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Re: object-model: Wrapping Subversion C-structs in C++

From: Branko Čibej <brane_at_xbc.nu>
Date: Thu, 23 Sep 2010 21:20:41 +0200

 On 22.09.2010 21:41, Hyrum K. Wright wrote:
> [ apologizes for the somewhat stream-of-conscious nature of these mails ]
> On Wed, Sep 22, 2010 at 7:16 PM, Hyrum K. Wright
> <hyrum_wright_at_mail.utexas.edu> wrote:
>> On Wed, Sep 22, 2010 at 5:35 PM, Hyrum K. Wright
>> <hyrum_wright_at_mail.utexas.edu> wrote:
>>> For the C++ folks out there, I've got a question about an approach to
>>> take on the object-model branch. At issue is how to wrap the various
>>> C structures returned to callers, particularly in a backward
>>> compatible manner. Currently, I'm looking at svn_wc_notify_t *. As I
>>> see it, there are a few options:
>>> 1) Just wrap the pointer to the C struct as a member of the wrapper class.
>>> Pros: Easy to implement; lightweight constructor.
>>> Cons: Getters would need to translate to C++ types; would need to
>>> implement a copy constructor which deep copies the C struct; would
>>> also introduce pools, since creating and duplicating C structs
>>> requires them.
>>> 2) Wrap each C struct member individually
>>> Pros: C->C++ complexity is constrained to the constructor,
>>> everything else is C++ types
>>> Cons: Hard to extend for future compatibility
>>> 3) Just pass the C-struct pointer around; don't even bother with a class
>>> Pros: Dead simple.
>>> Cons: Requires more memory management thought by consumers; not
>>> C++-y enough; may introduce wrapping difficulties.
>>> I'd like to come up with something consistent, which would be used
>>> throughout the C++ bindings. I'm also interested in a solution which
>>> ensures the C++ bindings can be used as the basis for other
>>> object-oriented bindings models (Python, Perl, etc.)
>> After lunch, and some thought, it feels like #1 is the best solution.
>> This doesn't change the external class interface, which is good, and
>> can still provide C++ values to callers who want them. The pool
>> issues are a bit messy, but at least the object can manage it's own
>> memory (albeit at a significant overhead).
> This could get ugly.
> Creating and destroying pools all over the place could get ugly, but
> it's necessary evil because all of our object creation / duplication
> functions all require a pool. An alternative would be a set of
> functions returning the size of the object, and then another which
> puts the object in a pre-allocated memory location. (These could
> theoretically replace the pool argument version of the API, but that'd
> be *a lot* of churn.)
> The approach would let the C++ allocate the memory (of the correct
> size) using whatever scheme it wants, and then do a "placement
> initialize" using the second API. If we do go this route, I'd
> recommend exposing these as private-to-Subversion, at least initially.
> The other option is just pass the C-struct pointer around everywhere,
> but then the bindings consumers have to work about this exact same
> issue. In other words, it solves it now, but really just pushes the
> problem elsewhere.
> -Hyrum

Memory management with pools and C++ -- keep away from doing it in
per-object ctor/dtor pairs is all I can say. Lifetimes would get so
messy and mucked up you could hardly believe it.

IMHO the best way to wrap the C structures in C++ is to subclass 'em.

class svn_some_thing : public svn_some_thing_t { .... };

This way you can pass C++ pointers directly to the C implementation,
most methods become just inlined wrappers. Callbacks are a bit more
hairy, but not all that much.

As to pools ... I'd once thought that a proper C++ wrapper for a pool
would be a reference-counted smart pointer. That turns out to be too
much overhead and too much of a good thing, since you *cannot* allow
pool lifetime to depend on some reference counting order of execution.
Best just use plain pool pointers, or maybe wrap them minimally so that
destructors do the pool cleanup reliably.

-- Brane
Received on 2010-09-23 21:21:28 CEST

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