issue regarding deleting an object in qt with delete and free

SherifOmran

I am trying to delete an object that has been created. When i gave delete p it did not work, but when i set p=NULL and then delete p it worked. I don't see why ? however when i give free(p) it also works.

it does not work without p=NULL

stack *p = new stack();
for (int i=0; i<4; i++)
{
    p->push(i);
}
for (int i=0; i<4; i++)
{
    p->pull();
}

p=NULL; delete p;

error message deleting an object that does not exist if i don't use p=NULL. If i use free (p) instead of p=NULL and delete it works. But what is the explaination?

sierdzio

What is stack? Do you mean std::stack (it does not have pull() method, so probably not)?

jsulm

@SherifOmran said in issue regarding deleting an object in qt with delete and free:

error message deleting an object that does not exist

Can you post the whole error message?

JonB

@SherifOmran

but when i set p=NULL and then delete p it worked

It "worked", in the sense that in this case it does not delete anything!

You should never free() an object created by new, only delete.

error message deleting an object that does not exist if i don't use p=NULL

Sounds more like an error in the destructor for stack referring to something else (e.g. there's something wrong in your push-me-pull-you code)!

As the others say: post whole message, and show what is going on inside stack?

SherifOmran

This post is deleted!

SherifOmran

This is the whole error message when i remove the null

linkelist(34449,0x7fffa95e1380) malloc: *** error for object 0x3: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug

SherifOmran

I found the mistake
it was in

ClassList::~ClassList()
{
    //destructor
    //delete temp; /
    delete head;
}

it was delete temp

JonB

@SherifOmran

it was delete temp

I was just about to tell you that! Your temp wasn't even initialized! As you can see, as predicted the error was indeed inside the destructor for stack, which is only called if you delete stack.

sierdzio

@SherifOmran said in issue regarding deleting an object in qt with delete and free:

ClassList::ClassList()
{
//main
head = new Node;
head=NULL;

Huh? Looks like an instant memory leak.

(btw. you should use nullptr)

Frankly, I don't have time to analyse your whole code, sorry. But perhaps try running it through valgrind and address sanitizer - it should pick all the leaks for you.

jsulm

@SherifOmran As far as I can see you did not initialise temp and try to delete it! No wonder it does not work.

ClassList::ClassList()
{
    head = new Node;
    head=NULL; // Why do you assign NULL after creating instance?!
}

ClassList::~ClassList()
{
    delete temp; //Check whether temp points to anything before deleting
    delete head;
}

Change

private:
    Node* head = nullptr; //number 1
    Node* current = nullptr; //current node
    Node* temp = nullptr;
    Node* temp2 = nullptr;

SherifOmran

what caused confusion is that i delete L at line 51 and it was accepted

SherifOmran

@jsulm
because i want to check that the head it Null at the initialization

jsulm

@SherifOmran

delete L; // L became a null pointer

The comment is wrong: the value of the pointer does not change when you call delete on it.

delete L;
L = nullptr; // Now it is null pointer

SherifOmran

yes thank you i corrected this comment

jsulm

@SherifOmran said in issue regarding deleting an object in qt with delete and free:

@jsulm
because i want to check that the head it Null at the initialization

As @sierdzio already told you: you created a memory leak this way.
Just remove

head = new Node;

Else you create an instance and loose the pointer to it, so you never delete this instance -> memory leak.

SherifOmran

yes i see.
how i delete temp that was initialized in ClassList::InsertNode ?

jsulm

@SherifOmran said in issue regarding deleting an object in qt with delete and free:

how i delete temp that was initialized in ClassList::InsertNode ?

delete temp;

JonB

@SherifOmran

how i delete temp that was initialized in ClassList::InsertNode ?

As @jsulm says it's still delete temp, but make sure you initialize temp = nullptr in either the declaration or the constructor, ClassList::InsertNode might never get called....

SherifOmran

thank you guys for suggested corrections