Re: [Boost-users] [Boost.Iostreams] Question about gzip codec anddual_use filter

From: Jonathan Turkanis (turkanis_at_[hidden])
Date: 2008-01-19 14:45:30

Marc Viala Perso wrote:

>> You should never instantiate a
>> filtering_stream<boost::iostreams::dual_use>. DualUse is not a real
>> mode, it's a pseudo mode, provided as a convenience for filter
>> implementors. I should put in a STATIC_ASSERT somewhere to prevent this.
> 
> [Marc Viala] 
> Oups, sorry but maybe it will be very valuable to add some explanations on
> the Iostreams documentation to explain this type of restriction (and to add
> also the STATIC_ASSERT). 

I certainly will. It never occurred to me that someone might try to
instantiate filtering_stream<dual_use>, but in retrospect I see how it 
could easily happen.

> More generally, my feelings after some days diving
> into the Iostream documentation, is that adding some use cases or examples
> would be relevant. Your library is very important/great in my sense and I'm
> sure that I'm not able to embrace all its features up to now, maybe due to
> the documentation. 

That's a reasonable request. However, I did provide a large amount of
documentation, including a tutorial, user guide, and reference. If
something is not explained well, it's probably because it seems obvious
to me, as the library author. So it would be very helpful for you to
point out areas that need more examples.

>> If you need to use an external interface that expects an iostream, you
>> could use a filtering_stream<bidirectional>, together with
>> boost::iostreams::combination (http://tinyurl.com/24zwqy) to producing a
>> bidirectional filter. Depending on your use case, which I don't fully
>> understand, you could combine two compression filters to produce a
>> bidirectional compression filter, or a compression filter with a dummy
>> filter that always throws, to produce a bidirectional stream that at
>> runtime can only be used for unidirectional i/o.
> 
> [Marc Viala] 
> Ok, I've implemented your suggestion and its works fine. Just to be sure
> that I didn't miss anything, do you confirm the implementation hereafter?
> 
> class gzfstream: 
>   public boost::iostreams::filtering_stream<boost::iostreams::bidirectional>
> {
> public:
>   template <class Path>
>   gzfstream(const Path& file_path, std::ios_base::openmode mode):
>     _file(file_path, mode)
>   {
>     namespace fs = boost::filesystem ;
>     namespace io = boost::iostreams ;
>     
>     const typename Path::string_type& ext = fs::extension(file_path) ;
>     if( gz_extension_exists(ext) ) 
>       push(io::combine(io::gzip_decompressor(), io::gzip_compressor())) ;
>     push(_file) ;
>   }  
> 
>   ~gzfstream()
>   { reset() ; }
>   
>   static bool gz_extension_exists(const std::string& ext)
>   { return ext.size() == 3 &&  ext == ".gz" ; }
> 
>   static bool gz_extension_exists(const std::wstring& ext)
>   { return ext.size() == 3 &&  ext == L".gz" ; }
> 
> private:
>   boost::filesystem::fstream _file ;
> } ;

Looks good. The call to reset() in the destructor is unnecessary, but 
harmless.

> Jonathan thanks again for your library and your very quick help. 

np

> Best regards,
> 
> Marc VIALA

-- 
Jonathan Turkanis
CodeRage
http://www.coderage.com