How to delete *ptr correctly?

uus99

I made a very simple class, and when the default destructor is called, i get a debug assertion failure. (i'm using VC++ SP5) if i call a function like this void Testfunction() { uImage test; test.Create(100); } the function exits properly, with the default destructor deleting the arrays successfully, however,if i call a function like this, that is without allocating the arrays, void Testfunction() { uImage test; } The function exits with a Debug Assertion Failure, _CrtIsValidHeapPointer What i'm confused is that, i'm trying to do cleanup in the destructor by deleting all the dynamic arrays. Although i havent created the arrays, there should be no problem coz i checked whether the pointers are NULL or not (when performing delete). So, why is there a debug assertion? My class header is #ifndef UIMAGE_H #define UIMAGE_H class uImage { private: int *R; int *G; int *B; public: uImage(); ~uImage(); void Create(int x); }; #endif My Class is #include "StdAfx.h" #include "uImage.h" uImage::uImage() { } uImage::~uImage() { if(R!=NULL) delete R; if(G!=NULL) delete G; if(B!=NULL) delete B; } void uImage::Create(int x) { R=new int[x]; G=new int[x]; B=new int[x]; }

David Crow

uus99 wrote: if(R!=NULL) delete R; if(G!=NULL) delete G; if(B!=NULL) delete B; Should be:

if(R!=NULL) delete [] R;
if(G!=NULL) delete [] G;
if(B!=NULL) delete [] B;

The differences between delete and delete [] were discussed here about a week ago. Search this forum for that thread.

"When I was born I was so surprised that I didn't talk for a year and a half." - Gracie Allen

uus99

Yes, you are right, i realized it. Even after i corrected that, there is still the same problem. Why?

Antony M Kancidrowski

You have not initialised the pointers.

uImage::uImage() : R(NULL), G(NULL), B(NULL)
{

}

or

uImage::uImage()
{
R = NULL;
G = NULL;
B = NULL;
}

Given this you do not need to check for R != NULL etc before calling delete as delete does the check for you. Secondly you have missed deleting the array properly.

delete [] R;
R = NULL;

etc... I would set R = NULL; after the delete also. But that's just me!;) Ant. I'm hard, yet soft.
I'm coloured, yet clear.
I'm fuity and sweet.
I'm jelly, what am I? Muse on it further, I shall return! - David Williams (Little Britain)

uus99

Thanks! The initialization of those pointers to Null solved my problem right away! I never knew i had to do that! Thanks again!

Jeremy Falcon

As a rule of thumb, always initialize pointers to null (unless you have a reason to initialize them with a value). When it becomes a habit, It'll make testing against them much easier in the future. Jeremy Falcon

Henry miller

You have to do that. Your pointers were not NULL, they were undefined which means any value is legal. My debugger expliactly sets everything to 0xAAAAAAAA to make sure that they are not null. You are lucky, on many systems they will be NULL 95% of the time and you will tear your hair out trying to figgure out why your program crashes one time in 20.

digwizfox

That's interesting. I would think that the proper usage of delete contributed to the solution as well. Both are absolutely critical to ensuring that your code works properly. Not using new and delete properly will cause undefined behavior (sometimes your code will crash, sometimes it won't). I've seen code like this: #include "gconst.h" struct globalmem { int counter; int[SIZE] nDataBuffer; //just as an example, the real data struct had many //other attributes as well } //now later, in some other piece of code globalmem* gpGlobalMem = new globalmem; //Now even later upon application exiting delete[] globalmem; //oops! use delete globalmem //remember, if new was used, delete should be used for //destruction. Always use the same format of new and //delete (even though there is an array in global mem //you still instantiated only 1 globalmem structure //using new. Now what was really weird was that the delete[] globalmem statement was in our code for years, even though it was wrong and never caused problems. But after adding even more variables to the data structure during the enhancement to a system, the software just started crashing when trying to exit the application.