On Wednesday, 14 November 2012 at 22:07:10 UTC, Jonathan M Davis
wrote:
Postblit doesn't work with const objects and there's not
currently any way to
make it work with const objects. So, any attempt to copy any
element in your
AA won't work. So, if the AA implementation has any code in it
which requires
copying an element (and it probably does), then a type with a
postblit in it
won't work if it's const.
Jonathan, thanks for taking the time on this and all my other
questions. Coming from C++ and trying to hammer out how to use D
I've hit this postblit many times and many ways. I agree that
postblit does not work with const - and my feeling is maybe it
should not and I hope there is a work around (for structs). For
instance:
------------
import opmix.mix;
import std.stdio;
struct Big {
char [] c ;
}
void main () {
const ( Big ) s = { " test " };
Big other = s.gdup;
writeln(other);
}
------------
At present const and postblit don't get allow at all, and it
sounds like the
solution that Andrei and Walter have been cooking up will
probably involve
I think that is fine they don't get along. postblit can not
guarantee no data is shared after the call, so they should not.
In the example here I don't access the map at all and the call
works fine in non-property form.
adding copy constructors, but nothing has actually happened yet.
I do not see how copy constructors can help. Receiving const(T)
or const(T) ref to anything and hoping to copy it is the
challenge and doing it from a copy constructor is no easier than
a postblit. I think bearophile pointed it out best:
In practice to copy something const to something that's
not const you need a deep copy function, because inside
the array there can be other arrays that are const,
etc. Transitive const requires transitive copy, it's
easy :-)
If you have any extra cycles I would love feedback on (dup and
Global Dup) in:
https://github.com/patefacio/d-help/blob/master/doc/canonical.pdf
Actually feedback on all would be great because it establishes
how I want to work with structs and being new to D I'd like
assurances it is safe/correct.
Thanks
Dan
postblit
fundamentally doesn't work with const or immutable though and
can't, because
you can't modify const or immutable objects, and postblit
requires you to
alter the object. So, as far as const and immutable go,
postblit is
fundamentally flawed.
- Jonathan M Davis
