[antlr-interest] Heterogeneous AST nodes (was: 2.7.2a4 ANTLR/C# - build ok, panic shutdown)
parrt at jguru.com
Fri Dec 6 13:30:27 PST 2002
On Thursday, December 5, 2002, at 09:30 PM, slightlynew wrote:
>> Perhaps you grabbed 11-29's version not the 30th :)
> Yeah, or made a dumber error than that!
> If I do get it to work I'll let you know. I might just wait for the
> C# version since my work is in C# these days.
The 30th version has the C# integrated :) Try it out :)
> By the way, I have a question about heterogeneous ASTs that relates
> to the "third arg". From recent posts I gather that the generated
> code used to do something like this (I'm simplifying here):
> ASTNode n = new MyTypeOfNode();
> and that this has been changed to something like this:
> ASTNode n = getASTFactory().createInstance("MyTypeOfNode");
> Internally, the factory appears to be using reflection to go from the
> string type name to a new object of that type. (I should qualify
> that I'm talking about the C# code, and assuming that the Java code
> does something similar.)
> Isn't creating an object in this way significantly more expensive?
> If you're creating, say, thousands of nodes of many different types,
> wouldn't one expect slower performance?
Yes. Oddly, the java version runs faster than before for homogeneous
creation (the only real test I had), which I didn't bother to
investigate. It uses the reflection thing too I believe.
> Given that the type
> information *is* known at compile time, why pay the overhead of run-
> time object creation?
Consistency for one. Flexibility for two: no point in a factory if you
don't use ;)
> This isn't an argument against using a factory. The factory could do
> a big switch statement (or the equivalent) on the name, like this:
> case "MyTypeOfNode":
> return new MyTypeOfNode();
I'd have to generate it though. You are free to build your own that
does this though. Literally I tried to add an IF to check for the
CommonAST type and then do "new CommonAST" and it was slower!
> and then resort to the run-time way at the end:
> return createInstance("MyTypeOfNode");
> Although, come to think of it, it's not obvious to me exactly what
> the advantage of the factory approach is in this context?
Only that you can track or otherwise play with the node creation w/o
changing the generated code.
Creator, ANTLR Parser Generator: http://www.antlr.org
Lecturer in Comp. Sci., University of San Francisco
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