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.  


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