[Rprotobuf-yada] placeholder for lookup mechanism

[Romain François]

Hi,

I've added some placeholders (lookup.R, lookup.cpp) for the lookup 
mechanism. Follow the white rabbit from the lookup.R file.

The idea is that R asks our package when it looks up for symbols in the 
search path, if the desriptor pool [1] knows a message with the given 
name, we build a "protobufDescriptor" object in R.

Combined with previous (also placeholder) code, this means we can do 
things like this:

new( Person )

given the descriptor pool knows the "Person" message type.

I need to get my hands on DescriptorPool to understand how to implement 
the interface (get, exists, objects, assign, remove) and we are good to go.

[1] 
http://code.google.com/apis/protocolbuffers/docs/reference/cpp/google.protobuf.descriptor.html#DescriptorPool

-- 
Romain Francois
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http://romainfrancois.blog.free.fr
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