Conference turris::c_plus_plus

>          str(char *p)     // constructor with one arguments
>           {
>              p = new char[strlen(s)];
>              strcpy(p,s);
>           }
        
        There is a memory corrupter there; you need to 'new'
        strlen(s)+1 characters in order to hold the terminating
        '\0' byte.
        
        Dan

Hi,

I think the problem has to do with how the qtail
and qhead classes were designed (or misdesigned (:). 
Let me explain.

A particular queue consists of a qtail object 
and a qhead object. There is a one-to-one correspondence
between these two objects, i.e. a qhead contains
a pointer to the qtail and the qtail contains a
pointer to the qhead.

Now let's see what you're trying to do with code
like:

qh = *((qhead*) qt.head());

You're trying to assign a qhead object to another
qhead object.

I don't see how this could ever work. If the user
is allowed to assign or copy qhead/qtail objects 
into other qhead/qtail objects then there
would be no way to maintain the one-to-one correspondence
between a qhead and a qtail. I actually think the
library should have declared a private assignment operator
and copy constructor for qhead and qtail to prevent
users from attempting something like this since I
don't think (without redesigning the whole class)
that these functions could ever be written to work as
the user expects they should.

So I think your code should be changed to something
like the code below.. (I also fixed up the str class.)

Judy 


#include <iostream.h>
#include <task.hxx>

class str : public object
{
    char *s;
public:
     str(char *p)     // constructor with one arguments
     {
         s = new char[strlen(p)+1];
         strcpy(s,p);
     }

      ~str() { delete [] s; } // destructor
      void prnt()          // print the deleted element
      {
            cout << " Removed entry : " << s << endl;
      }
};

void main()
{
   qtail qt(WMODE,10);    // qt is object of type qtail
   qhead qh;              // qh is object of type qhead

   qtail* qtp=&qt;
   qhead* qhp=&qh;
 
   str a("hello");       
   str b("queue");
   str c("cpp"); 
   str *a1,*a2,*a3;
  a1 = &a;
  a2 = &b;
  a3 = &c;

 // set the maximun size of queue

   qtp->setmax(10);

cout << " Before inserting ::  Free slots : " << qtp->rdspace() 
     << " Entries : " << qhp->rdcount() << endl;
 
// inserting the element into the queue

if(!qtp->put(a1))
 cout << " error in iserting 1 " << endl;
 qhp = qtp->head();

 cout << " Before inserting ::  Free slots : " << qtp->rdspace() 
	<< " Entries : " << qhp->rdcount() << endl;

// inserting the second element

if(!qtp->put(a2))
 cout << " error in iserting 1 " << endl;
 qhp = qtp->head();

 cout << " Before inserting ::  Free slots : " << qtp->rdspace() 
	<< " Entries : " << qhp->rdcount() << endl;
 
 // deleting the element from the queue

 a1 = (str*) qhp->get();
 a1 -> prnt();   // printing the element
 qtp = qhp->tail();
 cout << " Before inserting ::  Free slots : " << qtp->rdspace() 
	<< " Entries : " << qhp->rdcount() << endl;

// delete the second element

 a2 = (str*) qhp->get();
 a2 -> prnt();   // printing the element
 qtp = qhp->tail();
 cout << " Before inserting ::  Free slots : " << qtp->rdspace() 
	<< " Entries : " << qhp->rdcount() << endl;

// inserting third element
// problem with inserting the element into the queue

if(!qtp->put(a3))
 cout << " error in iserting 1 " << endl;
 qhp = qtp->head();

 cout << " Before inserting ::  Free slots : " << qtp->rdspace() 
	<< " Entries : " << qhp->rdcount() << endl;

// delete the third element
 
 a3 = (str*) qhp->get();
 a3 -> prnt();   // printing the element
 qtp = qhp->tail();
 cout << " Before inserting ::  Free slots : " << qtp->rdspace() 
	<< " Entries : " << qhp->rdcount() << endl;

}