 | | From: | jsiehler | | Subject: | Using a yacc parser in Objc | | Date: | 13 Jan 2005 09:54:43 -0800 |
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 | I have an Objective C class called 'Expr' with several subclasses, and
a yacc parser with reduction rules like
exp: exp '+' exp { $$ = [[BinaryOpExpr alloc] initAs: PLUSOP of:$1
and:$3];}
to build up a complicated Expr from a formula that has been input.
When yyparse() finishes, I would like it to return the completed Expr
to the calling program.
I tried modifying the yyparse header to
int yyparse (Expr *result)
and amending the final reduction action to include a line like
result = yyvsp[-1]
which does make result point to the completed object (and within
yyparse() I can do things like have result print or evaluate itself),
but upon return to the calling program, result is apparently pointing
to garbage, as if all the instances created during parsing get trashed
when yyparse() completes.
What's the right way to get my completed Expr out of yyparse() and back
to the calling program?
Thanks.
js
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| | From: | jsiehler | | Subject: | Re: Using a yacc parser in Objc | | Date: | 13 Jan 2005 10:40:44 -0800 |
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| Ach, what a dumb mistake I made. Got it figured out now.
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| | From: | jsiehler | | Subject: | Re: Using a yacc parser in Objc | | Date: | 13 Jan 2005 12:49:29 -0800 |
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| >I'm sure you mean Expr **result here.
Yes, that was my stupid mistake.
Your alternative is cleaner and you're precisely right about it needing
less maintenance since I have to edit the result if I should change the
grammer and re-yacc. Thanks.
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| | From: | Michael Ash | | Subject: | Re: Using a yacc parser in Objc | | Date: | Thu, 13 Jan 2005 12:43:17 -0600 |
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| jsiehler wrote:
> I have an Objective C class called 'Expr' with several subclasses, and
> a yacc parser with reduction rules like
>
> exp: exp '+' exp { $$ = [[BinaryOpExpr alloc] initAs: PLUSOP of:$1
> and:$3];}
>
> to build up a complicated Expr from a formula that has been input.
>
> When yyparse() finishes, I would like it to return the completed Expr
> to the calling program.
>
> I tried modifying the yyparse header to
> int yyparse (Expr *result)
I'm sure you mean Expr **result here. You want to "return" an Expr *, so
you need to pass a pointer to *that*.
> and amending the final reduction action to include a line like
> result = yyvsp[-1]
This would then become *result = yyvsp[-1].
> What's the right way to get my completed Expr out of yyparse() and back
> to the calling program?
My technique was to use a global variable to get the final result. In the
extra declarations section of my .y file, I put:
id parsedTree;
And then in the top-level parse statement:
program : method_list
{ parsedTree = $1; $$ = $1; }
;
This might be easier to maintain than your solution, and it seems a little
cleaner. But then again, this is yacc, and nothing is going to be very
clean.
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