What happens when you deallocate a pointer twice or more in C++?

Question

int main() {
    Employee *e = new Employee();

    delete e;
    delete e;
    ...
    delete e;
    return 0;
}

Arthralgia

out of cucumber error restart universe. – Brunelleschi

Why the downvote? Its a simple & straightforward question. Just because you think it's an elementary question doesn't mean it should be d/v'ed. – Externalism

Answer 1

You get undefined behaviour if you try to delete an object through a pointer more than once.

This means that pretty much anything can happen from 'appearing to work' to 'crashing' or something completely random.

Intwine

I've tried it and it doesn't crash. But what I think is that you might deallocate memory that other part of your program is using. – Arthralgia

It might not crash at that moment. But if it corrupts part of the heap, then there's a significant chance that a crash could happen at some arbitrary point afterwards. However, it could become something of a lurking time-bomb. Not causing any problem until later when some seemingly unrelated action happens to touch the corrupted part and then boom – Rile

Answer 2

It's undefined behavior, so anything can happen.

What's likely to happen is bad. Typically, the free store is a carefully managed system of free and allocated blocks, and new and delete do bookkeeping to keep everything in a consistent state. If you delete again, the system is likely to do the same bookkeeping on invalid data, and suddenly the free store is in an inconsistent state. This is known as "heap corruption".

Once that happens, anything you do with new or delete may have unpredictable results, which can include attempting to write outside the application's memory area, silently corrupting data, erroneously thinking there's no more memory, or double or overlapping allocation. If you're lucky, the program will crash soon, although you'll still have problems figuring out why. If you're unlucky, it will continue to run with bad results.

Madalena

Answer 3

Aside from the old saw about "undefined behavior" meaning anything could happen from nothing to a gateway to the seventh circle of the inferno opening up in main memory, in practice what will usually happen in most implementations is that the program will continue to run past the deletes, and then mysteriously crash sometime later in some unrelated memory allocation.

Owings

Yes, and that's one of the better things that can happen. – Madalena

often When I get the mysterious memory crashes, I WISH a gateway to the seventh circle of the inferno had opened up in main memory instead... – Brunelleschi

Answer 4

You are likely venturing into 'undefined behavior' territory.

On many systems this will cause a crash; for example, on my Linux machine:

*** glibc detected *** ./cctest: double free or corruption (fasttop): 0x0000000000d59900 ***
======= Backtrace: =========
/lib/libc.so.6[0x7f399f4cbdd6]
/lib/libc.so.6(cfree+0x6c)[0x7f399f4d074c]
./cctest[0x400a7a]
/lib/libc.so.6(__libc_start_main+0xfd)[0x7f399f474abd]
./cctest[0x400959]

Porshaport

Answer 5

If you're really lucky it will crash. What normally happens is it stores up karma until your CEO is demonstrating the code to your most important new customer when it will corrupt/destroy all of their data.

In checked or debug builds often this kind of thing is caught, but it can go completely undetected and cause havoc later. This is especially profound when multiple threads get involved.

Kreda

Answer 6

If you are worried this might happen in your apps, either stop using raw pointers completely, so that you don't need delete (eg switch over to shared_ptr) or always set pointers to NULL (or 0, or better still nullptr) after you delete them. Calling delete on a null pointer is guaranteed to do nothing.

Woolgrower

I've always disliked the latter solution. Deleting a pointer twice is bad, and setting pointers to null is just going to hide the problem. – Kettledrum

Answer 7

It's not safe, and it's undefined what might actually happen:

http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.2

Shellieshellproof

Answer 8

Even though sometimes we can access the memory location after deleting ptr. we shouldn't delete same pointer again or assign value to that pointer( leads to inconsistent behavior).

But we can use same pointer variable to point different memory address ( valid memory)

int *p = new int(10);
std::cout << "in main" << std::endl;
std::cout <<*p << std::endl;
std::cout << p << std::endl;
std::cout << &p<< std::endl;
delete p;
std::cout << "in main2 after delete" << std::endl;
std::cout <<*p << std::endl;
std::cout << p << std::endl;
std::cout << &p<< std::endl;

p = new int(100);
std::cout << "in main3" << std::endl;
std::cout <<*p << std::endl;
std::cout << p << std::endl;
std::cout << &p<< std::endl;

leads to output

in main
10
0x558b597a8eb0
0x7fff8f7a5ba0
in main2 after delete
0
0x558b597a8eb0
0x7fff8f7a5ba0
in main3
100
0x558b597a8eb0
0x7fff8f7a5ba0

Hobo