[cxx-abi-dev] trivial __dynamic_cast fails?

[David Abrahams]

David Baraff <deb at pixar.com> writes:


>> From: Daveed Vandevoorde <daveed at edg.com>
>
> Please note that Mark Mitchell already pointed me in the right
> direction, though it shifts a lot of implementation burden to me.  I
> can use the base_type_info to compute the address shift as I chain
> upward from derived to base, which is all I'm really after in this
> case.

Could you please outline the details?

>> There has been occasional talk among WG21/
>> J16 actives about adding a facility like
>> what David B. is looking for, but so far
>> no concrete proposals.  I'm going to CC
>> David Abrahams who mentioned the desire
>> for this once.  Perhaps he has some insight
>> into how widespread the need for this is.
>
> Here's a biggy.  Consider the boost::any class.  Currently, if it
> holds a Derived* and you ask it for a Base*, I think it says "no can
> do."  Imagine if it could actually say "yes, I can give you back a
> Base*", because it could dynamically look up the parent chain, and do
> the address shift.
>
> That would make boost::any a heck of a lot more useful than it is today.

boost::any could already be modified to do this, since it "knows" the
"Derived" type.  I've enclosed an implementation.  Seems to reveal EH
bugs in quite a few compilers, tho ;-)

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-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com