Easy pointer question

Hi all,

This is a great site. I'm doing the tutorials now, and I ran into a minor hitch:

Quote:

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The delete operator frees up the memory allocated through new. To do so, the syntax is as in the example.

Code:

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delete ptr;

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After deleting a pointer, it is a good idea to reset it to point to 0.

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Deleting the pointer before resetting it to 0 gives me this error:
Debug Assertion Failed!
Expression: _BLOCK_TYPE_IS_VALID(pHead -> nBlockUse)

Resetting the pointer to 0 before I delete it seems to work, but I'm wondering - is there more to this than meets the eye?

code:

Code:

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int main()
{
        int pointee = 1010;
        int *pointer = new int;
        cout<<"\nPointee is " << pointee << "";
        pointer = &pointee;
        cout<<"\nPointer is " << pointer << "";
        pointer = 0;
        cout<<"\nPointer is " << pointer << "";
        delete pointer; //if I try deleting pointer before setting it to 0, I get the error
}

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Quote:

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Originally Posted by Edo

Deleting the pointer before resetting it to 0

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You actually use delete after setting the pointer to 0 (which is wrong), but delete on a null pointer should have no effect, so I am not sure why you get such an error.

Code:

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int main()
{
        int pointee = 1010;
        int *pointer = new int;
        cout<<"\nPointee is " << pointee << "";
        pointer = &pointee;
        cout<<"\nPointer is " << pointer << "";
        delete pointer; //if I try deleting pointer before setting it to 0, I get the error
}

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This is what crashes, right?

That's because the red line changes what your pointer points at, so that it now points at your local variable pointee. That is not what you got back from new, so you can't delete it.

It would be like you take a book out of the library, and then return a completely different book - it doesn't matter if it's a "better" or "crappy" book you give back, you are not returning what you borrowed, so you are likely to get the "pay for lost books" penalty - which is roughly what that error message means.

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Mats

Quote:

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Originally Posted by matsp

. . .the red line changes what your pointer points at, so that it now points at your local variable pointee. That is not what you got back from new, so you can't delete it.

It would be like you take a book out of the library, and then return a completely different book

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Thanks, that was a great analogy! Now I understand what went wrong, and I'll delete the pointer first.