Re: [Boost-users] seekable input filter definition

Subject: Re: [Boost-users] seekable input filter definition
From: llonesmiz (jorge.navalon_at_[hidden])
Date: 2016-04-07 10:36:25

Cherotek Music wrote
> Hey, I'm new to boost and need a seekable input filter that will operate
> on
> bytes read from an ifstream. This is what I currently have:
> 
> struct my_filter : boost::iostreams::seekable_filter{
>     template
> <typename Source>
>     int get(Source& src)
>     {
>         int byte = boost::iostreams::get(src);
>         if(byte != EOF && byte != boost::iostreams::WOULD_BLOCK)
>         {
>             // Do something with byte
>         }
>         return byte;
>     }
> 
>     template
> <typename Sink>
>     bool put(Sink&, char)
>     {
>         // No need to actually implement put because this filter is
> only used with ifstream
>         return true;
>     }
> 
>     template
> <typename T>
>     std::streampos seek(T& t, boost::iostreams::stream_offset off,
> std::ios_base::seekdir way, std::ios_base::openmode which =
> std::ios_base::in | std::ios_base::out)
>     {
>         return boost::iostreams::seek(t, off, way, which);
>     }};
> 
> It works but I would like to get rid of the useless put method. The boost
> documentation here
> http://www.boost.org/doc/libs/1_60_0/libs/iostreams/doc/concepts/filter.html
> says
> there is an InputSeekableFilter refinement of Filter but I can't seem to
> understand how to use it (there are no input_seekable_filter_tag or
> input_seekable_filter structs that I could use).
> 
> Thanks

"boost::iostreams::seekable_filter" seems to just be a typedef for
"boost::iostreams::filter<boost::iostreams::seekable>". I don't know for
sure, but I think it would make sense that deriving your "struct my_filter"
from "boost::iostreams::filter<boost::iostreams::input_seekable>" would
accomplish what you want.

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