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SmartPointers.md

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SerenityOS smart pointers

Introduction

There are three main C++ smart pointer types used in SerenityOS. Each type describes the ownership (or lack thereof) of the pointee.

The reason for using these pointers is to make it explicit through code who owns which resources, and how ownership is transferred. They also serve as a guard against memory leaks and use-after-free bugs.

OwnPtr and NonnullOwnPtr

OwnPtr is used for single-owner objects. An object held in an OwnPtr is owned by that OwnPtr, and not by anybody else.

This means that the OwnPtr is responsible for deleting the pointee when the OwnPtr goes out of scope.

NonnullOwnPtr is a special variant of OwnPtr with one additional property: it cannot be null. NonnullOwnPtr is suitable as a return type from functions that are guaranteed to never return null, and as an argument type where ownership is transferred, and the argument may not be null. In other words, if OwnPtr is "*", then NonnullOwnPtr is "&".

There is a make<T>() helper that creates a new object and returns it wrapped in an NonnullOwnPtr.

{
    NonnullOwnPtr<Foo> my_object = make<Foo>();
    my_object->do_stuff();
    // my_object goes out of scope here, and the Foo will be deleted.
}

Note: A NonnullOwnPtr can be assigned to an OwnPtr but not vice versa. To transform an known-non-null OwnPtr into a NonnullOwnPtr, use OwnPtr::release_nonnull().

RefPtr and NonnullRefPtr

RefPtr is used for multiple-owner objects. An object held by a RefPtr is owned together by every pointer pointing to that object.

Shared ownership is implemented via reference counting.

NonnullRefPtr is a special variant of RefPtr with one additional property: it cannot be null. NonnullRefPtr is suitable as a return type from functions that are guaranteed to never return null, and as an argument type where the argument may not be null. In other words, if RefPtr is "*", then NonnullRefPtr is "&".

Objects can only be held by RefPtr if they meet certain criteria. Specifically, they need to implement the functions ref() and unref().

To make a class T reference-counted, you can simply make it inherit from RefCounted<T>. This will add all the necessary pieces to T.

Note: When constructing an object that derives from RefCounted, the reference count starts out at 1 (since 0 would mean that the object has no owners and should be deleted.) The object must therefore be "adopted" by someone who takes responsibility of that 1. This is done through the global adopt_ref() function:

class Bar : public RefCounted<Bar> {
    ...
};

RefPtr<Bar> our_object = adopt_ref(*new Bar);
RefPtr<Bar> another_owner = our_object;

In the above example, the Bar object will only be deleted once both our_object and another_owner are gone.

Note: A NonnullRefPtr can be assigned to a RefPtr but not vice versa. To transform an known-non-null RefPtr into a NonnullRefPtr, either use RefPtr::release_nonnull() or simply dereference the RefPtr using its operator*.

WeakPtr

WeakPtr is used for objects that somebody else owns. When the pointee of a WeakPtr is deleted, the WeakPtr will magically become null.

Behind the scenes, this is implemented using the Weakable template. If you want to make it possible for a class T to be weakly-pointed-to, have it inherit from Weakable<T>.

To create a WeakPtr to a weakable object, use make_weak_ptr():

class Baz : public Weakable<Baz> {
    ....
};

WeakPtr<Baz> a_baz;
{
    OwnPtr<Baz> my_baz = make<Baz>();
    a_baz = my_baz->make_weak_ptr();
    // a_baz now points to my_baz
}
// a_baz is now null, since my_baz went out of scope.