In some situations it may be necessary to intercept the insertion and extraction of Ice objects. For example, the Ice extension for PHP is implemented using Ice for C++ but represents Ice objects as native PHP objects. The PHP extension accomplishes this by manually encoding and decoding Ice objects as directed by the data encoding rules. However, the extension obviously cannot pass a native PHP object to the C++ stream function writeObject. To bridge this gap between object systems, Ice supplies the classes ObjectReader and ObjectWriter:
namespace Ice {
class ObjectReader : public Ice::Object {
public:
virtual void read(const InputStreamPtr&, bool) = 0;
// ...
};
typedef ... ObjectReaderPtr;
class ObjectWriter : public Ice::Object {
public:
virtual void write(const OutputStreamPtr&) const = 0;
// ...
};
typedef ... ObjectWriterPtr;
}
A foreign Ice object is inserted into a stream using the following technique:
- A C++ "wrapper" class is derived from
ObjectWriter. This class wraps the foreign object and implements thewritemember function. - An instance of the wrapper class is passed to
writeObject. (This is possible becauseObjectWriterderives fromIce::Object.) Eventually, thewritemember function is invoked on the wrapper instance. - The implementation of
writeencodes the object's state as directed by the data encoding rules for classes.
It is the application's responsibility to ensure that there is a one-to-one mapping between foreign Ice objects and wrapper objects. This is necessary in order to ensure the proper encoding of object graphs.
Extracting the state of a foreign Ice object is more complicated than insertion:
- A C++ "wrapper" class is derived from
ObjectReader. An instance of this class represents a foreign Ice object. - An object factory is installed that returns instances of the wrapper class. Note that a single object factory can be used for all Slice types if it is registered with an empty Slice type ID.
- A C++ callback class is derived from
ReadObjectCallback. The implementation ofinvokeexpects its argument to be either nil or an instance of the wrapper class as returned by the object factory. - An instance of the callback class is passed to
readObject. - When the stream is ready to extract the state of an object, it invokes
readon the wrapper class. The implementation ofreaddecodes the object's state as directed by the data encoding rules for classes. The boolean argument toreadindicates whether the function should invokereadTypeIdon the stream; it is possible that the type ID of the current slice has already been read, in which case this argument isfalse. - The callback object passed to
readObjectis invoked, passing the instance of the wrapper object. All other callback objects representing the same instance in the stream (in case of object graphs) are invoked with the same wrapper object.