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From: Jan Gaspar (jano_gaspar_at_[hidden])
Date: 2004-03-07 16:31:40

--- David Abrahams <dave_at_[hidden]> wrote:
> Alberto Barbati <abarbati_at_[hidden]> writes:
> > Suppose that T *is* assignable, is destroy+copy
> construction better
> > than assignment? Probably no, for several reasons,
> including:
> >
> > 1) assigment is probably more efficient. In
> particular it might avoid
> > destruction/reconstruction of sub-objects non
> related with the
> > class invariant (for example: buffers, mutexes,
> etc.).
I agree the assignment is more efficient. If you dig
in the code little bit you will find that there is
assignment operation provided for the primitive types.

> >
> > 2) with the dtor/ctor idiom, if the ctor throws an
> exception, the old
> > element is lost and you can't do anything about
> it, so the
> > container cannot provide more than the basic
> guarantee for any
> > method that might overwrite elements. However,
> the user might
> > implement assigment with the strong guarantee,
> the container might
> > leverage on that and provide the strong
> guarantee too at least for
> > methods that might overwrite a single element
> such as
> > push_back/push_front. (Of course, the user
> might implement
> > assignment so badly that it doesn't provide
> even the basic
> > guarantee and the container could not recover
> from that, but a
> > library such not try to outsmart the user...
> too much ;).
> >
> > Just my opinion,
The idea behind the dtor/ctor idiom is like this:
suppose you have full circular_buffer and you want to
push_back a new element (instance of some class). That
means the fron-most element is about to be
overwritten. Now if there will be the assignment idiom
applied the front-most will be assigned to the new one
- it will be not destroyed (no destructor will be
called). IMHO this is not correct. The old element
will not disappear - just its value/state will be
different. I think that the overwrite operation means
destruction of the old object, NOT assignment. What do
you think?


> Well said.
> --
> Dave Abrahams
> Boost Consulting
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