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

25 Jul 2013

 > Gesendet: Donnerstag, 25. Juli 2013 um 02:41 Uhr
...
Von: "Stephen Greenfield" 
An: "boost-users@lists.boost.org" 
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_streambufboost::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_streambufboost::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' 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