Re: C++ objects and flex

gus gassmann <>
Sat, 02 Mar 2013 23:27:18 -0400
Let me try this again, before the original message disappears from the
news server...

On 11/02/2013 4:46 PM, Jorgen Grahn wrote:

On Fri, 2013-02-08, Gus Gassmann wrote:

Not sure if this is a C++ question. If not, I apologize. Is there a
group where I could direct questions that deal with flex?

I don't think so. Most people use flex/bison rarely.

My situation is this: I have a (fairly complicated) C++ object (call it
Obj, perhaps) that I need to populate by parsing a string. I use
flex/bison to parse the string, and I currently have another C++ class,
ObjReader, containing a method readObj(std::string objStr) that calls
the parser and stores the information read into the various elements of
a private member Obj* ObjReader->my_obj.

My problem is that I constantly get into memory trouble (variously leaks
and segfaults), because I ultimately want to access the data from my own
instance of Obj and often trigger (viz., construct/destruct) the reader
in a subroutine.

I don't understand the reference to subroutines. I think you mean a
"function", but everything you do in C or in C++ is done in a

OK. Fair enough. I guess my Fortran roots are showing. So let me try
again, with a little bit more of the code showing.

First, I have a complicated object class Obj, with code file Obj.cpp and
header file Obj.hpp. Let's say this consists of 5000 lines of nested
objects, and let's leave it at that. I hope I can explain the rest
without going into the details.

I then have an ObjReader class that looks like this:


class ObjReader

     Obj *readObj(const std::string &objstring);

   Obj* p_obj;


#include "ObjReader.h"

//bison function
void yygetObj(const char *objstring, Obj* obj;

//lex functions
int objlex_init(void** ptr_yy_globals);
int objlex_destroy (void* scanner );
void objset_extra (void* yyscanner );

ObjReader::ObjReader( )

     if(p_obj != NULL) delete p_obj;
     p_obj = NULL;

Obj* ObjReader::readObj(const std::string& objstring)
     if (objstring.length() > 0)
         const char *ch = objstring.c_str();
         yygetObj( ch, my_obj);
     return p_obj;
}//end readObj


void method1(Obj* my_obj);

void main()
     Obj* my_obj = new Obj();

void method1(Obj* my_obj)
     ObjReader *objreader = new ObjReader();
     my_obj = objreader->readObj("some XML string");

I do want to get the parsing out of main, just to make the code more
structured and easier for me to read and maintain. (I did not create the
design, I just inherited it.)

My trouble is: I newed the objreader in method1, but obj is only a
pointer to the private p_obj in the objreader.

Hence I have two choices:

I do not call the ObjReader destructor in method1, and I have created a
memory leak;

I do call the ObjReader destructor in method1, and I have created a
segfault as soon as I try to do anything with my_obj outside of method1.

I suppose I can get around it by thinking passing the ObjReader into
method1, but typically there are several layers of code involved, and
sometimes even different classes that are wrapped one inside the other.
It seems unnatural to me to pass the reader, and hence I forget, leading
to all sorts of frustrations. Is there no way to make the ObjReader a
member of the Obj class? I think (hope!) that this would fix my problem.
(And yes, I think I am stuck with flex/bison for this.)

I feel --- given what little I know of C++ and OOP ---
that I would be better off if I could make the ObjReader a member of the
Obj class.

However, I am quite unsure of how to handle the flex construct yyextra
in this context. I have tried to find examples on the web for anything
similar to what I want to do, and I have so far turned up nothing.

My only advice is:
- Don't use flex in its C++ mode (flex -+) -- it didn't seem well
   supported last time I checked.
- Treat the flex part of the problem as a C/flex problem, then wrap it
   in C++ when you're done.
- Get familiar with tools to handle memory leaks and crashes.


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