Re: CMap - can it do this?

From:

"Doug Harrison [MVP]" <dsh@mvps.org>

Newsgroups:

microsoft.public.vc.mfc

Date:

Sat, 07 Jun 2008 09:14:28 -0500

Message-ID:

<7t5l44ldqq9rt9lbdl21ch07vn5b7sv4ic@4ax.com>

On Fri, 6 Jun 2008 19:12:19 -0700, "David Ching"
<dc@remove-this.dcsoft.com> wrote:

Just because RemoveKey() takes a 'key', that doesn't mean the implementation
can't be smart and cache the key/value returned by GetNextAssoc(), and if
the 'key' passed to RemoveKey matches that, then the implementation knows to
use the cached value without doing a lookup of 'key' again.

I don't know of the MFC CMap does this... but it's such a common pattern
when iterating the contents of a map to immediately use the returned key to
do something, I'd be very surprised if it did not.

Maintaining state in a collection during iteration tends to cause problems
for thread safety and "nested" iteration by a single thread, so on general
principle, I'd be very surprised if that was done. It would be much better
to provide an erase(POSITION) member function instead.

BTW, the .NET Dictionary
class can have the same usage pattern.

I hope it's not the only one. What CMap should support is an erase function
that takes a POSITION, but POSITION objects seem useful only as magic
cookies in CMap's bizarre iteration model.

--
Doug Harrison
Visual C++ MVP