On 2/1/11 10:44 AM, Michel Fortin wrote:
On 2011-02-01 11:12:13 -0500, Andrei Alexandrescu
<seewebsiteforem...@erdani.org> said:
On 1/28/11 8:12 PM, Michel Fortin wrote:
On 2011-01-28 20:10:06 -0500, "Denis Koroskin" <2kor...@gmail.com> said:
Unfortunately, this design has big issues:
void fill(Appender appender)
{
appender.put("hello");
appender.put("world");
}
void test()
{
Appender<string> appender;
fill(appender); // Appender is supposed to have reference semantics
assert(appender.length != 0); // fails!
}
Asserting above fails because at the time you pass appender object to
the fill method it isn't initialized yet (lazy initialization). As
such, a null is passed, creating an instance at first appending, but
the result isn't seen to the caller.
That's indeed a problem. I don't think it's a fatal flaw however, given
that the idiom already exists in AAs.
That said, the nice thing about my proposal is that you can easily reuse
the Impl to create a new container to build a new container wrapper with
the semantics you like with no loss of efficiency.
As for the case of Appender... personally in the case above I'd be
tempted to use Appender.Impl directly (value semantics) and make fill
take a 'ref'. There's no point in having an extra heap allocation,
especially if you're calling test() in a loop or if there's a good
chance fill() has nothing to append to it.
That's the issue with containers. The optimal semantics always change
depending on the use case.
Yep, yep, I found myself wrestling with the same issues. All good
points. On one hand containers are a target for optimization because
many will use them. On the other hand you'd want to have reasonably
simple and idiomatic code in the container implementation because you
want people to understand them easily and also to write their own. I
thought for a while of a layered approach in which you'd have both the
value and the sealed reference version of a container... it's just too
much aggravation.
But are you not just pushing the aggravation elsewhere? If I need a by
value container for some reason (performance or semantics) I'll have to
write my own, and likely others will write their own too.
If semantics are the primary concern, you could (and in fact Phobos
could) provide a Value!C template that automatically calls dup in
this(this) etc.
For performance I agree there is stuff that class containers leave on
the table.
Using classes for containers is just marginally better than making them
by-value structs: you can use 'new' with a by-value struct if you want
it to behave as a class-like by-reference container:
struct Container {
...
}
auto c = new Container();
The only noticeable difference from a class container is that now c is
now a Container*.
Well one problem now is that if you have a Container* you don't know
whether it's dynamically allocated or the address of some
stack-allocated object. This is pretty big; a major issue that I believe
C++ has is that you can seldom reason modularly about functions because
C++ makes it impossible to represent reference semantics with
local/remote/shared/no ownership without resorting to convention.
A better solution is to define something like
auto c = new Classify!Container;
which transforms a value into a class object.
With this, the question becomes a matter of choosing the right default:
do we want values most of the time and occasional references, or vice
versa? I think most of the time you need references, as witnessed by the
many '&'s out there in code working on STL containers.
Personally, I'm really concerned by the case where you have a container
of containers. Class semantics make things really complicated as you
always have to initialize everything in the container explicitly; value
semantics makes things semantically easier but quite inefficient as
moving elements inside of the outermost container implies copying the
containers. Making containers auto-initialize themselves on first use
solves the case where containers are references-types; making containers
capable of using move semantics solves the problem for value-type
containers.
Neither values nor references are perfect indeed. For example, someone
mentioned, hey, in STL I write set< vector<double> > and it Just
Works(tm). On the other hand, if you swap the two names it still seems
to work but it's awfully inefficient (something that may trip even
experienced developers).
Isn't that solved by C++0x, using move semantics in swap?
This particular incarnation yes, but that doesn't automatically fix user
code that forgets the cost of copying. But that took a large language
change. My point was that values by default is not automatically a good
choice.
Andrei
