Re: [boost] [optional] problems with swap()

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2008-04-12 05:58:58

Corrado Zoccolo skrev:
> On Fri, Apr 11, 2008 at 9:38 AM, Sebastian Redl <
> sebastian.redl_at_[hidden]> wrote:
> 
>> If this variant of swap() is really more efficient, perhaps
>> metaprogramming could be used to support it *if* the type has a default
>> constructor. But I don't know how to detect this. TypeTraits doesn't
>> contain a has_default_constructor trait. It may be that this will only
>> work with 09 concepts - DefaultConstructible.
>>
> 
> We can use has_nothrow_default_constructor .
> This will also exclude the specialization when the default constructor
> performs some allocation, and could result in lower performance.

I can't imagine that it would result in lower performance in general ... 
have you seen many types for which default construction was more 
expensive than copy-construction?

has_nothrow_default_constructor is certainly useable, albeit it requires 
compiler inrinsics. Do we know how many compilers that do support them?

-Thorsten