toBuilder() makes only a shallow copy of the object. It does not copy the
contents of individual Strings, ByteStrings, sub-messages, etc.; it just
reuses the existing values. This means that it's generally reasonably
cheap, although obviously not quite as cheap as if the builders for
sub-objects were kept around by the parent. I'd suggest doing tests to see
if this is a problem for your app before trying to change protocol buffers.
On Wed, Dec 16, 2009 at 5:37 PM, Mark <mjsc...@gmail.com> wrote:
> Yes, this seems nice:
>
> Foo.Builder builder;
> builder.setBar(builder.getBar().toBuilder().setA(...).build());
>
> But I guess I'm a little worried about its hidden cost if I use it a
> lot. If message Bar was very large, then would converting it to a
> builder, setting one value, and converting it back to a message again
> incur a large cost? That is, is the time cost of setting one of a
> builder's sub-message's values proportional to the total size of the
> sub-message? That would be quite different than the cost of setting
> one of a builder's values, which is constant.
>
> If I know that a message of type Foo contains some Foo-specific
> information, then I'd want to be able to write code that reads a
> message of type Foo and gives me back that information. Like in your
> example, I know that a Foo message has some "a" information and some
> "b" information. I could write a method GetA(Foo) : int32 ... well,
> Option<int32> anyway. (It might be null.) Then if I later decided to
> move the field "a" from Bar to Foo I wouldn't have to change client
> code of GetA(Foo) at all. It would be just as fast, too, because there
> really isn't any difference between msg.getBar().getA() and msg.getA
> (). I can do this right now, which is great.
>
> But writing is a different story. Let's say I moved "a" from Bar to
> Foo. I know that I can write to a builder of type Foo.Builder some "a"
> information and some "b" information. I could write a method SetA
> (Foo.Builder, int32) : unit that just invokes the setA method in
> Foo.Builder. But then, if I decided to move "a" back to Bar, then my
> implementation of SetA(Foo.Builder, int32) : unit has to change a lot.
> I might have to convert Foo's bar from a message to a builder, set "a"
> in it, and rebuild the updated Bar message. That doesn't change the
> type signature of SetA but it might have a big effect on the cost of
> calling the method.
>
> If builders could contain builders, however, then I'd know for sure
> that I could change where "a" lives without significantly affecting
> the performance of methods like SetA!
>
> And I do those kinds of changes to my protocol a lot--I'm always
> rearranging how information relates to each other as my applications
> evolve. Is it just me?
>
> On Dec 16, 11:55 am, Jason Hsueh <jas...@google.com> wrote:
> > You should be able to pass a builder to some method, and have the method
> > modify some value of the builder just fine.
> >
> > Modifying a value in a sub-message is a bit more inconvenient, but still
> > doable. If you have some type Bar in type Foo like:
> > message Bar {
> > optional int32 a = 1;
> > optional int32 b = 2;
> >
> > }
> >
> > message Foo {
> > optional Bar bar = 1;
> >
> > }
> >
> > Foo.Builder builder;
> > builder.setBar(builder.getBar().toBuilder().setA(...).build());
> >
> > Would that work for you?
> >
> >
> >
> > On Wed, Dec 16, 2009 at 9:24 AM, Mark <mjsc...@gmail.com> wrote:
> > > I wish I could pass a builder object to a method and have the method
> > > modify either a value of the builder or a value of a sub-message in
> > > the builder!
> >
> > > I came across this thread, which described exactly the problem I have.
> > > The Car/Engine example in the thread is perfectly illustrative of my
> > > scenario.
> >
> > >http://groups.google.com/group/protobuf/browse_thread/thread/16997910.
> ..
> >
> > > Has anyone else been in this same situation? What have you done to
> > > ameliorate the problem?
> >
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