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Deleting a pointer?

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Join Date: Jan 2009

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BevoX

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Junior Poster in Training

Re: Deleting a pointer?

Jan 4th, 2009

I have a rather simple question, and I' ve been looking for the answer for awhile, but did not find any.
So I would like to know, what is the proper way of deleting a pointer.

For example I have a class with some kind of array, which I want to extend later. But what if, for some reason, I won't have to. Like, I am opening a file, but it is not there, so I don't need to allocate any memory. Eveything is OK, then destructor starts working and then the whole program crashes. Altough It works fine if it had some amount of memory allocated before.
So would you please show me the proper way of deleting pointers?

Thank you very much.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
myClass::~myClass()
{
	if( my_array != 0 )
	{
		 delete []my_array;
	}	
}
myClass::~myClass()
{
	if( my_array != 0 )
	{
		 delete []my_array;
	}	
}

I didn't want to off, so if it is just kill it. 'SOORY'

"Just because I'm losing, doesn't mean I'm lost. Doesn't mean I'll stop."

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Narue

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Code Goddess

Re: Deleting a pointer?

Jan 4th, 2009

Initialized the pointer to 0 if you aren't allocating memory. You can safely delete a null pointer and it will effectively be a no-op.

I'm here to prove you wrong.

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StuXYZ

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Posting Whiz

Re: Deleting a pointer?

Jan 4th, 2009

The question should be asked "how do I delete a pointer if I allocate the memory using xxxx?"

Let us review:

if you write

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 c++ Syntax (Toggle Plain Text) 
double* Ptr=new double[50];    // Allocate space for 50 double s
// ... other code
delete [] Ptr;                             // Delete the pointer to 50 doubles
// CARE:
// Ptr is still pointing to a value
double* Ptr=new double[50];    // Allocate space for 50 double s
// ... other code
delete [] Ptr;                             // Delete the pointer to 50 doubles
// CARE:
// Ptr is still pointing to a value

but if you write

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 c++ Syntax (Toggle Plain Text) 
double* Ptr=new double;        // allocate 1 double 
//.... other code
delete Ptr;
double* Ptr=new double;        // allocate 1 double 
//.... other code
delete Ptr;

Note as pointed out, delete removes the memory for the pointer BUT does not change the value of the pointer variable. e.g.

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 C++ Syntax (Toggle Plain Text) 
;
std::cout<<"Ptr memory location == "<<reinterpret_cast<long int>(Ptr)<<std::endl; 
delete [] Ptr;
std::cout<<"Ptr memory location == "<<reinterpret_cast<long int>(Ptr)<<std::endl; 
// 
// THIS CAUSES A RUNTIME ERROR:
// DO NOT DELETE TWICE
//
delete [] Ptr;
;
std::cout<<"Ptr memory location == "<<reinterpret_cast<long int>(Ptr)<<std::endl; 
delete [] Ptr;
std::cout<<"Ptr memory location == "<<reinterpret_cast<long int>(Ptr)<<std::endl; 
// 
// THIS CAUSES A RUNTIME ERROR:
// DO NOT DELETE TWICE
//
delete [] Ptr;

Note in the above example , if you set Ptr=0; after the first delete then all is well.

Finally::

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 c++ Syntax (Toggle Plain Text) 
double X[50];        
//.... code
// NO DELETE Required:
// THIS CAUSES A RUNTIME ERROR:
delete [] X;
double X[50];        
//.... code
// NO DELETE Required:
// THIS CAUSES A RUNTIME ERROR:
delete [] X;

If the pointer is from an object that is not allocated via a new then don't delete it.

[Note: that if you are using malloc etc, then you must use free. If you are not using malloc, then don't start using it. ]

Hope this helps

experience is the most expensive way to learn anything