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Subject: Re: Are clang++ and libc++ compatible?
From: David Chisnall <theraven@FreeBSD.org>
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Date: Wed, 13 Nov 2013 10:04:36 +0000
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References: <20131112163219.GA2834@troutmask.apl.washington.edu>
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To: John Baldwin <jhb@FreeBSD.org>
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Cc: Eitan Adler <lists@eitanadler.com>, freebsd-current@FreeBSD.org,
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On 12 Nov 2013, at 18:21, John Baldwin <jhb@freebsd.org> wrote:

>> struct foo {
>> struct foo bar;
>> }
>=20
> Except it isn't.  It's declaring the head of a container.  This is =
more
> like:
>=20
> 	struct foo {
> 		TAILQ_HEAD(, foo) messages;
> 	};

Eitan is correct here.  The definition of std::deque is that it copies =
the value that is the template argument and does not require =
modifications to the layout.  A deque is more akin to an array, so in C =
it would be something like:

struct foo {
  struct foo bar[10];
};

This is clearly nonsense - you can't have a structure that contains =
itself.  The same is true for the deque.  It's not clear what the pan =
people actually wanted, but an efficient implementation of a deque would =
most likely contain space for a small number of the template argument =
elements, so they are literally defining a structure containing a =
structure containing the parent structure.  The same would be true if =
they did:

struct Entry {
    std::vector<Entry> v;
};

An implementation of the vector class might allocate all of the elements =
on the heap lazily, but it's not required to and could equally have =
space for a small number inside the object, expanding to something like =
this:

struct Entry {
   struct MangledNameOfVectorOfEntry {
      size_t size;
      Entry small[4];
      Entry *ptr;
   };
};

It would make sense to have a std:deque<Entry&> or std:deque<Entry*>, =
because then you're only storing references or pointers to the outer =
structure in the inner structure. =20

David