Re: [boost] ASIO into the standard (was: Re: C++ committee meeting report)

Subject: Re: [boost] ASIO into the standard (was: Re: C++ committee meeting report)
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-07-08 18:44:48

On 8 Jul 2014 at 17:03, Jonathan Wakely wrote:

> On 5 July 2014 21:00, Niall Douglas wrote:
> > I am grateful for the clarification, but I think my original
> > statement was correct yes? STL containers don't have a noexcept move
> > constructor in C++ 11 due to allocators?
> 
> No, that's not correct.
> 
> It's because some implementations either need at least one node even
> in an empty container (which requires the moved-from object to
> allocate memory after it's move from) or because debugging "safe STL"
> implementations perform memory allocation during a move.
> 
> Although those reasons are related to memory allocation, they're not
> to do with Allocators, the same reasons would apply if everything used
> malloc().

I think we might be at cross purposes here due to my sloppy phrasing.

I didn't mean that allocators per se are at fault, I did mean that 
the present design of STL allocators are at fault. If they had a 
different design the issues you mentioned wouldn't constrain 
noexcept, though they probably would constrain something else even 
more important.

For example, a better allocator design could guarantee that zero 
sized allocations always succeed, or that known temporary allocations 
always use alloca(), or guarantee non-relocating reallocs, or copy on 
write page table duplicate maps. That sort of thing. Such flexibility 
would eliminate the constraints the present design imposes upon their 
users.

Niall

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