On 12/23/2014 3:03 PM, Jeff Walden wrote:
On 12/23/2014 10:48 AM, L. David Baron wrote:
Our convention has always been to pass raw pointers, generally with
the assumption that the caller is expected to ensure the pointer
lives across the function call.
Like Eric, I would like us to move away from this convention over time, or at
least stay away from it in the places that are fresher and better-adjusted.
Passing references to smart pointers doesn't work if you need
implicit conversions from derived class to base class (e.g., if the
function takes Base*, and the caller has nsRefPtr<Derived>), which
is pretty important in many areas of our codebase.
Passing references to smart pointers is also mildly more expensive because you
have an extra dereference operation, in the general case. But that almost
certainly far outweighs not addrefing/releasing through even a well-predicted
vtable, as you go on to say.
Really, tho, I don't think we have to forego a smart pointer argument type. We just
need a separate smart pointer type ArgumentPtr<T> (name not seriously proposed,
just to have a handle on the concept), implicitly constructible from the various
smart pointer types of U, where U derives from T.
And concerning the binary compatibility argument -- which I don't think really
matters any more, and if it does a11y could have its own MSCOM smart pointer
class for its own special-snowflake uses -- it seems fairly likely to me that
there's a smart pointer class that could preserve that binary compatibility.
Although I haven't investigated deeply to conclude so for sure.
For what it's worth, if we are willing to do a real mass-conversion on
our codebase (to deal with XPIDL-related API), we could probably replace
a lot of our T* arguments with use of "the world's dumbest smart
pointer" (see
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4282.pdf>). If
we designed it right, it would be ABI-compatible for raw pointers on
argument passing (but probably not return values, if I recall my C++
ABIs properly). We could arguably also replace our T** arguments with
nsRefPtr/nsCOMPtr & arguments at the same time if we're massively
rewriting our codebase.
--
Joshua Cranmer
Thunderbird and DXR developer
Source code archæologist
_______________________________________________
dev-platform mailing list
dev-platform@lists.mozilla.org
https://lists.mozilla.org/listinfo/dev-platform
