How expensive is reflection?

[Leandro Lucarella]

Hello.

I'm facing a situation where using reflection could be useful, but I'm
wondering if this imposes a performance loss.

Would this (sightly modified extract from the C++ API[1]):

Foo foo = new Foo;
Descriptor* descriptor = foo->GetDescriptor();
FieldDescriptor* text_field = descriptor->FindFieldByName("text");
foo->ParseFromString(data);
Message::Reflection* reflection = foo->GetReflection();
std::cout << reflection->GetString(text_field);

be less efficient than this:

Foo foo;
foo.ParseFromString(data);
std::cout << foo.text();

?

Thank you.

[1] http://code.google.com/apis/protocolbuffers/docs/reference/cpp/google.protobuf.message.html

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Leandro Lucarella (luca) | Blog colectivo: http://www.mazziblog.com.ar/blog/
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[Kenton Varda]

(Please don't BCC this mailing list.  It's confusing to the recipients.)

Yes, it's less efficient, since the regular (generated) accessor is an inline method, whereas the Reflection-based accessor is virtual.  However, this only matters if you're using it in an inner loop in a performance-sensitive app.

[Leandro Lucarella]

Kenton Varda, el 27 de agosto a las 10:40 me escribiste:

> (Please don't BCC this mailing list. It's confusing to the recipients.)

I'm sorry, I've posted the message using gmane NNTP interface but for some
reason it didn't arrived so I bounced it to the list and I forgot the To:
header is not set for newsgroups =)

> Yes, it's less efficient, since the regular (generated) accessor is an
> inline method, whereas the Reflection-based accessor is virtual. However,
> this only matters if you're using it in an inner loop in a
> performance-sensitive app.

Ok, so it's just a matter of function call only, there's no extra
processing or lookups when using the reflection API?

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Leandro Lucarella (luca) | Blog colectivo: http://www.mazziblog.com.ar/blog/
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[Kenton Varda]

On Wed, Aug 27, 2008 at 11:46 AM, Leandro Lucarella <llu...@gmail.com> wrote:

Ok, so it's just a matter of function call only, there's no extra
processing or lookups when using the reflection API?

Just a simple array lookup to find the byte offset.  No hashtables or anything like that.

Note that Descriptor::FindFieldByName() *does* do a hashtable lookup, but you only need to do this once.  After you have the FieldDescriptor for the field you want, you can reuse it for every message.