Re: [boost] lightweight_once

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-08-27 12:33:11

Tobias Schwinger:

> Most processors have linear write buffers and if 'initialized' is seen
> as true the object has been written, too.

The write buffer in thread 1 doesn't affect the reads in thread 2, which can 
still be reordered.

True if by "most processors" you mean "x86", though, absent compiler 
optimizations.

To be on the safe side one needs:

if( atomic_load_acquire( &initialized ) != 0 )
{
    // access object
}

in thread 2 and

// initialize object
atomic_store_release( &initialized, 1 );

in thread 1.

Hasn't Anthony Williams already implemented a header-only call_once? I'm not 
sure I see a reason to reinvent that particular wheel. Once boost::mutex is 
made header-only, there'd be no need for lightweight_mutex either and I'll 
be able to retire it as well.