Subject: Re: [Boost-users] Can boost help me organize/parse lots of binary data?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2013-02-21 12:04:47
On 02/20/13 19:21, Chris Stankevitz wrote:
> On Wed, Feb 20, 2013 at 3:27 PM, Richard
> <legalize+jeeves_at_[hidden]> wrote:
>> Boost.Spirit.Qi to parse the stream into data structures
>> Boost.Spirit.Karma to emit the necessary output from the data structures
>
> Richard,
>
> Thank you. I am unfamiliar with Spirit and Karma. Your comment not
> only introduced me to them but highlighted just how vague my question
> was. I was thinking less about the actual decoding (who would have
> known based on what I wrote) and more on how I would "generically" and
> "extensibly" handle the many fields I am about to receive.
>
> For example, consider the following horrible code. Is there a "boost"
> way to do this kind of thing... (perhaps Spirit/Karma is the answer
> and I am just more out of touch than I imagine):
>
> Good things about the classes below:
> It is easy to add a new parameter. Just push one onto the back of
> EmployeePacket::Items
> The class EmployeePacket doesn't have hundreds of data members such as
> "string name_, int age_"
>
> Bad things about the classes below:
> Extracting data from the EmployeePacket requires hideous dynamic_casts
> and hard-coded vector indices
>
> Thank you again for your comments/criticisms/suggestions,
>
> Chris
>
> ===
>
> class EmployeePacket
> {
> std::vector<Item*> Items;
>
> EmployeePacket()
> {
> Items.push_back(new String("name", "John Doe"));
> Items.push_back(new Double("salary", "USD", 1, 1));
> }
>
> void Decode(istream& Stream)
> {
> for (auto pItem : Items) pItem->Decode(Stream)
> }
>
> double GetSalary()
> {
> return dynamic_cast<Double*>(Items[1])->value;
> }
> }
>
> class Item
> {
> virtual void Decode(istream& Stream) = 0;
> }
>
> class Double : public Item
> {
> 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 : public Item
> {
> string name;
> string value;
> void Decode(istream& Stream);
> }
Hi Chris,
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?
-regards,
Larry