# shared_ptr

ISO C++ shared_ptr

The shared_ptr class template stores a pointer, usually obtained via new, and implements shared ownership semantics.

## Requirements

The standard deliberately doesn't require a reference-counted implementation, allowing other techniques such as a circular-linked-list.

## Design Issues

The shared_ptr code is kindly donated to GCC by the Boost project and the original authors of the code. The basic design and algorithms are from Boost, the notes below describe details specific to the GCC implementation. Names have been uglified in this implementation, but the design should be recognisable to anyone familiar with the Boost 1.32 shared_ptr.

The basic design is an abstract base class, `_Sp_counted_base` that does the reference-counting and calls virtual functions when the count drops to zero. Derived classes override those functions to destroy resources in a context where the correct dynamic type is known. This is an application of the technique known as type erasure.

## Implementation

### Class Hierarchy

A shared_ptr<T> contains a pointer of type T* and an object of type __shared_count. The shared_count contains a pointer of type _Sp_counted_base* which points to the object that maintains the reference-counts and destroys the managed resource.

_Sp_counted_base<Lp>

  The base of the hierarchy is parameterized on the lock policy (see below.) _Sp_counted_base doesn't depend on the type of pointer being managed, it only maintains the reference counts and calls virtual functions when the counts drop to zero. The managed object is destroyed when the last strong reference is dropped, but the _Sp_counted_base itself must exist until the last weak reference is dropped.

_Sp_counted_base_impl<Ptr, Deleter, Lp>

  Inherits from _Sp_counted_base and stores a pointer of type `Ptr` and a deleter of type `Deleter`. _Sp_deleter is used when the user doesn't supply a custom deleter. Unlike Boost's, this default deleter is not "checked" because GCC already issues a warning if `delete` is used with an incomplete type. This is the only derived type used by tr1::shared_ptr<Ptr> and it is never used by std::shared_ptr, which uses one of the following types, depending on how the shared_ptr is constructed.

_Sp_counted_ptr<Ptr, Lp>

  Inherits from _Sp_counted_base and stores a pointer of type Ptr, which is passed to `delete` when the last reference is dropped. This is the simplest form and is used when there is no custom deleter or allocator.

_Sp_counted_deleter<Ptr, Deleter, Alloc>

  Inherits from _Sp_counted_ptr and adds support for custom deleter and allocator. Empty Base Optimization is used for the allocator. This class is used even when the user only provides a custom deleter, in which case allocator is used as the allocator.

_Sp_counted_ptr_inplace<Tp, Alloc, Lp>

  Used by `allocate_shared` and `make_shared`. Contains aligned storage to hold an object of type Tp, which is constructed in-place with placement `new`. Has a variadic template constructor allowing any number of arguments to be forwarded to Tp's constructor. Unlike the other _Sp_counted_* classes, this one is parameterized on the type of object, not the type of pointer; this is purely a convenience that simplifies the implementation slightly.

C++11-only features are: rvalue-ref/move support, allocator support, aliasing constructor, make_shared & allocate_shared. Additionally, the constructors taking auto_ptr parameters are deprecated in C++11 mode.

### Thread Safety

The Thread Safety section of the Boost shared_ptr documentation says "shared_ptr objects offer the same level of thread safety as built-in types." The implementation must ensure that concurrent updates to separate shared_ptr instances are correct even when those instances share a reference count e.g.

    shared_ptr<A> a(new A);
    shared_ptr<A> b(a);

    // Thread 1     // Thread 2
       a.reset();      b.reset();

The dynamically-allocated object must be destroyed by exactly one of the threads. Weak references make things even more interesting. The shared state used to implement shared_ptr must be transparent to the user and invariants must be preserved at all times. The key pieces of shared state are the strong and weak reference counts. Updates to these need to be atomic and visible to all threads to ensure correct cleanup of the managed resource (which is, after all, shared_ptr's job!) On multi-processor systems memory synchronisation may be needed so that reference-count updates and the destruction of the managed resource are race-free.

The function `_Sp_counted_base::_M_add_ref_lock()`, called when obtaining a shared_ptr from a weak_ptr, has to test if the managed resource still exists and either increment the reference count or throw bad_weak_ptr. In a multi-threaded program there is a potential race condition if the last reference is dropped (and the managed resource destroyed) between testing the reference count and incrementing it, which could result in a shared_ptr pointing to invalid memory.

The Boost shared_ptr (as used in GCC) features a clever lock-free algorithm to avoid the race condition, but this relies on the processor supporting an atomic Compare-And-Swap instruction. For other platforms there are fall-backs using mutex locks. Boost (as of version 1.35) includes several different implementations and the preprocessor selects one based on the compiler, standard library, platform etc. For the version of shared_ptr in libstdc++ the compiler and library are fixed, which makes things much simpler: we have an atomic CAS or we don't, see Lock Policy below for details.

### Selecting Lock Policy

There is a single _Sp_counted_base class, which is a template parameterized on the enum __gnu_cxx::_Lock_policy. The entire family of classes is parameterized on the lock policy, right up to __shared_ptr, __weak_ptr and __enable_shared_from_this. The actual std::shared_ptr class inherits from __shared_ptr with the lock policy parameter selected automatically based on the thread model and platform that libstdc++ is configured for, so that the best available template specialization will be used. This design is necessary because it would not be conforming for shared_ptr to have an extra template parameter, even if it had a default value. The available policies are:

1. `_S_Atomic`

   Selected when GCC supports a builtin atomic compare-and-swap operation on the target processor (see Atomic Builtins.) The reference counts are maintained using a lock-free algorithm and GCC's atomic builtins, which provide the required memory synchronisation.

2. `_S_Mutex`

   The _Sp_counted_base specialization for this policy contains a mutex, which is locked in add_ref_lock(). This policy is used when GCC's atomic builtins aren't available so explicit memory barriers are needed in places.

3. `_S_Single`

   This policy uses a non-reentrant add_ref_lock() with no locking. It is used when libstdc++ is built without `--enable-threads`.

For all three policies, reference count increments and decrements are done via the functions in `ext/atomicity.h`, which detect if the program is multi-threaded. If only one thread of execution exists in the program then less expensive non-atomic operations are used.

### Related functions and classes

`dynamic_pointer_cast`, `static_pointer_cast`, `const_pointer_cast`

  As noted in N2351, these functions can be implemented non-intrusively using the alias constructor. However the aliasing constructor is only available in C++11 mode, so in TR1 mode these casts rely on three non-standard constructors in shared_ptr and __shared_ptr. In C++11 mode these constructors and the related tag types are not needed.

`enable_shared_from_this`

  The clever overload to detect a base class of type `enable_shared_from_this` comes straight from Boost. There is an extra overload for `__enable_shared_from_this` to work smoothly with `__shared_ptr<Tp, Lp>` using any lock policy.

`make_shared`, `allocate_shared`

  `make_shared` simply forwards to `allocate_shared` with `std::allocator` as the allocator. Although these functions can be implemented non-intrusively using the alias constructor, if they have access to the implementation then it is possible to save storage and reduce the number of heap allocations. The newly constructed object and the _Sp_counted_* can be allocated in a single block and the standard says implementations are "encouraged, but not required," to do so. This implementation provides additional non-standard constructors (selected with the type `_Sp_make_shared_tag`) which create an object of type `_Sp_counted_ptr_inplace` to hold the new object. The returned `shared_ptr<A>` needs to know the address of the new `A` object embedded in the `_Sp_counted_ptr_inplace`, but it has no way to access it. This implementation uses a "covert channel" to return the address of the embedded object when `get_deleter<_Sp_make_shared_tag>()` is called. Users should not try to use this. As well as the extra constructors, this implementation also needs some members of _Sp_counted_deleter to be protected where they could otherwise be private.

## Use

### Examples

Examples of use can be found in the testsuite, under `testsuite/tr1/2_general_utilities/shared_ptr`, `testsuite/20_util/shared_ptr` and `testsuite/20_util/weak_ptr`.

### Unresolved Issues

The shared_ptr atomic access clause in the C++11 standard is not implemented in GCC.

The _S_single policy uses atomics when used in MT code, because it uses the same dispatcher functions that check `__gthread_active_p()`. This could be addressed by providing template specialisations for some members of _Sp_counted_base<_S_single>.

Unlike Boost, this implementation does not use separate classes for the pointer+deleter and pointer+deleter+allocator cases in C++11 mode, combining both into _Sp_counted_deleter and using allocator when the user doesn't specify an allocator. If it was found to be beneficial an additional class could easily be added. With the current implementation, the _Sp_counted_deleter and __shared_count constructors taking a custom deleter but no allocator are technically redundant and could be removed, changing callers to always specify an allocator. If a separate pointer+deleter class was added the __shared_count constructor would be needed, so it has been kept for now.

The hack used to get the address of the managed object from `_Sp_counted_ptr_inplace::_M_get_deleter()` is accessible to users. This could be prevented if `get_deleter<_Sp_make_shared_tag>()` always returned NULL, since the hack only needs to work at a lower level, not in the public API. This wouldn't be difficult, but hasn't been done since there is no danger of accidental misuse: users already know they are relying on unsupported features if they refer to implementation details such as _Sp_make_shared_tag.

tr1::_Sp_deleter could be a private member of tr1::__shared_count but it would alter the ABI.

## Acknowledgments

The original authors of the Boost shared_ptr, which is really nice code to work with, Peter Dimov in particular for his help and invaluable advice on thread safety. Phillip Jordan and Paolo Carlini for the lock policy implementation.

## Bibliography

Improving shared_ptr for C++0x, Revision 2

N2351

C++ Standard Library Active Issues List

N2456

Working Draft, Standard for Programming Language C++

N2461

Boost C++ Libraries documentation, shared_ptr

N2461