On Fri, Apr 2, 2010 at 2:40 PM, Navin <ng06d...@gmail.com> wrote:
> I believe I get the piece about constructing a message for a specific
> type given a descriptor file.
>
> About your last statement:
> "If person.pb contains the serialized (binary) data for your proto,
> you
> should be using one of the Message::ParseFrom* routines. "
>
>
> My understanding was I needed a "Person" object, before I could use
> the "Parse" routines and hence I went through this trail of getting a
> descriptor file and then constructing a Message(of type Person) and
> then using that to Parse.
>
I was just trying to differentiate between the serialized format and the
text format. In your original email you stated that the file contained the
serialized data. However, the code snippet you gave was using
TextFormat::Parse. You should only use that method to parse data produced by
TextFormat::Print. The routines in Message deal with the serialized format.
>
> Are you saying that I do not need any descriptors etc at all. Given
> ANY data file (which is a serialized proto buf file) I can use
> Message::Parse and print it out ???
>
Message is an abstract class. Given the serialized data and an instance of
Message, you can invoke ParseFrom*. The trick is getting the instance of
Message - you either need to use the correct generated class (in this case
Person), or else use DynamicMessage to emulate that type. Since you want
your code to work with arbitrary types, and not be constrained to reading
encoded data for a Person message, you should use DynamicMessage. Does that
make sense?
>
> For e.g. I see that protoc --decode_raw < person.pb does indeed print
> it out correctly (without any descriptor OR any .proto file), so I
> think this may be possible.
>
This just reads the wire format without any type information. The wire
format is a dense encoding and many field types may be represented using the
same encoding, so doing this will not necessarily give you the data you
want. For instance, doubles and fixed size 64-bit integers are encoded in
the same way on the wire. If you had serialized a message containing a
double field, --decode_raw would interpret that data as a 64-bit little
endian value - it doesn't know that the value is actually a double.
>
> In summary all I want to do is given ANY serialized message in a file,
> I want to load it and print its contents out. That all.
>
> Thanks again.
>
>
>
>
>
>
>
> On Apr 2, 1:42 pm, Jason Hsueh <jas...@google.com> wrote:
> > On Fri, Apr 2, 2010 at 9:13 AM, Navin <ng06d...@gmail.com> wrote:
> > > 1. For the person example, I create a descriptor as follows:
> > > "E:\Desktop\prototype_MAIN\protoc.exe" person.proto --
> > > descriptor_set_out=msg.protocomp
> >
> > > 2. I create a Person object and serialized it to file. "person.pb"
> >
> > > 3. I want to write a program to "print" the Person object using ONLY
> > > its descriptor. I know there is a way, but I cannot seem to get all
> > > the APIs correct.
> >
> > > 4. Here is what I have done.
> > > ifstream desc_file("E:\\Visual Studio 2008\\Projects\
> > > \proto_buf1\\msg.protocomp",ios::in);
> > > if (desc_file.bad()) {
> > > std::cout << "desc file not read...exiting" <<
> std::endl;
> > > return -1;
> > > }
> >
> > > FileDescriptorSet f;
> > > f.ParseFromIstream(&desc_file);
> >
> > f.PrintDebugString();
> >
> >
> >
> > > FileDescriptorProto fdp = f.file(0);
> > > fdp.PrintDebugString();
> >
> > > DescriptorProto dp = fdp.message_type(0);
> > > dp.PrintDebugString();
> >
> > > Now what ? I need a copy of "Message" that I can pass as input to
> > > Parse, I think. I also may need DynamicMessage etc, and possibly also
> > > the following:
> >
> > Yes, you should construct a DynamicMessage. To do this, you'll need to
> build
> > the descriptor protos into a pool. For instance:
> >
> > DescriptorPool descriptor_pool;
> > for (int i = 0; i < f.file_size(); ++i) {
> > descriptor_pool.BuildFile(f.file(i));}
> >
> > const Descriptor* descriptor =
> descriptor_pool.FindMessageTypeByName("...");
> > DynamicMessageFactory factory;
> > Message* my_message = factory.GetPrototype(descriptor)->New();
> > ...
> > delete my_message;
> >
> > (You should probably check the return values of most of these calls -
> > BuildFile() may return NULL, for instance. You also probably want to have
> > some mechanism for specifying the message type, since you generally won't
> be
> > able to assume that it's the first message in the first file)
> >
> >
> >
> > > ifstream input_file("E:\\Visual Studio 2008\\Projects\
> > > \proto_buf1\\Debug\\person.pb",ios::in);
> > > if (input_file.bad()) {
> > > std::cout << "file not read...exiting" << std::endl;
> > > return -1;
> > > }
> >
> > > IstreamInputStream is(&input_file);
> >
> > > TextFormat::Parse(&is,(msg));
> >
> > If person.pb contains the serialized (binary) data for your proto, you
> > should be using one of the Message::ParseFrom* routines.
> >
> > > msg->PrintDebugString();
> >
> > > Any simple example is GREATLY appreciated. Thanks.
> >
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