new D;

which set off alarms to check for a virtual destuctor (though often I use
one anyway, in case I'm tempted to do that later). In any event, I tend to
use polymorphism a lot less than general derivation and encapsulation. [I
do use multiple inheritance quite a lot, though less than I used to, and I
avoid doing it on MFC classes these days.]

So, I read the source of std::vector implementation in VC7.1, and it seems
to me (I may be wrong...) that the destructor is not virtual.
In this case, would inheriting from std::vector be unsafe?

I do it all over the place and it has never caused me problems.

I also export the derived classes from DLLs which tends to generate one or
two warnings unless one suppresses them. But the general wisdom is that
you *can* export these from DLLs (though vector may be unique among STL
containers in this respect).

David Webber
Author of 'Mozart the Music Processor'
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