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From: Howard Hinnant (hinnant_at_[hidden])
Date: 2005-04-26 12:44:12

On Apr 26, 2005, at 12:08 PM, David Abrahams wrote:

> I had this disagreement with Thorsten in Lillehammer (or was it
> Oxford?) but he was unimpressed with my arguments. I was wondering
> what Boost as a whole would think:
> My Thesis: the library should support copying and assignment of
> pointer containers using the usual copy/assignment syntax, rather
> than only allowing a copy via a clone() function.
> Rationale: Not being able to copy and assign pointer containers is an
> arbitrary restriction that limits usefulness and expressivity. The
> argument Thorsten gives against copyability is that the elements
> would have to be clone()d rather than copied, which is expensive
> because there's a dynamic allocation per element. I don't get it;
> we don't arbitrarily prohibit copying of std::vector<std::string>
> even though that incurs a dynamic allocation per element.
> Example: Consider std::pair<int,some_pointer_container>. Almost
> nothing works. IIUC, it's not copyable, can't be initialized, and
> can't be clone()d.

Why not just a clone_ptr<T> which calls clone on copy construction /
assignment? Then you could std::container<clone_ptr<T>>. And such a
container is copyable (clones on copy). An indirection iterator
adaptor could ease use of this container with algorithms, as could
indirect comparitors.

std::list<clone_ptr<T>> might be the container of choice in the short
term since list doesn't copy around its elements at the drop of a hat.

In C++0X, (I believe) none of the containers will unnecessarily copy
clone_ptr's, choosing to internally move them instead (moving a
clone_ptr would be cheap).

On your thesis: The library should only support copying and assignment
of pointer containers using the usual copy/assignment syntax if a
non-destructive copy of the pointer can be made. The proposed
std::container<unique_ptr<T>> won't be such a container. Having
std::container<auto_ptr<T>> fail at compile time is still a good idea.
Otherwise it would destructively copy with copy syntax.

<sigh> I realize these thoughts are a little ahead of their time at
the moment due to the lack of compilers/libs which would make
std::vector<clone_ptr<T>> acceptably efficient.


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