delete this & private destructor

Question

I've been thinking about the possible use of delete this in c++, and I've seen one use.

Because you can say delete this only when an object is on heap, I can make the destructor private and stop objects from being created on stack altogether. In the end I can just delete the object on heap by saying delete this in a random public member function that acts as a destructor. My questions:

1) Why would I want to force the object to be made on the heap instead of on the stack?

2) Is there another use of delete this apart from this? (supposing that this is a legitimate use of it :) )

Answer 1

Any scheme that uses delete this is somewhat dangerous, since whoever called the function that does that is left with a dangling pointer. (Of course, that's also the case when you delete an object normally, but in that case, it's clear that the object has been deleted). Nevertheless, there are somewhat legitimate cases for wanting an object to manage its own lifetime.

It could be used to implement a nasty, intrusive reference-counting scheme. You would have functions to "acquire" a reference to the object, preventing it from being deleted, and then "release" it once you've finished, deleting it if noone else has acquired it, along the lines of:

class Nasty {
public:
    Nasty() : references(1) {}

    void acquire() {
        ++references;
    }
    void release() {
        if (--references == 0) {
            delete this;
        }
    }
private:
    ~Nasty() {}
    size_t references;
};

// Usage
Nasty * nasty = new Nasty; // 1 reference
nasty->acquire();          // get a second reference
nasty->release();          // back to one
nasty->release();          // deleted
nasty->acquire();          // BOOM!

I would prefer to use std::shared_ptr for this purpose, since it's thread-safe, exception-safe, works for any type without needing any explicit support, and prevents access after deleting.

More usefully, it could be used in an event-driven system, where objects are created, and then manage themselves until they receive an event that tells them that they're no longer needed:

class Worker : EventReceiver {
public:
    Worker() {
        start_receiving_events(this);
    }    
    virtual void on(WorkEvent) {
        do_work();
    }
    virtual void on(DeleteEvent) {
        stop_receiving_events(this);
        delete this;
    }
private:
    ~Worker() {}
    void do_work();
};

Answer 2

1) Why would I want to force the object to be made on the heap instead of on the stack?

1) Because the object's lifetime is not logically tied to a scope (e.g., function body, etc.). Either because it must manage its own lifespan, or because it is inherently a shared object (and thus, its lifespan must be attached to those of its co-dependent objects). Some people here have pointed out some examples like event handlers, task objects (in a scheduler), and just general objects in a complex object hierarchy.

2) Because you want to control the exact location where code is executed for the allocation / deallocation and construction / destruction. The typical use-case here is that of cross-module code (spread across executables and DLLs (or .so files)). Because of issues of binary compatibility and separate heaps between modules, it is often a requirement that you strictly control in what module these allocation-construction operations happen. And that implies the use of heap-based objects only.

2) Is there another use of delete this apart from this? (supposing that this is a legitimate use of it :) )

Well, your use-case is really just a "how-to" not a "why". Of course, if you are going to use a delete this; statement within a member function, then you must have controls in place to force all creations to occur with new (and in the same translation unit as the delete this; statement occurs). Not doing this would just be very very poor style and dangerous. But that doesn't address the "why" you would use this.

1) As others have pointed out, one legitimate use-case is where you have an object that can determine when its job is over and consequently destroy itself. For example, an event handler deleting itself when the event has been handled, a network communication object that deletes itself once the transaction it was appointed to do is over, or a task object in a scheduler deleting itself when the task is done. However, this leaves a big problem: signaling to the outside world that it no longer exists. That's why many have mentioned the "intrusive reference counting" scheme, which is one way to ensure that the object is only deleted when there are no more references to it. Another solution is to use a global (singleton-like) repository of "valid" objects, in which case any accesses to the object must go through a check in the repository and the object must also add/remove itself from the repository at the same time as it makes the new and delete this; calls (either as part of an overloaded new/delete, or alongside every new/delete calls).

However, there is a much simpler and less intrusive way to achieve the same behavior, albeit less economical. One can use a self-referencing shared_ptr scheme. As so:

class AutonomousObject {
  private:
    std::shared_ptr<AutonomousObject> m_shared_this;

  protected:
    AutonomousObject(/* some params */);

  public:

    virtual ~AutonomousObject() { };

    template <typename... Args>
    static std::weak_ptr<AutonomousObject> Create(Args&&... args) {
      std::shared_ptr<AutonomousObject> result(new AutonomousObject(std::forward<Args>(args)...));
      result->m_shared_this = result;  // link the self-reference.
      return result;  // return a weak-pointer.
    };

    // this is the function called when the life-time should be terminated:
    void OnTerminate() {
      m_shared_this.reset( NULL );  // do not use reset(), but use reset( NULL ).
    };
};

With the above (or some variations upon this crude example, depending on your needs), the object will be alive for as long as it deems necessary and that no-one else is using it. The weak-pointer mechanism serves as the proxy to query for the existence of the object, by possible outside users of the object. This scheme makes the object a bit heavier (has a shared-pointer in it) but it is easier and safer to implement. Of course, you have to make sure that the object eventually deletes itself, but that's a given in this kind of scenario.

2) The second use-case I can think of ties in to the second motivation for restricting an object to be heap-only (see above), however, it applies also for when you don't restrict it as such. If you want to make sure that both the deallocation and the destruction are dispatched to the correct module (the module from which the object was allocated and constructed), you must use a dynamic dispatching method. And for that, the easiest is to just use a virtual function. However, a virtual destructor is not going to cut it because it only dispatches the destruction, not the deallocation. The solution is to use a virtual "destroy" function that calls delete this; on the object in question. Here is a simple scheme to achieve this:

struct CrossModuleDeleter;  //forward-declare.

class CrossModuleObject {
  private:
    virtual void Destroy() /* final */;

  public:
    CrossModuleObject(/* some params */);  //constructor can be public.

    virtual ~CrossModuleObject() { };  //destructor can be public.

    //.... whatever...

    friend struct CrossModuleDeleter;

    template <typename... Args>
    static std::shared_ptr< CrossModuleObject > Create(Args&&... args);
};

struct CrossModuleDeleter {
  void operator()(CrossModuleObject* p) const {
    p->Destroy();  // do a virtual dispatch to reach the correct deallocator.
  };
};

// In the cpp file:

// Note: This function should not be inlined, so stash it into a cpp file.
void CrossModuleObject::Destroy() {
  delete this;
};

template <typename... Args>
std::shared_ptr< CrossModuleObject > CrossModuleObject::Create(Args&&... args) {
  return std::shared_ptr< CrossModuleObject >( new CrossModuleObject(std::forward<Args>(args)...), CrossModuleDeleter() );
};

The above kind of scheme works well in practice, and it has the nice advantage that the class can act as a base-class with no additional intrusion by this virtual-destroy mechanism in the derived classes. And, you can also modify it for the purpose of allowing only heap-based objects (as usually, making constructors-destructors private or protected). Without the heap-based restriction, the advantage is that you can still use the object as a local variable or data member (by value) if you want, but, of course, there will be loop-holes left to avoid by whoever uses the class.

As far as I know, these are the only legitimate use-cases I have ever seen anywhere or heard of (and the first one is easily avoidable, as I have shown, and often should be).

Answer 3

"Why would I want to force the object to be made on the heap instead of on the stack?"

Generally when you force that it's not because you want to as such, it's because the class is part of some polymorphic hierarchy, and the only legitimate way to get one is from a factory function that returns an instance of a different derived class according to the parameters you pass it, or according to some configuration that it knows about. Then it's easy to arrange that the factory function creates them with new. There's no way that users of those classes could have them on the stack even if they wanted to, because they don't know in advance the derived type of the object they're using, only the base type.

Once you have objects like that, you know that they're destroyed with delete, and you can consider managing their lifecycle in a way that ultimately ends in delete this. You'd only do this if the object is somehow capable of knowing when it's no longer needed, which usually would be (as Mike says) because it's part of some framework that doesn't manage object lifetime explicitly, but does tell its components that they've been detached/deregistered/whatever[*].

If I remember correctly, James Kanze is your man for this. I may have misremembered, but I think he occasionally mentions that in his designs delete this isn't just used but is common. Such designs avoid shared ownership and external lifecycle management, in favour of networks of entity objects managing their own lifecycles. And where necessary, deregistering themselves from anything that knows about them prior to destroying themselves. So if you have several "tools" in a "toolbelt" then you wouldn't construe that as the toolbelt "owning" references to each of the tools, you think of the tools putting themselves in and out of the belt.

[*] Otherwise you'd have your factory return a unique_ptr or auto_ptr to encourage callers to stuff the object straight into the memory management type of their choice, or you'd return a raw pointer but provide the same encouragement via documentation. All the stuff you're used to seeing.

Answer 4

delete this;

can be useful at times and is usually used for a control class that also controls the lifetime of another object. With intrusive reference counting, the class it is controlling is one that derives from it.

The outcome of using such a class should be to make lifetime handling easier for users or creators of your class. If it doesn't achieve this, it is bad practice.

A legitimate example may be where you need a class to clean up all references to itself before it is destructed. In such a case, you "tell" the class whenever you are storing a reference to it (in your model, presumably) and then on exit, your class goes around nulling out these references or whatever before it calls delete this on itself.

This should all happen "behind the scenes" for users of your class.

Answer 5

1) Why would I want to force the object to be made on the heap instead of on the stack?

Because its life span isn't determined by the scoping rule.

2) Is there another use of delete this apart from this? (supposing that this is a legitimate use of it :) )

You use delete this when the object is the best placed one to be responsible for its own life span. One of the simplest example I know of is a window in a GUI. The window reacts to events, a subset of which means that the window has to be closed and thus deleted. In the event handler the window does a delete this. (You may delegate the handling to a controller class. But the situation "window forwards event to controller class which decides to delete the window" isn't much different of delete this, the window event handler will be left with the window deleted. You may also need to decouple the close from the delete, but your rationale won't be related to the desirability of delete this).

Answer 6

The general reason is that the lifetime of the object is determined by some factor internal to the class, at least from an application viewpoint. Hence, it may very well be a private method which calls delete this;.

Obviously, when the object is the only one to know how long it's needed, you can't put it on a random thread stack. It's necessary to create such objects on the heap.

Answer 7

Once upon a time i was writing some plugin code. I believe i mixed build (debug for plugin, release for main code or maybe the other way around) because one part should be fast. Or maybe another situation happened. Such main is already released built on gcc and plugin is being debugged/tested on VC. When the main code deleted something from the plugin or plugin deleted something a memory issue would occur. It was because they both used different memory pools or malloc implementations. So i had a private dtor and a virtual function called deleteThis().

-edit- Now i may consider overloading the delete operator or using a smart pointer or simply just state never delete a function. It will depend and usually overloading new/delete should never be done unless you really know what your doing (dont do it). I decide to use deleteThis() because i found it easier then the C like way of thing_alloc and thing_free as deleteThis() felt like the more OOP way of doing it

Answer 8

It's generally an exceptionally bad idea. There are a very few cases- for example, COM objects have enforced intrusive reference counting. You'd only ever do this with a very specific situational reason- never for a general-purpose class.

Answer 9

A good rule of thumb is not to use delete this.

Simply put, the thing that uses new should be responsible enough to use the delete when done with the object. This also avoids the problems with is on the stack/heap.