Re: [Boost-users] Can boost help me organize/parse lots of binary data?

Subject: Re: [Boost-users] Can boost help me organize/parse lots of binary data?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2013-02-21 13:10:24

On 02/21/13 10:47, Larry Evans wrote:
> On 02/21/13 08:15, Antony Polukhin wrote:
>> This looks like a perfect place for Boost.Variant. Here is some pseudo code:
>>
>> #include <boost/variant.hpp>
>>
>> struct DecoderVisitor: boost ::static_vizitor<> {
>> istream& Stream;
>>
>> explicit DecoderVisitor(istream& Stream): Stream(Stream){}
>>
>> void operator()(String& s) const {
>> s.Decode(Stream);
>> }
>>
>> void operator()(Double& d) const {
>> d.Decode(Stream);
>> }
>>
>> // or you may just write
>> // template <class T>
>> // void operator()(T& val) const {
>> // val.Decode(Stream);
>> //}
>> };
>>
>>
>>   class EmployeePacket
>>    {
>> typedef boost::variant<String, Double> variant_t;
>>      std::vector<variant_t> Items;
>>
>>      EmployeePacket()
>>      {
>>        Items.push_back(String("name", "John Doe"));
>>        Items.push_back(Double("salary", "USD", 1, 1));
>>      }
>>
>>      void Decode(istream& Stream)
>>      {
>>        for (auto pItem : Items)
>> boost::apply_visitor(DecoderVisitor(Stream), pItem)
>>      }
>>
>>      double GetSalary()
>>      {
>>        return boost::get<Double>(Items[1]).value;
>>      }
>>    }
>>
>>  class Double
>>  {
>>    string name;
>>    string units;
>>  //  double decode_conversion_scale;
>>  //  double decode_conversion_translate;
>>  //  unsigned number_of_bits_used_to_encode
>>  //  double scale_factor
>>    double value;
>>    void Decode(istream& Stream)
>>     {
>>      Stream.read(&value, 8);
>>      value = value * decode_conversion_scale + decode_conversion_translate;
>>   }
>>
>>    class String
>>    {
>>      string name;
>>      string value;
>>      void Decode(istream& Stream);
>>    }
>>
>>
> Hi Anthony,
> 
> I haven't a clear idea how this would work because I assumed that
> the binary input stream would have to have something that
> 1st would be decoded into the tag for the variant.
Or, something like the serialization library's "external key":

http://www.boost.org/doc/libs/1_53_0/libs/serialization/doc/extended_type_info.html

AFAICT, such an external key serves a purpose similar
to variant's tag in that it's used to tag which value
is stored in some "storage medium'.  In the case of variant,
the "storage medium" is a character buffer inside variant.
In the case of the serialization library, it's the
serialization's archive file.

Of course maybe I'm oversimplifying.
[snip]