Bison error handling

[Bernd Clausen]

I have the following destructor specified for strings like IDENT.

%destructor { printf ("free at line %d: %s\n",@$.first_line, $$->c_str()); 
delete($$); } <str>

Now I have the following input. The first line is valid, the second line isn't 
(identifier is missing):

    ConfigParam p;
    ConfigParam;

Output:

    In input 2:12 - 2:12 : syntax error
    free at line 2: p
    free at line 1: statementlist


Because of an error in the second line, Bison complains, returns a syntax error 
and calls the destructors of all objects on the parsing stack.
Now I don't understand why the destructor of identifier "p" from the first line 
is called? It belongs to the first line and it was successfully parsed. The 
problem is that p is used in the main program and should be not removed by 
Bison.

If I choose as an (invalid) statement an arbitrary string ("foo") p is not 
removed by Bison.

    ConfigParam p;
    foo;

    In input 2:1 - 2:3 : syntax error
    free at line 1: statementlist
    free at line 2: foo

Why this works?

Thank you,
Bernd