shared_ptrISO 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.
ImplementationClass 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:
_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.
_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.
_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 classesdynamic_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_sharedmake_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.
UseExamples
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.
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