On Wednesday, 14 October 2015 at 13:12:00 UTC, Rainer Schuetze
wrote:
On 14.10.2015 14:57, Szymon Gatner wrote:
extern(C++)
void freeSubtract(Operation animal) {
auto cat = cast(Subtract) animal; <<====== cast yields null
if(cat !is null) {
destroy(cat);
free(cast(void*) cat);
}
}
extern(C++)
void useOperation(Operation t) {
auto res = t.execute(1, 2);
}
Everything works fine except the marked line. Subtract
instance seems to
be created correctly on D side as it returns valid result in
the
execute() call on C++ side but then when trying to free the
instance on
D side again, downcast from Operation to Subtract fails
resulting in null.
To get compatible class layout, the D compiler has to omit it's
class info entry in the vtable of C++ classes. In addition D
doesn't know about C++ RTTI (I don't know if this is planned to
add), so it cannot do the dynamic cast from Operation to
Subtract.
I have not tried, but calling the virtual destructor instead of
destroy() might just work?
Mint that this code comes from Adam Ruppe's book, and it is
suppose to be working code. Also mind that Subtract is D class
and instantiated on D side, it is only briefly passed to C++ for
usage.
