From 554fd8c5195424bdbcabf5de30fdc183aba391bd Mon Sep 17 00:00:00 2001 From: upstream source tree Date: Sun, 15 Mar 2015 20:14:05 -0400 Subject: obtained gcc-4.6.4.tar.bz2 from upstream website; verified gcc-4.6.4.tar.bz2.sig; imported gcc-4.6.4 source tree from verified upstream tarball. downloading a git-generated archive based on the 'upstream' tag should provide you with a source tree that is binary identical to the one extracted from the above tarball. if you have obtained the source via the command 'git clone', however, do note that line-endings of files in your working directory might differ from line-endings of the respective files in the upstream repository. --- libstdc++-v3/doc/xml/manual/shared_ptr.xml | 543 +++++++++++++++++++++++++++++ 1 file changed, 543 insertions(+) create mode 100644 libstdc++-v3/doc/xml/manual/shared_ptr.xml (limited to 'libstdc++-v3/doc/xml/manual/shared_ptr.xml') diff --git a/libstdc++-v3/doc/xml/manual/shared_ptr.xml b/libstdc++-v3/doc/xml/manual/shared_ptr.xml new file mode 100644 index 000000000..4ef5f72b6 --- /dev/null +++ b/libstdc++-v3/doc/xml/manual/shared_ptr.xml @@ -0,0 +1,543 @@ +
+ + +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. + + + + At the time of writing the C++0x working paper doesn't mention how + threads affect shared_ptr, but it is likely to follow the existing + practice set by boost::shared_ptr. The + shared_ptr in libstdc++ is derived from Boost's, so the same rules + apply. + + + + +
+ +
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 shared_ptr<Ptr> +and it is never used by 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. + + + + + + +
+ +
Thread Safety + + + +C++0x-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++0x mode. + + + +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. + +
+ +
Dual C++0x and TR1 Implementation + + + +The interface of tr1::shared_ptr was extended for C++0x +with support for rvalue-references and the other features from N2351. +The _Sp_counted_base base class is implemented in +tr1/boost_sp_shared_count.h and is common to the TR1 +and C++0x versions of shared_ptr. + + + +The classes derived from _Sp_counted_base (see Class Hierarchy +above) and __shared_count are implemented separately for C++0x +and TR1, in bits/shared_ptr.h and +tr1/shared_ptr.h respectively. + + + +The TR1 implementation is considered relatively stable, so is unlikely to +change unless bug fixes require it. If the code that is common to both +C++0x and TR1 modes needs to diverge further then it might be necessary to +duplicate _Sp_counted_base and only make changes to +the C++0x version. + +
+ +
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++0x mode, so in TR1 mode these casts rely on three non-standard +constructors in shared_ptr and __shared_ptr. +In C++0x 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++0x working draft 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++0x 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 + + + + + shared_ptr + + + Boost C++ Libraries documentation, shared_ptr + + + N2461 + + + + + +
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