[osg-users] How do I Subclass a file loader?

[brettwiesner]

Robert,

OK, I'm reading up on basic_streambuf<char> now. In the meantime, maybe 
you can help me understand how I can use a derived streambuf with the 
existing IVE loader. Aren't I going to run into the same problem? When 
the IVE loader comes across a proxy node, it will write out the node 
using the IVE loader and it won't matter that I originally made the call 
to writeNode() with my specialized ostream. It's going to create a file 
stream and write out the proxy node and there's no way for me to change 
that.

Thanks,
Brett

Robert Osfield wrote:
> Hi Brett,
>
> You need to read up about deriving your own custom std::streambuf,
> once you understand this part then adapting existing plugins to use
> your encrypted stream will be straightforward.  You could even be lazy
> a use a stringstream like the curl plugin does.  Please look into this
> stuff - this will answer your need, and there is certainly no need to
> subclass from plugins, C++ std::stream streambuf extensibility exist
> to help solve problem like yours, and the OSG takes advantage of this.
>
> Robert.
>
> On Wed, Oct 1, 2008 at 9:27 PM, brettwiesner <brettwiesner at gmail.com> wrote:
>   
>> Hey Robert,
>>
>> I'm confused about what you mean here. Many plugins have read/ write methods
>> that take istreams/ ostreams (ReaderWriterIVE being one of them). However,
>> since I can't subclass them I can't attach to the stream. For example, in
>> ReaderWriterIVE::writeNode() a std::ofstream is created but there's no way
>> for me to attach it to a stream buffer.
>>
>> It seems like the osga plugin would have the same problem. The doread()/
>> dowrite() methods call up to whatever ReaderWriter was passed in and I can't
>> subclass those.
>>
>> Thanks,
>> Brett
>>
>> Robert Osfield wrote:
>>     
>>> Hi Brett,
>>>
>>> A number of the plugins support reading a writing from istream/ostream
>>> which means you can implement your std::streambuffer and attach to the
>>> stream and then pass this to the plugin.  This technique allows you to
>>> do items like compression/decompression/encription/decription.  Have a
>>> look at the osga and curl plugins as they provide examples of using
>>> streams this way.
>>>
>>> Robert.
>>>
>>> On Wed, Oct 1, 2008 at 5:02 PM, brettwiesner <brettwiesner at gmail.com>
>>> wrote:
>>>
>>>       
>>>> Hi,
>>>>
>>>> I've got a requirement to ship certain 3rd party model data only in an
>>>> encrypted format. So I wrote my own loader that does the encryption/
>>>> decryption but uses the IVE loader for everything. This works except for
>>>> files that reference other files. The master file is encrypted, but the
>>>> referenced files are saved out as .ive's.
>>>>
>>>> Ideally all I would have to do is subclass ReaderWriterIVE and override
>>>> the
>>>> following stream methods:
>>>>
>>>>  virtual osgDB::ReaderWriter::ReadResult readObject(std::istream& fin,
>>>>    const osgDB::ReaderWriter::Options* options) const;
>>>>
>>>>  virtual osgDB::ReaderWriter::ReadResult readImage(std::istream& fin,
>>>>    const osgDB::ReaderWriter::Options* options) const;
>>>>
>>>>  virtual osgDB::ReaderWriter::ReadResult readNode(std::istream& fin,
>>>>    const osgDB::ReaderWriter::Options* options) const;
>>>>
>>>>  virtual osgDB::ReaderWriter::WriteResult writeObject(const osg::Object&
>>>> object,
>>>>    std::ostream& fout, const osgDB::ReaderWriter::Options* options)
>>>> const;
>>>>
>>>>  virtual osgDB::ReaderWriter::WriteResult writeImage(const osg::Image&
>>>> image,
>>>>    std::ostream& fout, const osgDB::ReaderWriter::Options* options)
>>>> const;
>>>>
>>>>  virtual osgDB::ReaderWriter::WriteResult writeNode(const osg::Node&
>>>> node,
>>>>    std::ostream& fout, const osgDB::ReaderWriter::Options* options)
>>>> const;
>>>>
>>>> For writing I could get the raw data from the ive loader encrypt it and
>>>> write it out. For reading I could decrypt the data, then pass the
>>>> unencrypted data up to the ive loader for use.
>>>>
>>>> There is one fatal flaw... I can't subclass the ReaderWriterIVE plugin.
>>>> :(
>>>>
>>>> 1) Has anyone besides me ever wanted to derive from a file loader? Would
>>>> it
>>>> make sense to keep the logic in a lib with headers, then have another
>>>> library that is just the plugin?
>>>>
>>>> 2) Is there some other mechanism in OSG that will let me do this?
>>>>
>>>> Thanks,
>>>> Brett
>>>> _______________________________________________
>>>> osg-users mailing list
>>>> osg-users at lists.openscenegraph.org
>>>> http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
>>>>
>>>>
>>>>         
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