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 14:42:35

On 02/21/13 12:53, Chris Stankevitz wrote:
> On Thu, Feb 21, 2013 at 9:04 AM, Larry Evans <cppljevans_at_[hidden]> wrote:
>> How do you decide whether the Stream argument to Decode contains a
>> String or a Double?  Is that already known?  IOW, if you open the
>> istream, do you *know* what types it contains already and only need
>> to fill in the values?
> 
> 
> Thank you all for your interest and help.  I LOVE boost and have been
> using it for almost a year now.  I cannot live without signals2,
> thread, and shared_ptr.  They have changed the way I.  I am eager to
> learn about the other components.
> 
> I created a simple program that basically does what I want to do.  Of
> course in my real application the objects are more complicated and
> this is just one small piece of everything I need to do.
> 
> What I hope to highlight with this sample cpp is:
> 
> 1. I am decoding data.  The format is about as simple and "fixed" as
> you can imagine.
> 
> 2. The data is not "POD-encoded" values.  Sometimes I have to perform
> conversions on the values.  For example "temperature" in the example.
> 
> 3. I would like to keep track of some static known-at-compile-time
> meta-data for each of the values such as the units they are in.  This
> I use for display purposes.
> 
> 4. Some values have meta data that other values do not, such as
> floating point values which have a "number of significant digits after
> the decimal point" which I illustrate with the "temperature" example.
> 
> 5. I will have many values.  ~60.  In my example I attempt to
> highlight what a mess the class will turn into if I make no attempt to
> use boost::variant, boost::any, polymorphism, etc.
> 
> 6. An external class "TCTimer" in my example will want access to
> Get/Set values in the class.
> 
> 7. I appear to be confusing/merging several concepts: 1) decoding 2)
> tracking meta data for members 3)
> replace-lots-of-members-and-with-a-vector-of-something-like-boost::variant

This 7th feature is not represented by your example code.  The example
code suggests the type being read is already known.  OTOH, your
previous example code had a vector of pointers to an abstract class:

 class EmployeePacket
  {
    std::vector<Item*> Items;

suggesting that *maybe* the concrete classes are only known at runtime.
OTOH, that same code had a CTOR:

    EmployeePacket()
    {
      Items.push_back(new String("name", "John Doe"));
      Items.push_back(new Double("salary", "USD", 1, 1));
    }

suggesting the actual values were really known and there was really
no need for the abstract Item class or, AFAICT, any need for something
like boost::variant.  Could you please clarify?

Also, the code:

  std::istringstream Stream(Bytes.substr(0, ByteCount));

  unsigned short Temperature;
  Stream.read(reinterpret_cast<char*>(&Temperature), 2);

  Stream.read(reinterpret_cast<char*>(&mAge), 4);

worries me because it assumes 2 bytes are used to store a short
and 4 bytes are used to store an int. I'm not sure how portable this is.
In addition, the comments:

  // First two bytes are a temperature encoded as an uint16.
  // 0=>25 DegF
  // 2^16=>125 DegF
  //
  // Next four bytes Age is encoded as an int32
  //
  // Byte Value
  //    0 Temperature LSB
  //    1 Temperature MSB
  //    2 Age LSB
  //    3 Age ...
  //    4 Age ...
  //    5 Age MSB
  //
  // Total bytes: 6

suggest that there's a specific encoding of the values which implies
that there needs to be a specific decoding of the bit's in
the bytes in the Byte argument.  IOW, I don't think the reinterpret_cast
and using of istream.read will give you what you want according to
the comments.

HTH.

-regards,
Larry

> 
> Again, thank you,
> 
> Chris
> 
> 
> 
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