On Wednesday, 26 September 2018 at 21:24:07 UTC, Adam D. Ruppe
wrote:
On Wednesday, 26 September 2018 at 20:41:38 UTC, Chad Joan
wrote:
I'm implementing a deep-copy method for a tree of templated
class instances. As part of this, I need some way to copy
each node.
[...]
that isn't already handled by their deepCopy method.
I would strongly suggest just using that virtual method and
having the child classes override it, then you call it from any
of them and get the right result.
The tree nodes are potentially very diverse, but the tree
structure itself will be very homogeneous. I'm writing a parser
generator backend and the tree is expressions of various
operators (Sequence, OrderedChoice, UnorderedChoice, Repetition,
etc). I'm trying to keep it simple: everything is an expression,
and expressions can contain other expressions (though some are
always leaves in the tree). At some level, if I let things
implement their own deepCopy, then it means there are potentially
other classes and state out there to iterate that the rest of the
code doesn't know about. That could be bad, and expressions
shouldn't contain anything besides expressions!
This probably contrasts a lot with other use-cases, like
serialization. And I wouldn't be surprised if things change
later on and I end up with some kind of auxiliary virtual copy
function that does what you suggest, but is specifically for
handling special out-of-band mutable reference data that the
expressions might need to carry someday.
I suppose I've never considered just how hard/impossible it is to
have a generic way to copy things. Well, maybe a little bit at
various points, but not this bad ;) There are so many dimensions
to the problem and it seems like the context and requirements
will always be really important. So it can be made simple under
the right constraints (ex: everything is immutable!), but the
constraints are always different depending on the requirements
(ex: ... but in this hypothetical, we need mutability, so then
it's gotta happen a different way).
Object.factory kinda sux and I'd actually like to remove it
(among other people). There's no plan to actually do that, but
still, just on principle I want to turn people away.
But even as you can see, the implementation is lacking and it
isn't great design anyway - the interface with virtual methods
does better work. It also wouldn't work in ctfe anyway,
object.factory relies on runtime stuff.
Good to know!
I don't think I've even used it much, if at all. I suppose I
won't miss it if it goes ;)
If Object.factory is incapable of this, is there some other
CTFE-friendly way to copy templated class instances?
I think you can copy typeinfo().init and then call
typeinfo().defaultConstructor - this is iirc what
Object.factory does, but once you already have the typeinfo you
can use it directly and bypass the string lookup.
I'm having trouble looking this up. Could you link me to the
docs for this?
But you'd really be better off with a virtual copy method. I
say those string factory things should only be used if you are
starting with a string, like deserialization.
interface Copyable {
Copyable copy();
}
class Whatever(T) : Copyable {
Whatever!T copy() {
auto c = new Whatever!T();
c.tupleof = this.tupleof;
return c;
}
}
that kind of method. the template implements the interface so
little boilerplate and it works and can be customized etc and
fits in well. If you call it from the interface, you get an
instance of the interface (tho note since it is virtual, the
underlying type is still what you need). If you call from the
child static type, you get it right back. Yay, fits liskov and
works!
As above, I think this might be a very clean and effective
solution for a different class of use-cases :) I'll keep it in
mind though.
If I have to, I can probably make these things register
themselves in some list of delegates that can be used to
instantiate the correct class. Or something like that. But I
am hoping that there is a better way that involves less
boilerplate.
that's not a terrible idea if you need delegates keyed to
strings...
Right. At some level I just need a function that I can call like
this:
auto newThing = makeAnother(originalThing);
and perhaps makeAnother(...) can just lookup originalThing's
classname in an associative array of delegates. Or maybe I can
just hash some part of originalThing's type information. I can
put all of the ugly registration boilerplate into a mixin
template and somehow force that to always be mixed-into any
descendant classes. OK, it's probably getting too far into the
weeds now, but it seems doable and I'll reach for that if I need
to.
...
Things at least seem much more clear already. Thanks a bunch!
