Re: [boost] Boost should not be saying "_FPOSOFF"

Subject: Re: [boost] Boost should not be saying "_FPOSOFF"
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-09-26 15:53:51

On 9/26/2016 7:02 AM, alanbirtles2 wrote:
> some context to the use of _FPOSOFF in boost might be useful.
> up to and including VS 2015 std::streamoff(std::streampos()) returned _Myoff
> + _FPOSOFF(_Fpos).
> there is no other way to retrieve the value of _Myoff from std::streampos.
> until VS 2012 _FPOSOFF was a cast to long.
> therefore the only way to safely get the stream offset from a std::streampos
> was to do:
> pos.seekpos + std::streamoff( pos ) - _FPOSOFF( pos.seekpos ) == _Fpos +
> _Myoff + _FPOSOFF(_Fpos) - _FPOSOFF( _Fpos ) == _Fpos + _Myoff
> after VS 2012 the _FPOSOFF macro is a cast to long long so calling
> std::streamoff(std::streampos()) returns the correct value and the above
> workaround is no longer necessary

I appreciate the historical explanation but it is confusing to me. 
Especially:

"pos.seekpos + std::streamoff( pos ) - _FPOSOFF( pos.seekpos ) == _Fpos 
+_Myoff + _FPOSOFF(_Fpos) - _FPOSOFF( _Fpos ) == _Fpos + _Myoff"