On 10/29/10 15:18 CDT, Pillsy wrote:
Andrei Alexandrescu Wrote: [...]
FWIW this matter is still tormenting me. It gridlocks work on
ranges, algorithms, and containers.
Here's my take: it seems in both cases you're arguing about
contorting something in the name of a relatively niche construction.
In one case, it's designing libraries around the possibility that you
might have an expensive-to-copy value type, and in the other you seem
to be seriously contorting the very idea of "value type" itself.
Very well put. (Why don't you post more often?)
I'm sort of wondering where all these expensive-to-copy value types
are going to come from. It's not like writing an expensive-to-copy
struct is likely to be something you do by accident, right? I can't
think of a case where someone just does it because they know better.
The typical case is value types of variable length: strings (the
built-in offering notwithstanding), BigInt, BigFloat, STL-style
containers, possibly even vectors and matrices (though that's almost
always a design mistake). And then of course come types that contain
such. So "expensive-to-copy" is a transitive property that rots
everything upwards. In fact let me just add this to the list of pros and
cons:
1. Arbitrary cost copy construction:
- Propagates through the membership relation, i.e. a struct with at
least one costly-to-copy member becomes itself costly-to-copy even
though it wasn't planning to or wasn't aware of.
2. Constant-cost copy construction (via mandatory refcounting for value
types):
+ Transparent for the member-of relation, i.e. a constant copy member
keeps its containing struct also constant copy.
A programmer coming from C is likely to naively treat D structs like
C structs, and be fine.
Except as noted above. A C programmer might plop a couple of BigInts in
a struct and think that they're in good shape.
A programmer coming from Java# or a dynamic scripting language is
probably going to be happier sticking to classes and their comforting
and familiar reference semantics when they're getting started.
That leaves programmers coming from C++, who are likely to know what
they're getting themselves into, or more experienced D programmers,
who again are likely to know what they're getting themselves into.
I think this is a definite worse-is-better situation. Just assume
that value types don't have randomly expensive copies in the standard
library code, while letting people who want to take the risk of
shooting themslves in the foot shoot themselves in the foot by having
this(this) open an HTTP collection to Alpha Centauri if that's what
they really want.
Cheers, Pillsy
Thanks for the vote.
Andrei
