Re: History of and support for std::basic_string::back()

James Kanze <>
Tue, 6 Aug 2013 09:40:14 -0700 (PDT)
On Monday, 5 August 2013 21:23:20 UTC+1, Paavo Helde wrote:

=D6=F6 Tiib <> wrote in

On Monday, 5 August 2013 09:37:42 UTC+3, Paavo Helde wrote:

CoW requires multi-threaded synchronization deep in the inners of
std::string, where it would be totally on the wrong level.

That is lock-less synchronization so it is fine to hide it.

As far as I have understood this is not really the case unfortunately.
Lock-less synchronization might be better than locks, but they still have=


to use proper memory barriers, to avoid code reordering by hardware. Memo=


barriers may become quite expensive, especially on NUMA.

The synchronization certainly isn't free. But the alternative
is deep copy, and most memory allocators will require
synchronization as well, and come at a much larger cost than the
atomic counting which can be used in CoW. (In our application,
at least, CoW is a definite win.)


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