[Boost-users] [iostreams] making a symmetric_filter flushable

From: Kessner, Darren E. (Darren.Kessner_at_[hidden])
Date: 2007-12-18 21:32:39

Hi there,

 

First of all thank you to Jonathan Turkanis for the iostreams library -
it has been very useful.

 

I am using a filter to calculate a hash on data as it is sent to an
ostream, using symmetric_filter.  The problem is that even when I flush
the filtering_ostream, the extra bytes in the symmetric_filter
destination buffer aren't flushed out to the sink until the
filtering_ostream goes out of scope (or reset() or pop() is called).

 

I found the following in the FAQ:

"Note, however, that there is no guarantee that all data written to a
filtering_stream will be forwarded to the final Sink until the stream is
closed, unless all the Filters in the underlying chain are Flushable."

 

My questions:

1)      How does one make a symmetric_filter Flushable?

2)       Is this the best way to do this sort of on-the-fly hash
calculation?

 

I've included example code below.

 

Thank you in advance for any advice!

 

 

Darren

 

 

#include "boost/iostreams/filtering_stream.hpp"

#include "boost/iostreams/filter/symmetric.hpp"

#include <iostream>

using namespace std;

 

class MySymmetricFilter 

{

    public:

    typedef char char_type;

 

    bool filter(const char*& src_begin, const char* src_end,

                char*& dest_begin, char* dest_end, bool flush)

    {

        // just copy everything

        for (; src_begin!=src_end && dest_begin!=dest_end

             ; ++src_begin, ++dest_begin)

            *dest_begin = *src_begin;

 

        // do some hash calculation on bytes copied 

 

        return false;

    }

 

    void close() {}

};

 

struct MyFilter 

    : public boost::iostreams::symmetric_filter<MySymmetricFilter>

{

    typedef boost::iostreams::symmetric_filter<MySymmetricFilter>
base_type;

    MyFilter(int bufferSize) : base_type(bufferSize) {}

};

 

int main()

{

    boost::iostreams::filtering_ostream fos;

    fos.push(MyFilter(40)); // buffer size == 40

    fos.push(cout);

 

    fos << "The quick brown fox jumps over the lazy dog" << flush; //
length == 43

    cout << "123456789 ";

    flush(fos); // no effect?

 

    return 0;

 

    // prints "The quick brown fox jumps over the lazy 123456789 dog"

    // last 3 bytes "dog" are printed when fos goes out of scope

}

 

 

Darren Kessner

Scientific Programmer

Darren.Kessner_at_[hidden]

 

Spielberg Family Center for Applied Proteomics

Cedars-Sinai Medical Center

http://www.sfcap.cshs.org/

 

 

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