How to clear the memory correctly?

Question

I have some code:

MemoryManager mm;
char *a = new (mm) char [len +1];

How can I free all the memory by pointer a?

What are you trying to do? And there's no need to `delete` the memory pointed to by `a` (if that's what you mean by *free*) — Praetorian, Apr 21 '15 at 20:57
As @Praetorian, you do not need free memory, because you not allocated memory. — gomons, Apr 21 '15 at 20:58
@Praetorian: I need to **delete a;** But it will be the memory leak. — user3818229, Apr 21 '15 at 21:03
@Eric not really. I have only one pointer. MemoryManager is a class with overloaded operator "new". — user3818229, Apr 21 '15 at 21:05
Ok, [this question](http://stackoverflow.com/questions/5857240/why-there-is-no-placement-delete-expression-in-c) — Eric, Apr 21 '15 at 21:05
@user3818229, you use placement new using object locally created, why do you need `delete a;`? — gomons, Apr 21 '15 at 21:06
@Eric 2nd one looks like it answers this question. Mjölnir used. — Yakk - Adam Nevraumont, Apr 21 '15 at 21:13

Eric · Accepted Answer · 2015-04-21T21:19:01.997

3

Firstly, there's no way that anything builtin can know how to delete your a data - you're the one that allocated it with placement new, so it's your job to clean it up.

Unfortunatly, there doesn't appear to be a way of overloading delete for this. Just use a method name of your choice:

MemoryManager mm;
char *a = new (mm) char [len +1];

mm.cleanup(a); // since mm allocated us the memory, only it knows how to destroy it.
               // Note that this also needs to do obj.~ClassName() for class types

See 'is there a placement delete' in the isocpp faq.

edited Apr 21 '15 at 21:19

answered Apr 21 '15 at 21:07

Eric

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Can somebody explain this example in more details? – gomons Apr 21 '15 at 21:12
@gomons: They're creating a `char[]` using a custom memory manager. They want to tell this memory manager to clean up the memory. – Eric Apr 21 '15 at 21:13
So `char *a = new (mm) char [len +1];` calls `operator new()` overloaded in `MemoryManager`? – gomons Apr 21 '15 at 21:14
I think it calls `operator new[](sizeof(char[len+1]), mm)` – Eric Apr 21 '15 at 21:17
and you are right, so I can't understand the sense of MemoryManager class. – gomons Apr 21 '15 at 21:21
In result, I've concluded the answer is: `operator delete (a, mm);` (to call delete like as the usual method) – user3818229 Apr 22 '15 at 04:49
@user3818229 You'll want to define that anyhow (in case that the construction throws). But wait -- what happens if the type has a virtual destructor? Is the proper `delete` called? ... so that doesn't work. `operator delete(a,mm)` *should not call the destructor of `a`*. Partly because if the constructor of `a` throws, it will be automatically called with a destroyed `a`. – Yakk - Adam Nevraumont Apr 23 '15 at 14:44
In short, `operator delete(a,mm)` can only be called after `a.~A()` is called. It may not call `a.~A()` or it will lead to UB if you use custom placement-new on any type with a throwing constructor. You could call `operator delete(a, mm, please_destroy_as_well{})` (ie, pass a tag type), but that seems rude. – Yakk - Adam Nevraumont Apr 23 '15 at 14:50

How to clear the memory correctly?

1 Answers1