Re: [protobuf] how to get RepeatedField object
On Wed, Feb 17, 2010 at 12:19 AM, Romain Francois
romain.francois.r.enthusi...@gmail.com wrote:
On 02/17/2010 12:51 AM, Kenton Varda wrote:
The Reflection interface already provides a way -- FieldSize() and
GetRepeatedX(). The only problem is that it's a bit slower than the
generated accessors because these methods aren't inlineable.
Sure. I meant STL algorithms iterating.
You could easily write an STL-like iterator on top of these if you really
need it.
BTW, have you observed an actual performance problem or are you just
speculating that this performance difference may be a problem for you?
In similar (non protobuf-related) settings, we have observed quite a bit of
difference between using plain loops and accessors as opposed to iterators.
It would obviously depend on the data structure involved. For example, on
an stl vector, the following two loops will have equivalent performance:
vectorint v;
for (int i = 0; i v.size(); ++i) {
DoSomehting(v[i]);
}
for (vectorint::const_iterator i = v.begin(); i != v.end(); ++i) {
DoSomehting(*i);
}
In fact, a good compiler may even produce identical assembly code for both
loops.
But this is mainly speculation on the performance.
It's generally a bad idea to try to fix theoretical performance problems.
But when you use std::vector, it is best to first reserve the target size as
opposed to create an empty vector and push_back each element.
It's slightly faster, but either way is still O(n). Reserving is often not
worth the effort, especially if you are good about reusing objects, in which
case they will already have space reserved from the previous use.
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Re: [protobuf] how to get RepeatedField object
On 02/17/2010 12:51 AM, Kenton Varda wrote:
On Tue, Feb 16, 2010 at 2:47 PM, Romain Francois
romain.francois.r.enthusi...@gmail.com
mailto:romain.francois.r.enthusi...@gmail.com wrote:
Thanks for the feedback.
On 02/16/2010 10:44 PM, Kenton Varda wrote:
GeneratedMessageReflection is an internal class used by the protobuf
implementation. Currently, users are not allowed to use it
directly,
because we may change it at any time. You're suggesting that we
promote
it to a public interface, which has implications with regard to
maintenance costs and implementation agility.
I understand that.
I'm not necessarily suggesting to make this particular class public,
but am looking for some way to iterate over the elements of a
repeated field.
The Reflection interface already provides a way -- FieldSize() and
GetRepeatedX(). The only problem is that it's a bit slower than the
generated accessors because these methods aren't inlineable.
Sure. I meant STL algorithms iterating.
BTW, have you observed an actual performance problem or are you just
speculating that this performance difference may be a problem for you?
In similar (non protobuf-related) settings, we have observed quite a bit
of difference between using plain loops and accessors as opposed to
iterators.
But this is mainly speculation on the performance.
Also, this looks nicer :
double sum = std::accumulate( y.begin(), y.end(), 0.0 ) ;
than this:
double x = 0.0 ;
for( int i=0; in; i++){
x += ref.GetRepeatedDouble( message, field, i ) ;
}
... but I suppose we can wrap these in our own iterators, but then I
guess we'd have to resort to some sort of proxy to deal with
GetRepeatedDouble and SetRepeatedDouble.
On the same score, if I want to increase the number of elements in a
repeated field, I have to do it one by one right ? I can't do
something like first reserve space, and then fill the generated
space. Does that mean that memory is reallocated each time ?
No, adding an element is O(1), even if you haven't reserved space. It
works the same way std::vector works.
But when you use std::vector, it is best to first reserve the target
size as opposed to create an empty vector and push_back each element.
What you would really need to
use is dynamic_cast. My golden rule of dynamic_cast is that it
should
only be used for optimization, and you must provide an
implementation
for the case where dynamic_cast always returns NULL.
But dynamic_cast is a runtime thing, where TMP dispatch happens at
compile time.
GetReflection() returns Reflection*, NOT GeneratedMessageReflection*.
Therefore the compile has no idea which type was returned, therefore
you *must* use a runtime check. That's my point -- your template
dispatch doesn't accomplish anything in this context.
Ah ok. Sorry about that.
In your case, you
are doing this, so that should be fine.
On Sat, Feb 13, 2010 at 1:16 AM, Romain Francois
romain.francois.r.enthusi...@gmail.com
mailto:romain.francois.r.enthusi...@gmail.com
mailto:romain.francois.r.enthusi...@gmail.com
mailto:romain.francois.r.enthusi...@gmail.com wrote:
Hello,
Thanks for the answers.
Maybe I should give some more background on why this is of
interest
to me. In RProtoBuf, we essentially only use the reflection
api so
that we can dynamically load new proto message types at
runtime, etc
... we don't use protoc and therefore have no access to the
generated classes.
In the class GeneratedMessageReflection, there are
templates such as :
template typename Type
inline Type GetRepeatedField(const Message message,
const FieldDescriptor* field,
int index) const;
but they are private ? Why ?
From what I can read of the code, methods like GetRepeatedInt32
get expanded out of :
PASSTYPE GeneratedMessageReflection::GetRepeated##TYPENAME(
\
const Message message, \
const FieldDescriptor* field, int index) const { \
USAGE_CHECK_ALL(GetRepeated##TYPENAME, REPEATED,
CPPTYPE); \
if (field-is_extension()) { \
return GetExtensionSet(message).GetRepeated##TYPENAME(
\
field-number(), index); \
} else { \
return GetRepeatedFieldTYPE(message, field, index);
\
} \
} \
so doing things like this code
for( int i=0; isize; i++){
Re: [protobuf] how to get RepeatedField object
Thanks for the feedback.
On 02/16/2010 10:44 PM, Kenton Varda wrote:
GeneratedMessageReflection is an internal class used by the protobuf
implementation. Currently, users are not allowed to use it directly,
because we may change it at any time. You're suggesting that we promote
it to a public interface, which has implications with regard to
maintenance costs and implementation agility.
I understand that.
I'm not necessarily suggesting to make this particular class public, but
am looking for some way to iterate over the elements of a repeated field.
I see that this is available for classes that are generated by protoc,
which is what is mostly used, but unfortunately we don't/can't really do
that with R.
On the same score, if I want to increase the number of elements in a
repeated field, I have to do it one by one right ? I can't do something
like first reserve space, and then fill the generated space. Does that
mean that memory is reallocated each time ?
I'm open to considering making this change for performance purposes.
However, even them I'm hesitant to expose Repeated[Ptr]Field
references directly via this interface. I'd like to see what happens if
we simply make all of the existing public accessor methods inline, so
you could then do something like:
int size = reflection-FieldSize(message, field);
for (int i = 0; i size; i++) {
const Message sub_message =
reflection-GeneratedMessageReflection::GetRepeatedMessage(message,
field, i);
// Do something with sub_message.
}
If GetRepeatedMessage is inline then I believe the above loop would be
nearly as efficient as iterating directly over a RepeatedPtrField. Note
that the funny syntax for the method call avoids making a virtual call.
BTW, the has_templates template you suggest would not work as you
think -- how would you actually use it?
implement some sort of TMP dispatch :
template typename T
foo( T t ){
foo_dispatch( t, typename has_templatesT::value_type() ) ;
}
and then have
template typename T
foo( T t , false_type){
/* the less efficient version */
}
template typename T
foo( T t, true_type){
/* the more efficient version using the RepeatedField */
}
This is similar to how e.g std::distance dispatches depending on whether
it deals with random access iterator or some other iterator category.
What you would really need to
use is dynamic_cast. My golden rule of dynamic_cast is that it should
only be used for optimization, and you must provide an implementation
for the case where dynamic_cast always returns NULL.
But dynamic_cast is a runtime thing, where TMP dispatch happens at
compile time.
In your case, you
are doing this, so that should be fine.
On Sat, Feb 13, 2010 at 1:16 AM, Romain Francois
romain.francois.r.enthusi...@gmail.com
mailto:romain.francois.r.enthusi...@gmail.com wrote:
Hello,
Thanks for the answers.
Maybe I should give some more background on why this is of interest
to me. In RProtoBuf, we essentially only use the reflection api so
that we can dynamically load new proto message types at runtime, etc
... we don't use protoc and therefore have no access to the
generated classes.
In the class GeneratedMessageReflection, there are templates such as :
template typename Type
inline Type GetRepeatedField(const Message message,
const FieldDescriptor* field,
int index) const;
but they are private ? Why ?
From what I can read of the code, methods like GetRepeatedInt32
get expanded out of :
PASSTYPE GeneratedMessageReflection::GetRepeated##TYPENAME(
\
const Message message, \
const FieldDescriptor* field, int index) const { \
USAGE_CHECK_ALL(GetRepeated##TYPENAME, REPEATED, CPPTYPE); \
if (field-is_extension()) { \
return GetExtensionSet(message).GetRepeated##TYPENAME( \
field-number(), index); \
} else { \
return GetRepeatedFieldTYPE(message, field, index); \
} \
} \
so doing things like this code
for( int i=0; isize; i++){
INTEGER(res)[i] = (int) ref-GetRepeatedInt32(
*message,
fieldDesc,
i ) ;
}
is going to be not as efficient as if I could directly iterate over
the repeated field using RepeatedField::iterator
Instead of extending the Reflection interface, what about making the
templates public in GeneratedMessageReflection and then maybe use
some sort of trait to indicate whether that the instance of
Reflection I have access to has these templates.
Something like :
template typename _T, _T _V struct integral_constant {
static const _Tvalue = _V;
typedef _T value_type;
Re: [protobuf] how to get RepeatedField object
Why not ? It seems reasonnable to want to use e.g. std::copy and friends.
On the documentation it says :
Most users will not ever use a RepeatedField directly; they will use the
get-by-index, set-by-index, and add accessors that are generated for all
repeated fields
What if I do want to use RepeatedField ?
Romain
On 02/11/2010 06:50 PM, Jason Hsueh wrote:
No, there isn't a way to get the RepeatedField from the reflection
interface. You can only do so via the generated interface.
On Thu, Feb 11, 2010 at 5:57 AM, Romain Francois
romain.francois.r.enthusi...@gmail.com
mailto:romain.francois.r.enthusi...@gmail.com wrote:
Hello,
How can I get hold of a RepeatedField object to manage a repeated
field in C++.
In RProtoBuf, we do a lot of :
for( int i=0; isize; i++){
INTEGER(res)[i] = (int) ref-GetRepeatedInt32( *message, fieldDesc,
i ) ;
}
where essentially the INTEGER macro gives a pointer to the beginning
of the int array we are filling.
I'd like to replace this using e.g std::copy
RepeatedField field ;
std::copy( field.begin(), field.end(), INTEGER(res) ) ;
but I can't find how to actually get hold of a RepeatedField object.
Is it possible ?
Romain
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
|- http://tr.im/NrTG : Rcpp 0.7.5
|- http://tr.im/MPYc : RProtoBuf: protocol buffers for R
`- http://tr.im/KfKn : Rcpp 0.7.2
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Re: [protobuf] how to get RepeatedField object
+kenton
Kenton may have a better answer, but I surmise that it's to avoid tying the
Reflection interface to implementation details. A Message implementation
might not use RepeatedField at all. The original version of protobufs used a
different class to represent repeated fields, so it wouldn't have been
possible to implement Reflection for the original version if the interface
required access to RepeatedField. So maybe the reason is historical. Recent
changes have been made that would make this technically possible. However,
adding methods to get direct access to the RepeatedField would still expand
the Reflection interface quite a bit. I'll defer to Kenton on that.
On Fri, Feb 12, 2010 at 2:51 AM, Romain Francois
romain.francois.r.enthusi...@gmail.com wrote:
Why not ? It seems reasonnable to want to use e.g. std::copy and friends.
On the documentation it says :
Most users will not ever use a RepeatedField directly; they will use the
get-by-index, set-by-index, and add accessors that are generated for all
repeated fields
What if I do want to use RepeatedField ?
Romain
On 02/11/2010 06:50 PM, Jason Hsueh wrote:
No, there isn't a way to get the RepeatedField from the reflection
interface. You can only do so via the generated interface.
On Thu, Feb 11, 2010 at 5:57 AM, Romain Francois
romain.francois.r.enthusi...@gmail.com
mailto:romain.francois.r.enthusi...@gmail.com wrote:
Hello,
How can I get hold of a RepeatedField object to manage a repeated
field in C++.
In RProtoBuf, we do a lot of :
for( int i=0; isize; i++){
INTEGER(res)[i] = (int) ref-GetRepeatedInt32( *message, fieldDesc,
i ) ;
}
where essentially the INTEGER macro gives a pointer to the beginning
of the int array we are filling.
I'd like to replace this using e.g std::copy
RepeatedField field ;
std::copy( field.begin(), field.end(), INTEGER(res) ) ;
but I can't find how to actually get hold of a RepeatedField object.
Is it possible ?
Romain
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
|- http://tr.im/NrTG : Rcpp 0.7.5
|- http://tr.im/MPYc : RProtoBuf: protocol buffers for R
`- http://tr.im/KfKn : Rcpp 0.7.2
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Re: [protobuf] how to get RepeatedField object
No, there isn't a way to get the RepeatedField from the reflection
interface. You can only do so via the generated interface.
On Thu, Feb 11, 2010 at 5:57 AM, Romain Francois
romain.francois.r.enthusi...@gmail.com wrote:
Hello,
How can I get hold of a RepeatedField object to manage a repeated field in
C++.
In RProtoBuf, we do a lot of :
for( int i=0; isize; i++){
INTEGER(res)[i] = (int) ref-GetRepeatedInt32( *message, fieldDesc, i ) ;
}
where essentially the INTEGER macro gives a pointer to the beginning of the
int array we are filling.
I'd like to replace this using e.g std::copy
RepeatedField field ;
std::copy( field.begin(), field.end(), INTEGER(res) ) ;
but I can't find how to actually get hold of a RepeatedField object.
Is it possible ?
Romain
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
|- http://tr.im/NrTG : Rcpp 0.7.5
|- http://tr.im/MPYc : RProtoBuf: protocol buffers for R
`- http://tr.im/KfKn : Rcpp 0.7.2
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