Re: Dynamic Message and google::protobuf::Message incompatibility

[Feng Xiao]

On Thursday, January 17, 2013 8:15:53 AM UTC+8, m wrote:

Hi All,

I'm seeing an incompatibility between a dynamically generated google::protobuf::Message (DynamicMessage?) created with a google::protobuf::DynamicMessageFactory and a normal google::protobuf::Message.

What I'm running up against ultimately comes down to this: I can't dynamic_cast a DynamicMessage to a known derived type of google::protobuf::Message.  For example, let's say my proto file has the following message:

package my.package;

messsage Foo

{

  string blah = 1;

  uint32 bleh = 2;

}

and I successfully create a my::package::Foo with some code along the lines of:

const google::protobuf::Descriptor* descriptor = myImporter.pool()->FindMessageTypeByName( "my.package.Foo" );

google::protobuf::Message* message = myDynamicMessageFactory.GetPrototype( descriptor )->New();

after populating message successfully with other commands not shown here, I try to do something like:

my::package::Foo foo;

foo.CopyFrom( *message );

and I get a segfault in CopyFrom() because ultimately the following doesn't work (the pointer pFoo ends up being NULL down inside CopyFrom()/MergeFrom()):

CopyFrom() should be able to copy a DynamicMessage to a generated message if they share the same descriptor.

Can you post the whole generated method where the segfault happens?

 

my::package::Foo* pFoo = dynamic_cast< my::package::Foo* >( message );

It appears as though the derived type of message is not actually my::package::Foo, but instead google::protobuf::DynamicMessage (or something like that), so naturally the dynamic_cast fails.  I was sure to observe the output of message->DebugString() and message->GetTypeName() which returned all the appropriate information.

Is there any way to get a real my::package::Foo object from a dynamic one?

 

[m]

Thanks for your reply.

My application links against all the stub proto sources of the types I will be creating.  What I've found works is if I use the default(?) descriptor pool (google::protobuf::DescriptorPool::generated_pool()) and the default message factory (google::protobuf::MessageFactory::generated_factory()).  Given the type name and the content, I have a simple function that more or less does this:

const gpb::Descriptor* descriptor = gpb::DescriptorPool::generated_pool()->FindMessageTypeByName( "my.package.Foo" );

 

gpb::MessageFactory* factory = gpb::MessageFactory::generated_factory();

 

const gpb::Message* prototype = factory->GetPrototype( descriptor );

 

gpb::Message* message = prototype->New();

 

gpb::Message* m = gpb::TextFormat::ParseFromString( "blah: \"some string\" bleh: 3", message );

On Wednesday, January 16, 2013 4:15:53 PM UTC-8, m wrote:

Hi All,

I'm seeing an incompatibility between a dynamically generated google::protobuf::Message (DynamicMessage?) created with a google::protobuf::DynamicMessageFactory and a normal google::protobuf::Message.

What I'm running up against ultimately comes down to this: I can't dynamic_cast a DynamicMessage to a known derived type of google::protobuf::Message.  For example, let's say my proto file has the following message:

package my.package;

messsage Foo

{

  string blah = 1;

  uint32 bleh = 2;

}

and I successfully create a my::package::Foo with some code along the lines of:

const google::protobuf::Descriptor* descriptor = myImporter.pool()->FindMessageTypeByName( "my.package.Foo" );

google::protobuf::Message* message = myDynamicMessageFactory.GetPrototype( descriptor )->New();

after populating message successfully with other commands not shown here, I try to do something like:

my::package::Foo foo;

foo.CopyFrom( *message );

and I get a segfault in CopyFrom() because ultimately the following doesn't work (the pointer pFoo ends up being NULL down inside CopyFrom()/MergeFrom()):

[m]

Sorry, there's a typo on that last line.  It should be:

gpb::TextFormat::ParseFromString( "blah: \"some string\" bleh: 3", message );

[Feng Xiao]

Glad that you found what works for you. What baffles me is why CopyFrom() segfaults when given a DynamicMessage.  DynamicMessage can't be cast to Foo, but foo.CopyFrom() gets to be working with it. Did you find why it segfaults?

[m]

The furthest I followed it was looking down into the implementation of CopyFrom() which calls MergeFrom().  Inside of MergeFrom() there's a spot where it calls something like "dynamic_cast_if_available" which returns NULL (failing to dynamic_cast).

[Feng Xiao]

On Sat, Jan 19, 2013 at 2:13 AM, m <mike...@gmail.com> wrote:

The furthest I followed it was looking down into the implementation of CopyFrom() which calls MergeFrom().  Inside of MergeFrom() there's a spot where it calls something like "dynamic_cast_if_available" which returns NULL (failing to dynamic_cast).

Returning NULL shouldn't cause a segment fault. The generated code should be able to handle this NULL case properly.

 

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