Re: [Boost-users] [boost.any] Inline allocation of small objects in any.

Subject: Re: [Boost-users] [boost.any] Inline allocation of small objects in any.
From: Christopher Jefferson (chris_at_[hidden])
Date: 2011-02-10 08:17:35

On 9 Feb 2011, at 22:29, Kim Barrett wrote:

> On Feb 9, 2011, at 12:38 PM, Adarsh Soodan wrote:
>> I was curious if others have solved similar performance problems. I
>> couldn't use boost::variant because I needed it for a message passing
>> library where library user can pass in any message type.
>> boost::variant would have been restrictive if the user wanted to use
>> too many top level types of messages.
> 
> 
> I would be interested in having boost::any (or some variation on it) providing small buffer optimization.  That was done for boost::function back around boost 1.34, and something like that would work just fine for my usage.  I've given the SBO for boost::any problem a cursory look, but have not yet demonstrated a sufficiently bad performance problem for use there to convince my managers that I should work on that rather than something else.

I believe spirit/home/support/detail/hold_any.hpp contains something like boost::any, with a small buffer optimisation. This is not suitable for directly copying out into general boost. In particular I believe it assume any type held in the 'any' is streamable, and quite possibly doesn't consider issues of exception safety.

On the subject of exception safety, I quite like how boost::variant will put a 'boost::blank' in (if that exists in the variant) if the value is lost. A similar 'empty' state for boost::any might solve problems with exception safety, at least for me.

Chris