deleting base pointer

Mar 15, 2012 at 5:00am
pushpat (2)
Hi Folks :),

I need your help, I am getting segmentation fault for "delete bptr_1" ( please refer program written below ).

If i remove statements "delete dptr;" inside try{}, then the program executes otherwise it will give error.

Could you please tell me why shouldnot i delete "dptr" inside try{}.
Does delete dptr inside try{} deletes bptr_1 too?

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#include<iostream>
using namespace std;

class base
{
	public:
	virtual ~base()
	{ }

};

class derived: public base
{
	public:
	~derived()
	{ }

};

int main()
{
	base *bptr_1 = new derived();
	base *bptr_2 = new base();
	derived *dptr;

	try
	{
		dptr = dynamic_cast<derived*>(bptr_1);
		if(dptr == 0)
			cout<<"(1) base ptr1 not pointing to derived"<<endl;

              delete dptr; // deleting memory allocated. If i remove this line,then program works
	      dptr = 0;  // assigning the pointer to null, to reuse

		dptr = dynamic_cast<derived*>(bptr_2);
		if(dptr == 0)
			cout<<"(2) bptr_2 not pointing to derived"<<endl;
	}
	catch (exception& err)
	{
		cout<<"error catched :"<<err.what()<<endl;
	}


	cout<<"delete base ptr pointing to d"<<endl;
	delete bptr_1;   //SEGFAULT HERE

	cout<<"delete base ptr pointing to base"<<endl;
	delete bptr_2;

	cout<<"delete derived ptr"<<endl;
	delete dptr;
	return 0;
}
Edit & run on cpp.sh


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OUTPUT:

(2) bptr_2 not pointing to derived
delete base ptr pointing to d
Segmentation fault
Mar 15, 2012 at 5:05am
Peter87 (11251)
You delete the derived object on line 32 and on line 46 but you are not allowed to delete an object more than once.
Mar 15, 2012 at 5:07am
IceThatJaw (529)
it looks like you are trying to delete a pointer cast twice. I may be wrong but casting a pointer is not the same as allocating new memory using "new".

Mar 15, 2012 at 9:16am
cire (8284)
2 calls of new. 4 calls of delete... Not good.
Mar 15, 2012 at 1:51pm
clanmjc (666)
Line by line analysis of your code:

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int main()
{
	base *bptr_1 = new derived(); //bptr_1 points to some memory on heap
	base *bptr_2 = new base(); //bptr_2 points to some memory on heap
	derived *dptr;

	try
	{
		dptr = dynamic_cast<derived*>(bptr_1); //dptr now points to the same memory pointed to by bptr_1
		if(dptr == 0)
			//...

              delete dptr; //Delete the memory pointed to by dptr, bptr_1 is now dangling
	      dptr = 0;  //Notice, this should have also been done for bptr_1

		dptr = dynamic_cast<derived*>(bptr_2); //dynamic_cast fails so dptr is null
		if(dptr == 0)
			cout<<"(2) bptr_2 not pointing to derived"<<endl;
	}
	catch (exception& err)
	{
		cout<<"error catched :"<<err.what()<<endl;
	}


	cout<<"delete base ptr pointing to d"<<endl;
	delete bptr_1;   //SEGFAULT HERE, because bptr_1 is dangling, if you would have set bptr_1 to null like dptr this would be ok

	cout<<"delete base ptr pointing to base"<<endl;
	delete bptr_2; //This is ok, currently because bptr_2 points to a valid memory block

	cout<<"delete derived ptr"<<endl;
	delete dptr; //Currently ok because dptr is null
	return 0;
}


Deleting a null pointer has no effect if using the operator supplied by the standard library.
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