CODEWITHSUNDEEP
c++pointersmemory-managementdelete-operator
Nicolas H
Nicolas H

Reputation: 545

C++: memory issue when I delete a pointer created by new

I'm a little bit confused by the following code

int main()
{
    int* a = new int{12};
    int* b = new int;

    b = a;
    delete a;

    delete b;

    return 0;
}

The code returns an error that

a.out(27538,0x10ade7e00) malloc: *** error for object 0x7f8c18504160: pointer being freed was not allocated
a.out(27538,0x10ade7e00) malloc: *** set a breakpoint in malloc_error_break to debug
zsh: abort      ./a.out

My question is that, when I delete the a, does it automatically delete b? What is the mechanism here, I'm a little bit getting lost.

Upvotes: 0

Views: 574

Answers (3)

David Schwartz
David Schwartz

Reputation: 182761

Let's walk through the code line by line.

int* a = new int{12};

This creates a new variable called a of type int* and initializes it to point to a newly-allocated integer assigned the value 12.

int* b = new int;

This creates a new variable called b of type int* and initializes it to point to a newly-allocated integer.

b = a;

This changes b's value to point to a. The value returned in the second call to new is now lost and that memory is leaked since it is no longer possible to pass it to delete.

delete a;

This deletes the object a points to, the one allocated first.

delete b;

Oops, this tries to delete the object b points to, but b doesn't point to any object that currently exists. The one allocated first was just deleted and no pointer to the second one exists. So this is a bug.

I suspect you are thinking that delete a; deletes a. It does not. It deletes whatever object a points to and requires that a be a pointer and point to a valid object that was allocated with new.

Upvotes: 2

Ted Lyngmo
Ted Lyngmo

Reputation: 117298

b = a;    // here the int that b pointed at "leaks" (you have no way of deleting it)

delete a; // here "a" is deleted first

delete b; // here "a" is deleted a second time (undefined behavior)

When you assign the value of a to b, the value (address if you will) b previously contained, is forgotten. Both a and b then points at the same object. You then lost all possibilities to delete the original object b pointed at. When you then delete b you in fact try deleting a a second time.

Upvotes: 8

eerorika
eerorika

Reputation: 238351

When you delete a pointer, all pointers, references, iterators etc. pointing to the destroyed object are invalidated. Because of the assignment b = a, b points to the same object as a does, so when a is deleted, b becomes invalid. Deleting an invalid pointer has undefined behaviour.

Note that b = a makes it impossible to delete the allocation that b previously pointed at. This is called a memory leak.

Upvotes: 0

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