CODEWITHSUNDEEP
c++pointersmemory-managementallocationdelete-operator
flopex
flopex

Reputation: 454

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

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

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

Upvotes: 30

Views: 12216

Answers (8)

Sharath S Balehosur
Sharath S Balehosur

Reputation: 1

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

Upvotes: 0

CB Bailey
CB Bailey

Reputation: 792557

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.

Upvotes: 47

Kate Gregory
Kate Gregory

Reputation: 18964

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.

Upvotes: 3

Stewart
Stewart

Reputation: 4026

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.

Upvotes: 3

David Thornley
David Thornley

Reputation: 57056

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.

Upvotes: 21

Crashworks
Crashworks

Reputation: 41444

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.

Upvotes: 13

John Ledbetter
John Ledbetter

Reputation: 14203

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]

Upvotes: 4

John Weldon
John Weldon

Reputation: 40789

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

Upvotes: 2

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