Alignment, STL vector and bullet physics aligned allocator

Question

When I use this, I guess it's safe to say those objects are aligned:

std::vector<object_type> vect;

I spotted an allocator in bullet physics, and I don't know how they work. This also raises the question about std::vector.

In the demo here, line 42, http://code.google.com/p/bullet/source/browse/trunk/Demos/BasicDemo/BasicDemo.h#42

btAlignedObjectArray<btCollisionShape*> m_collisionShapes;

the type is a pointer, and later those pointer are assigned a new. Does it really guarantee alignment ? If the allocator is made to deal with pointers, I guess yes, but I don't have any allocator knowledge, on top of that I don't know what is obsolete or not.

What about std::vector ? If I declare

std::vector<object_type*> vect;

and assign later, will the compiler still align my objects ?

Question very similar to http://stackoverflow.com/questions/9053157/will-this-code-make-those-vector-aligned — Lightness Races in Orbit, Feb 11 '13 at 00:48
What alignment? 4-byte-alignment? 8-byte-alignment? 4KiB-alignment? Or are you misusing the term alignment? — , Feb 11 '13 at 00:48

s.bandara · Answer 1 · 2013-02-11T01:09:46.827

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The pointers will be aligned contiguously in this vector of pointers. For the objects these pointers point to, nothing can be said. They may be placed anywhere.

A striking example would be

object_type on_stack;
vect[0] = new object_type;
vect[1] = & on_stack;
vect[2] = new object_type;

where the first and third element of the vector are assigned pointers to objects instantiated with new on the heap, while the second element is assigned the address of yet another instance on the stack.

If you wish to "align" N objects on the heap, there is still new object_type[N];

edited Feb 11 '13 at 01:09

answered Feb 11 '13 at 00:49

s.bandara

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Or just `std::vector v(N);` – Lightness Races in Orbit Feb 11 '13 at 12:01

score -2 · Answer 2 · answered Feb 11 '13 at 00:47

-2

Do you mean, will the elements be packed contiguously?

Yes, they are guaranteed to be.

answered Feb 11 '13 at 00:47

Lightness Races in Orbit

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the objects ? or just the pointers ? Because I don't care about aligning pointers (or should I ?) – jokoon Feb 11 '13 at 01:05
Only the pointers. The "elements" in this case are of pointer type. The objects can be allocated anywhere. – s.bandara Feb 11 '13 at 01:07
Pointers are objects too. The elements of your container are packed contiguously, which is what I already said. – Lightness Races in Orbit Feb 11 '13 at 12:00

Alignment, STL vector and bullet physics aligned allocator

2 Answers2