7.2 KiB
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<T> and NonnullOwnPtr<T>
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.
These pointers cannot be copied. Transferring ownership is done by moving the pointer.
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 "&".
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().
Construction using helper functions
There is a make<T>() helper that constructs a new object and returns it wrapped in a NonnullOwnPtr. All arguments passed to it are forwarded to T's constructor. If it fails to allocate heap memory for the object, it terminates the program.
{
NonnullOwnPtr<Foo> my_object = make<Foo>();
my_object->do_stuff();
// my_object goes out of scope here, and the Foo will be deleted.
}
The try_make<T>() helper attempts to construct a new object wrapped in an OwnPtr. All arguments passed to it are forwarded to T's constructor. In case of allocation failure, a null pointer is returned. This allows the calling code to handle allocation failure as it wishes.
OwnPtr<Foo> my_object = try_make<Foo>();
if (!my_object) {
// handle allocation failure...
}
my_object->do_stuff();
Note: Objects constructed using try_make<T>() should only be dereferenced after a null check.
Manual construction
The helper functions cannot access private constructors, so in some cases, smart pointers need to be created manually. This is done by "adopting" a raw pointer, which moves its ownership to the smart pointer. Dereferencing the raw pointer or calling its destructor afterwards can cause undefined behavior.
Known non-null pointers can be turned into a NonnullOwnPtr by the global adopt_own() function.
NonnullOwnPtr<Foo> my_object = adopt_own(*new Foo);
It is safe to immediately dereference this raw pointer, as the normal new expression cannot return a null pointer.
Any (possibly null) pointer to T can be turned into an OwnPtr<T> by the global adopt_own_if_nonnull() function.
OwnPtr<Foo> my_object = adopt_own_if_nonnull(new (nothrow) Foo);
In this case, the non-throwing new should be used to construct the raw pointer, which returns null if the allocation fails, instead of aborting the program.
Note: Always prefer the helper functions to manual construction.
RefPtr<T> and NonnullRefPtr<T>
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.
class Bar : public RefCounted<Bar> {
...
};
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*.
Construction using helper functions
There is a make_ref_counted<T>() global helper function that constructs a new object and returns it wrapped in a NonnullRefPtr. All arguments passed to it are forwarded to T's constructor. If memory cannot be allocated for the object, the program is terminated.
NonnullRefPtr<Bar> our_object = make_ref_counted<Bar>();
NonnullRefPtr<Bar> another_owner = our_object;
The try_make_ref_counted<T>() function constructs an object wrapped in RefPtr<T> which may be null if the allocation does not succeed. This allows the calling code to handle allocation failure as it wishes. All arguments passed to it are forwarded to T's constructor.
RefPtr<Bar> our_object = try_make_ref_counted<Bar>();
if (!our_object) {
// handle allocation failure...
}
RefPtr<Bar> another_owner = our_object;
In the above examples, the Bar object will only be deleted once both our_object and another_owner are gone.
Manual construction
The helper functions cannot access private constructors, so in some cases, objects need to be manually wrapped into smart pointers. 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. The raw pointer must not be used after its ownership is transferred to the smart pointer.
A known non-null raw pointer can be turned into a NonnullRefPtr by the global adopt_ref() function.
NonnullRefPtr<Bar> our_object = adopt_ref(*new Bar);
Note: It is safe to immediately dereference this raw pointer, as the normal new expression cannot return a null pointer.
Any (possibly null) pointer to a reference-counted object can can be turned into a RefPtr by the global adopt_ref_if_nonnull() function.
RefPtr<Bar> our_object = adopt_ref_if_nonnull(new (nothrow) Bar);
In this case, the non-throwing new should be used to construct the raw pointer, which returns null if the allocation fails, instead of aborting the program.
Note: Always prefer the helper functions to manual construction.
WeakPtr<T>
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;
{
NonnullOwnPtr<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.