Re: RAII techniques in semantic actions leading to double deletion

[Akim Demaille]

Hi John,

Le 5 déc. 2012 à 21:43, John Horigan <address@hidden> a écrit :

> My C++ grammar file is littered with code like this:
> 
> sizetime:
>        MODTYPE modification_v2 {
>            exp_ptr mod($2);
>            driver.lexer->maybeVersion = token::CFDG2;
>            if ($1 != ASTmodTerm::size && $1 != ASTmodTerm::time) {
>                error(@1, "Syntax error");
>            } else {
>                static const std::string sizeVar("CF::Size");
>                static const std::string timeVar("CF::Time");
>                driver.NextParameter($1 == ASTmodTerm::size ? sizeVar
> : timeVar, mod, @1, @2);
>            }
>            $$ = 0;
>        }
>        ;
> 
> 
> exp_ptr is a typedef for std::auto_ptr<expression_t>
> 
> The general idea is that the semantic values get automatically deleted
> if nothing in the action takes ownership of them. This served me well
> until recently, when I had a bug where an action threw an exception.
> The std::auto_ptr<> deleted the semantic values and then the parser
> caught the exception and tried to delete the semantic values a second
> time.

Could you be more specific about this deletion?  It looks like
you are dealing with pointers, so the parser should not try to
delete anything, unless you specified a %destructor.

Also, what skeleton are you using?

I tend to think this is a lot of clutter, not to mention the
(small) penalty for constructing all these guys.  Bison comes
with warnings when you forget mentioning some rhs member that
has a value, and valgrind can help catching the missing ones.
Shouldn't that suffice?

> One way to think about this is that it is an error for the action to
> delete a semantic value if there is any chance that the action can
> exit through an exception (or YYERROR, YYABORT, etc). But I really
> prefer to use RAII whenever I can.
> 
> Can I solve the problem by changing all of my auto_ptr<> declarations from 
> this:
> 
>            exp_ptr mod($2);
> 
> to this:
> 
>            exp_ptr mod($2); $2 = 0;
> 
> to explicitly move ownership from the semantic value stack to my
> auto_ptr<>? Would this actually do what I think it does and can I
> still use @2 after erasing $2?

That would work.  $2 and @2 are unrelated, changing one won't
change the other.