Does shared_ptr allocate when constructed from raw pointer?

Question

As far as I understand shared_ptr it contains a pointer to a control block which contains the raw pointer strong count and weak count.

I'm in a situation where I need lock-free allocation, so I can allocate a raw pointer from the pool allocated memory and then make a unique pointer with a custom deleter to put it back in the pool, so far so good.

In the case of a shared_ptr however if I construct a shared pointer with one of these raw pointers and custom allocators does it allocate the control block at that point?

If it does is there a way to provide the memory for the control block in a lock-free way?

@DavidSchwartz [Recommended reading](https://stackoverflow.com/a/46356957/560648) — Lightness Races in Orbit, Nov 18 '19 at 18:00
@LightnessRaceswithMonica Thanks. That suggests, as I suspected, that there is nothing to do here. Your platform already does the right thing if it's not junk. — David Schwartz, Nov 18 '19 at 18:03

Lightness Races in Orbit · Accepted Answer · 2019-11-19T14:10:01.790

3

In the case of a shared_ptr however if I construct a shared pointer with one of these raw pointers and custom allocators does it allocate the control block at that point?

Yes.

If it does is there a way to provide the memory for the control block in a lock-free way?

shared_ptr's constructor has overloads that accept a custom allocator.

Refer to some shared_ptr documentation.

(I'm not entirely clear whether the deleter will also be allocated by your custom allocator.)

edited Nov 19 '19 at 14:10

answered Nov 18 '19 at 16:27

Lightness Races in Orbit

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Does this also allocate the memory for the type erased deleter? – Andreas Loanjoe Nov 19 '19 at 09:35
@AndreasLoanjoe That's an excellent question! It, er, doesn't seem to be specified :O – Lightness Races in Orbit Nov 19 '19 at 11:19
@AndreasLoanjoe I've [asked this separately](https://stackoverflow.com/q/58932991/560648). Will edit into this answer if I get an acceptable response! – Lightness Races in Orbit Nov 19 '19 at 11:26
Hehehe looks like a popular question so far – Lightness Races in Orbit Nov 19 '19 at 12:37

Does shared_ptr allocate when constructed from raw pointer?

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