dynamic_message.h

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// Protocol Buffers - Google's data interchange format
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// Author: [email protected] (Kenton Varda)
// Based on original Protocol Buffers design by
// Sanjay Ghemawat, Jeff Dean, and others.
//
// Defines an implementation of Message which can emulate types which are not
// known at compile-time.

#ifndef GOOGLE_PROTOBUF_DYNAMIC_MESSAGE_H__
#define GOOGLE_PROTOBUF_DYNAMIC_MESSAGE_H__

#include <memory>

#include <google/protobuf/message.h>
#include <google/protobuf/stubs/common.h>

namespace google {
namespace protobuf {

// Defined in other files.
class Descriptor; // descriptor.h
class DescriptorPool; // descriptor.h

// Constructs implementations of Message which can emulate types which are not
// known at compile-time.
//
// Sometimes you want to be able to manipulate protocol types that you don't
// know about at compile time. It would be nice to be able to construct
// a Message object which implements the message type given by any arbitrary
// Descriptor. DynamicMessage provides this.
//
// As it turns out, a DynamicMessage needs to construct extra
// information about its type in order to operate. Most of this information
// can be shared between all DynamicMessages of the same type. But, caching
// this information in some sort of global map would be a bad idea, since
// the cached information for a particular descriptor could outlive the
// descriptor itself. To avoid this problem, DynamicMessageFactory
// encapsulates this "cache". All DynamicMessages of the same type created
// from the same factory will share the same support data. Any Descriptors
// used with a particular factory must outlive the factory.
class LIBPROTOBUF_EXPORT DynamicMessageFactory : public MessageFactory {
 public:
 // Construct a DynamicMessageFactory that will search for extensions in
 // the DescriptorPool in which the extendee is defined.
 DynamicMessageFactory();

 // Construct a DynamicMessageFactory that will search for extensions in
 // the given DescriptorPool.
 //
 // DEPRECATED: Use CodedInputStream::SetExtensionRegistry() to tell the
 // parser to look for extensions in an alternate pool. However, note that
 // this is almost never what you want to do. Almost all users should use
 // the zero-arg constructor.
 DynamicMessageFactory(const DescriptorPool* pool);

 ~DynamicMessageFactory();

 // Call this to tell the DynamicMessageFactory that if it is given a
 // Descriptor d for which:
 // d->file()->pool() == DescriptorPool::generated_pool(),
 // then it should delegate to MessageFactory::generated_factory() instead
 // of constructing a dynamic implementation of the message. In theory there
 // is no down side to doing this, so it may become the default in the future.
 void SetDelegateToGeneratedFactory(bool enable) {
 delegate_to_generated_factory_ = enable;
 }

 // implements MessageFactory ---------------------------------------

 // Given a Descriptor, constructs the default (prototype) Message of that
 // type. You can then call that message's New() method to construct a
 // mutable message of that type.
 //
 // Calling this method twice with the same Descriptor returns the same
 // object. The returned object remains property of the factory and will
 // be destroyed when the factory is destroyed. Also, any objects created
 // by calling the prototype's New() method share some data with the
 // prototype, so these must be destroyed before the DynamicMessageFactory
 // is destroyed.
 //
 // The given descriptor must outlive the returned message, and hence must
 // outlive the DynamicMessageFactory.
 //
 // The method is thread-safe.
 const Message* GetPrototype(const Descriptor* type);

 private:
 const DescriptorPool* pool_;
 bool delegate_to_generated_factory_;

 // This struct just contains a hash_map. We can't #include <google/protobuf/stubs/hash.h> from
 // this header due to hacks needed for hash_map portability in the open source
 // release. Namely, stubs/hash.h, which defines hash_map portably, is not a
 // public header (for good reason), but dynamic_message.h is, and public
 // headers may only #include other public headers.
 struct PrototypeMap;
 scoped_ptr<PrototypeMap> prototypes_;
 mutable Mutex prototypes_mutex_;

 friend class DynamicMessage;
 const Message* GetPrototypeNoLock(const Descriptor* type);

 // Construct default oneof instance for reflection usage if oneof
 // is defined.
 static void ConstructDefaultOneofInstance(const Descriptor* type,
 const int offsets[],
 void* default_oneof_instance);
 // Delete default oneof instance. Called by ~DynamicMessageFactory.
 static void DeleteDefaultOneofInstance(const Descriptor* type,
 const int offsets[],
 void* default_oneof_instance);

 GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(DynamicMessageFactory);
};

} // namespace protobuf

} // namespace google
#endif // GOOGLE_PROTOBUF_DYNAMIC_MESSAGE_H__