Re: [Boost-users] [iostreams] Some basic questions about filtering_stream

Subject: Re: [Boost-users] [iostreams] Some basic questions about filtering_stream
From: Jens Weller (JensWeller_at_[hidden])
Date: 2013-07-25 04:57:39

 
 
 > Gesendet: Donnerstag, 25. Juli 2013 um 02:41 Uhr
> Von: "Stephen Greenfield" <StephenG_at_[hidden]>
> An: "boost-users_at_[hidden]" <boost-users_at_[hidden]>
> Betreff: [Boost-users] [iostreams] Some basic questions about filtering_stream
> Some basic questions about Boost filtering_streams.  I have dozens of functions that presently take a parameter of std::ofstream&.  For example:
>     void foo(std::ofstream& outStream)
>     {
>         // lots of operations, like this:
>           outStream << "various bits of text";
>     }
> Which then get used as thus:
>      void StreamSomeTextToFile(char* fileName)
>     {
>           ofstream myFileStream(fileName, ios::out | ios::app | ios::binary);
>           foo(myFileStream);
>           myFileStream.close();
>     }
> Now I'd like to use the boost filtering_stream to output to a compressed ZIP file.  I've compiled, linked and run the commonly cited boost filtering_stream test code for packing and unpacking and it worked perfectly for me.  Now I'd like to try to substitute my use of std::ofstream& with the filtering_stream:
>      void StreamSomeCompressedTextToFile(char* fileName)
>     {
>           ofstream myFileStream(destPath, std::ios_base::out | std::ios_base::app | std::ios_base::binary);
>           boost::iostreams::filtering_streambuf<boost::iostreams::output> myCompressedFileStream;
>           myCompressedFileStream.push(boost::iostreams::zlib_compressor());
>           myCompressedFileStream.push(myFileStream);
>
>           foo(myCompressedFileStream); // I can't just pass myCompressedFileStream to foo(std::ofstream&), right?
>           myFileStream.close();
>     }
> THREE QUESTIONS:
> 1) Do all my functions that previously accepted std::ofstream& outStream need to now accept a parameter of type boost::iostreams::filtering_streambuf<boost::iostreams::output>& ?   Or, is there a proper parameter type so those numerous ("foo") functions could work with EITHER type of stream type?
> 2) In my simple test cases, I was not able to use stream operator syntax with the filtering_streambuf:
>           myCompressedFileStream << "some text";
> this generated the the error: no match for 'operator<<'.  I similarly had compile errors with write():
>           `error: 'class boost::iostreams::filtering_streambuf<boost::iostreams::output, char, std::char_traits<char>, std::allocator<char>, boost::iostreams::public_>' has no member named 'write`'
> Could this be a missing header?  If so, I haven't been able to determine which one.
> 3) In the common test case example code (below), I was confused that I could not locate the file "hello.z" after it had been created.  The unpack code (also below) executed correctly and clearly references the created file -- so where can it be found?

to 1: Make your functions take a std::ostream& reference, that way you are not bound to the streamtype.

to 2: A streambuf is not a stream, its a buffer for a stream, please consult the documentation for usage :)

to 3: if you do not specify a concrete path, afaik the file should be created in the directory where the file is run from.

kind regards,

Jens Weller