[phobos] What does this code do?

[Andrei Alexandrescu]

Submitted a fix, please let me know which of the questions still stand.

Andrei

On 08/17/2010 08:31 PM, David Simcha wrote:
> Sorry for the double mail, Andrei.  I meant to send this to the whole
> list but my mail client screwed up.
>
> I ran into a few issues w/ emplace while trying to improve array() that
> I don't know how to fix:
>
> 1. Bug 4671 (http://d.puremagic.com/issues/show_bug.cgi?id=4671).
> Basically, emplace() doesn't work if args is arguments for a struct
> literal, not a struct c'tor. I assume that this is supposed to work. I'm
> not sure how to fix it because:
>
> a. Something like the following won't work if the struct contains
> const/immutable elements that are supposed to be initialized in the c'tor:
>
> foreach(ti, arg; args) {
> (*result).tupleof[ti] = arg;
> }
>
> b. Creating a new struct using args and then using memcpy to copy it to
> the buffer will do weird things if the struct has a destructor.
>
> c. Creating a new struct using args, initializing the buffer and then
> assigning it to *result using opAssign might do weird things if the
> semantics of opAssign and the semantics of the c'tor aren't the
> consistent, though this is poor enough design that anyone who does it
> probably deserves what they get.
>
> 2. I don't understand this code:
>
> else static if (Args.length == 1)
> {
> // T doesn't define a constructor, must be a primitive type
> static if (is(typeof(result.opAssign(args))))
> {
> static init = T.init;
> memcpy(p, &init, T.sizeof);
> }
> *result = args[0];
> }
>
> How can something be a primitive if it has an opAssign?
>
> 3. In the memcpy() line in the code above, I assume you meant
> memcpy(result, &init, T.sizeof)? There is no variable p in that scope.
>
>
>
> On 8/17/2010 2:07 PM, Andrei Alexandrescu wrote:
>> The idea is to create a typed object in uninitialized memory.
>> Generally using the assignment operator for that is suicide because
>> that guy assumes the object was previously initialized.
>>
>> Instead of placement new, a better way is to use emplace(). Grep for
>> it to see documentation and examples. Please let us know if you hit
>> another hitch.
>>
>>
>> Andrei
>>
>> On 08/16/2010 07:01 PM, David Simcha wrote:
>>> I'm trying to revamp array() to follow Bearophile's very good suggestion
>>> of letting any foreachable object be convertible to an array even if
>>> it's opApply-based, not a range. My efforts have been blocked by this
>>> cryptic piece of code:
>>>
>>> foreach (ref e; result)
>>> {
>>> // hacky
>>> static if (is(typeof(&e.opAssign)))
>>> {
>>> // this should be in-place construction
>>> new(&e) E(r.front);
>>> }
>>> else
>>> {
>>> e = r.front;
>>> }
>>> r.popFront;
>>> }
>>>
>>> It took me awhile to even wrap my head around what the point of that
>>> code is. First of all, why are we bypassing opAssign? Secondly, what if
>>> the E doesn't have a constructor that takes only another E as an
>>> argument?
>>> _______________________________________________
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>>> phobos at puremagic.com
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>>
>
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