Re: Exception safety of generated C++ parser

[Luca]

On 13/09/2012 22:52, Oleksii Taran wrote:
> 13.09.12 19:32, Hans Aberg wrote:
> > On 13 Sep 2012, at 17:54, Oleksii Taran wrote:
> >
> > > Does Bison guarantees exception safety in C++ parser, i.e. will it make
> > > any clean-up and free memory if an exception is thrown on rule action?
> > Yes, the default stack is std::deque.
> >
> > > I
> > > know about %destructor directive, but it seems irrelevant, since
> > > destructor code is called only if parser recovers from _syntax_ error.
> > This is for the C parser, since C does not provide automatic cleanup. I might 
> > be possible to compile this parser as C++ (supported at least in the past), in 
> > which case %destructor would be needed.
If "C" the parser doesn't recover from a syntax error context (i.e. the 
code is syntactically correct), you have to free yourself the memory. I 
just suggest to create a simple tree inside the actions, and to free it 
from the leaves up to the root. When a syntax error happens instead, you 
have to free many sub-trees.
Please keep also into account that freeing memory is really needed only 
when you design an interactive always running parser, like a command 
line one; for a "batch" compiler the memory will be freed when the 
process will exit.
> But what if deque holds not objects, but pointers to them? e.g. code
> like this:
>
> %union {
>    ::std::string *token;
> }
>
> How Bison knows that it should call something like `delete token;` ?
Using %destructor you can specify the action (delete) and the production 
to apply, for example:
%destructor{delete $$}  exp stmt
since $$ is a

::std::string *token;


for "exp" and "stmt".
Also using "C", you can always put a void* inside the union.




Luca