Can a call delete on the pointer which is allocated with the placement new?

Question

Can we call delete on the pointer which is allocated with the placement new? If no then why? Please explain in details.

I know that there is no placement delete. But I wonder why just delete opetator can not delete the memory without caring how that memory on which the pointer points is allocated?

delete is doing two things:

1. Calls destrucor
2. Frees memory

And I see no reaason for delete not to be able to call either of these two operations on the object which was created by placement new. Any idea about reasons?

Answer 1

You must only call delete on pointers that were created with operator new. If you use placement new with a memory location that was allocated by the normal operator new then you may safely use delete on it (provided you get the types and pointers right). However, you can use placement new on any memory, so you usually will manage that memory some other way and call the object's destructor manually.

For instance, in this convoluted and usually unnecessary scenario, it is safe to delete the memory you used placement new on, but only because you allocated it with new before:

char* mem = new char[sizeof(MyObject)];
MyObject* o = new (mem) MyObject;

// use o

o->~MyObject(); // with placement new you have to call the destructor by yourself

delete[] mem;

However, this is illegal:

char mem[16]; // create a buffer on the stack, assume sizeof(MyObject) == 16

MyObject* o = new (mem) MyObject; // use stack memory to hold a MyObject
                                  // note that after placement new is done, o == mem
                                  // pretend for this example that the point brought up by Martin in the comments didn't matter

delete o; // you just deleted memory in the stack! This is very bad

Another way to think of it is that delete only deallocates memory allocated previously by the normal new. With placement new, you do not have to use memory that was allocated by the normal new, so with the possibility of not having been allocated by normal new, delete cannot deal with it.