Re: [boost] boost::shared_ptr, std::swap

Subject: Re: [boost] boost::shared_ptr, std::swap
From: Ben Pope (benpope81_at_[hidden])
Date: 2012-05-26 05:13:13

On Saturday, May 26, 2012 04:54 PM, Ingo Loehken wrote:
> Hello,
>
> i just recognized, that std::swap is not overload for boost::shared_ptr,
> meaning that using swap with std namespace ends up with unexpected
> results in respect to performance.
>
> Is this by intent ?

If you look at the synopsis:
http://www.boost.org/doc/libs/1_49_0/libs/smart_ptr/shared_ptr.htm#Synopsis

You'll see a member swap and a free function swap in the boost 
namespace, this free function is intended to be found with ADL.

There is a note on that page:
"[swap is defined in the same namespace as shared_ptr as this is 
currently the only legal way to supply a swap function that has a chance 
to be used by the standard library.]"

If you look at the implementation of boost::swap you'll see something like:

namespace boost {
    void swap(T& a, T& b)
    {
       using std::swap;
       swap(a, b);
    }
}

Which allows you to swap with ADL or std::swap fallback.  I don't think 
you want to explicitly call ::std::swap from user code.

Details: http://www.boost.org/doc/libs/1_49_0/libs/utility/swap.html

Ben