On 12 Nov 2013, at 18:21, John Baldwin <j...@freebsd.org> wrote:
>> struct foo {
>> struct foo bar;
>> }
>
> Except it isn't. It's declaring the head of a container. This is more
> like:
>
> 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.
David
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