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ISO C++
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ISO C++
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shared_ptr
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shared_ptr
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shared_ptr
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shared_ptr
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The shared_ptr class template stores a pointer, usually obtained via new,
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The shared_ptr class template stores a pointer, usually obtained via new,
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and implements shared ownership semantics.
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and implements shared ownership semantics.
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Requirements
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Requirements
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The standard deliberately doesn't require a reference-counted
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The standard deliberately doesn't require a reference-counted
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implementation, allowing other techniques such as a
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implementation, allowing other techniques such as a
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circular-linked-list.
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circular-linked-list.
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At the time of writing the C++0x working paper doesn't mention how
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At the time of writing the C++0x working paper doesn't mention how
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threads affect shared_ptr, but it is likely to follow the existing
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threads affect shared_ptr, but it is likely to follow the existing
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practice set by boost::shared_ptr. The
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practice set by boost::shared_ptr. The
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shared_ptr in libstdc++ is derived from Boost's, so the same rules
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shared_ptr in libstdc++ is derived from Boost's, so the same rules
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apply.
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apply.
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Design Issues
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Design Issues
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The shared_ptr code is kindly donated to GCC by the Boost
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The shared_ptr code is kindly donated to GCC by the Boost
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project and the original authors of the code. The basic design and
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project and the original authors of the code. The basic design and
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algorithms are from Boost, the notes below describe details specific to
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algorithms are from Boost, the notes below describe details specific to
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the GCC implementation. Names have been uglified in this implementation,
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the GCC implementation. Names have been uglified in this implementation,
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but the design should be recognisable to anyone familiar with the Boost
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but the design should be recognisable to anyone familiar with the Boost
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1.32 shared_ptr.
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1.32 shared_ptr.
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The basic design is an abstract base class, _Sp_counted_base that
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The basic design is an abstract base class, _Sp_counted_base that
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does the reference-counting and calls virtual functions when the count
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does the reference-counting and calls virtual functions when the count
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drops to zero.
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drops to zero.
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Derived classes override those functions to destroy resources in a context
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Derived classes override those functions to destroy resources in a context
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where the correct dynamic type is known. This is an application of the
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where the correct dynamic type is known. This is an application of the
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technique known as type erasure.
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technique known as type erasure.
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Implementation
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Implementation
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Class Hierarchy
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Class Hierarchy
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A shared_ptr<T> contains a pointer of
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A shared_ptr<T> contains a pointer of
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type T* and an object of type
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type T* and an object of type
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__shared_count. The shared_count contains a
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__shared_count. The shared_count contains a
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pointer of type _Sp_counted_base* which points to the
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pointer of type _Sp_counted_base* which points to the
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object that maintains the reference-counts and destroys the managed
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object that maintains the reference-counts and destroys the managed
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resource.
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resource.
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_Sp_counted_base<Lp>
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_Sp_counted_base<Lp>
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The base of the hierarchy is parameterized on the lock policy alone.
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The base of the hierarchy is parameterized on the lock policy alone.
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_Sp_counted_base doesn't depend on the type of pointer being managed,
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_Sp_counted_base doesn't depend on the type of pointer being managed,
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it only maintains the reference counts and calls virtual functions when
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it only maintains the reference counts and calls virtual functions when
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the counts drop to zero. The managed object is destroyed when the last
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the counts drop to zero. The managed object is destroyed when the last
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strong reference is dropped, but the _Sp_counted_base itself must exist
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strong reference is dropped, but the _Sp_counted_base itself must exist
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until the last weak reference is dropped.
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until the last weak reference is dropped.
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_Sp_counted_base_impl<Ptr, Deleter, Lp>
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_Sp_counted_base_impl<Ptr, Deleter, Lp>
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Inherits from _Sp_counted_base and stores a pointer of type Ptr
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Inherits from _Sp_counted_base and stores a pointer of type Ptr
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and a deleter of type Deleter . _Sp_deleter is
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and a deleter of type Deleter . _Sp_deleter is
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used when the user doesn't supply a custom deleter. Unlike Boost's, this
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used when the user doesn't supply a custom deleter. Unlike Boost's, this
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default deleter is not "checked" because GCC already issues a warning if
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default deleter is not "checked" because GCC already issues a warning if
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delete is used with an incomplete type.
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delete is used with an incomplete type.
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This is the only derived type used by shared_ptr<Ptr>
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This is the only derived type used by shared_ptr<Ptr>
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and it is never used by shared_ptr, which uses one of
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and it is never used by shared_ptr, which uses one of
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the following types, depending on how the shared_ptr is constructed.
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the following types, depending on how the shared_ptr is constructed.
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_Sp_counted_ptr<Ptr, Lp>
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_Sp_counted_ptr<Ptr, Lp>
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Inherits from _Sp_counted_base and stores a pointer of type Ptr,
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Inherits from _Sp_counted_base and stores a pointer of type Ptr,
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which is passed to delete when the last reference is dropped.
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which is passed to delete when the last reference is dropped.
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This is the simplest form and is used when there is no custom deleter or
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This is the simplest form and is used when there is no custom deleter or
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allocator.
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allocator.
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_Sp_counted_deleter<Ptr, Deleter, Alloc>
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_Sp_counted_deleter<Ptr, Deleter, Alloc>
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Inherits from _Sp_counted_ptr and adds support for custom deleter and
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Inherits from _Sp_counted_ptr and adds support for custom deleter and
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allocator. Empty Base Optimization is used for the allocator. This class
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allocator. Empty Base Optimization is used for the allocator. This class
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is used even when the user only provides a custom deleter, in which case
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is used even when the user only provides a custom deleter, in which case
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allocator is used as the allocator.
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allocator is used as the allocator.
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_Sp_counted_ptr_inplace<Tp, Alloc, Lp>
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_Sp_counted_ptr_inplace<Tp, Alloc, Lp>
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Used by allocate_shared and make_shared .
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Used by allocate_shared and make_shared .
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Contains aligned storage to hold an object of type Tp,
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Contains aligned storage to hold an object of type Tp,
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which is constructed in-place with placement new.
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which is constructed in-place with placement new.
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Has a variadic template constructor allowing any number of arguments to
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Has a variadic template constructor allowing any number of arguments to
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be forwarded to Tp's constructor.
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be forwarded to Tp's constructor.
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Unlike the other _Sp_counted_* classes, this one is parameterized on the
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Unlike the other _Sp_counted_* classes, this one is parameterized on the
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type of object, not the type of pointer; this is purely a convenience
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type of object, not the type of pointer; this is purely a convenience
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that simplifies the implementation slightly.
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that simplifies the implementation slightly.
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Thread Safety
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Thread Safety
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The interface of tr1::shared_ptr was extended for C++0x
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The interface of tr1::shared_ptr was extended for C++0x
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with support for rvalue-references and the other features from
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with support for rvalue-references and the other features from
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N2351. As with other libstdc++ headers shared by TR1 and C++0x,
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N2351. As with other libstdc++ headers shared by TR1 and C++0x,
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boost_shared_ptr.h uses conditional compilation, based on the macros
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boost_shared_ptr.h uses conditional compilation, based on the macros
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_GLIBCXX_INCLUDE_AS_CXX0X and
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_GLIBCXX_INCLUDE_AS_CXX0X and
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_GLIBCXX_INCLUDE_AS_TR1, to enable and disable
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_GLIBCXX_INCLUDE_AS_TR1, to enable and disable
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features.
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features.
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C++0x-only features are: rvalue-ref/move support, allocator support,
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C++0x-only features are: rvalue-ref/move support, allocator support,
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aliasing constructor, make_shared & allocate_shared. Additionally,
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aliasing constructor, make_shared & allocate_shared. Additionally,
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the constructors taking auto_ptr parameters are
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the constructors taking auto_ptr parameters are
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deprecated in C++0x mode.
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deprecated in C++0x mode.
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The
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The
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Thread
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Thread
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Safety section of the Boost shared_ptr documentation says "shared_ptr
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Safety section of the Boost shared_ptr documentation says "shared_ptr
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objects offer the same level of thread safety as built-in types."
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objects offer the same level of thread safety as built-in types."
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The implementation must ensure that concurrent updates to separate shared_ptr
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The implementation must ensure that concurrent updates to separate shared_ptr
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instances are correct even when those instances share a reference count e.g.
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instances are correct even when those instances share a reference count e.g.
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shared_ptr<A> a(new A);
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shared_ptr<A> a(new A);
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shared_ptr<A> b(a);
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shared_ptr<A> b(a);
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// Thread 1 // Thread 2
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// Thread 1 // Thread 2
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a.reset(); b.reset();
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a.reset(); b.reset();
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The dynamically-allocated object must be destroyed by exactly one of the
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The dynamically-allocated object must be destroyed by exactly one of the
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threads. Weak references make things even more interesting.
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threads. Weak references make things even more interesting.
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The shared state used to implement shared_ptr must be transparent to the
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The shared state used to implement shared_ptr must be transparent to the
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user and invariants must be preserved at all times.
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user and invariants must be preserved at all times.
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The key pieces of shared state are the strong and weak reference counts.
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The key pieces of shared state are the strong and weak reference counts.
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Updates to these need to be atomic and visible to all threads to ensure
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Updates to these need to be atomic and visible to all threads to ensure
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correct cleanup of the managed resource (which is, after all, shared_ptr's
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correct cleanup of the managed resource (which is, after all, shared_ptr's
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job!)
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job!)
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On multi-processor systems memory synchronisation may be needed so that
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On multi-processor systems memory synchronisation may be needed so that
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reference-count updates and the destruction of the managed resource are
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reference-count updates and the destruction of the managed resource are
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race-free.
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race-free.
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The function _Sp_counted_base::_M_add_ref_lock(), called when
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The function _Sp_counted_base::_M_add_ref_lock(), called when
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obtaining a shared_ptr from a weak_ptr, has to test if the managed
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obtaining a shared_ptr from a weak_ptr, has to test if the managed
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resource still exists and either increment the reference count or throw
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resource still exists and either increment the reference count or throw
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bad_weak_ptr.
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bad_weak_ptr.
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In a multi-threaded program there is a potential race condition if the last
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In a multi-threaded program there is a potential race condition if the last
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reference is dropped (and the managed resource destroyed) between testing
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reference is dropped (and the managed resource destroyed) between testing
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the reference count and incrementing it, which could result in a shared_ptr
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the reference count and incrementing it, which could result in a shared_ptr
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pointing to invalid memory.
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pointing to invalid memory.
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The Boost shared_ptr (as used in GCC) features a clever lock-free
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The Boost shared_ptr (as used in GCC) features a clever lock-free
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algorithm to avoid the race condition, but this relies on the
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algorithm to avoid the race condition, but this relies on the
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processor supporting an atomic Compare-And-Swap
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processor supporting an atomic Compare-And-Swap
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instruction. For other platforms there are fall-backs using mutex
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instruction. For other platforms there are fall-backs using mutex
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locks. Boost (as of version 1.35) includes several different
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locks. Boost (as of version 1.35) includes several different
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implementations and the preprocessor selects one based on the
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implementations and the preprocessor selects one based on the
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compiler, standard library, platform etc. For the version of
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compiler, standard library, platform etc. For the version of
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shared_ptr in libstdc++ the compiler and library are fixed, which
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shared_ptr in libstdc++ the compiler and library are fixed, which
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makes things much simpler: we have an atomic CAS or we don't, see Lock
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makes things much simpler: we have an atomic CAS or we don't, see Lock
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Policy below for details.
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Policy below for details.
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Selecting Lock Policy
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Selecting Lock Policy
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There is a single _Sp_counted_base class,
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There is a single _Sp_counted_base class,
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which is a template parameterized on the enum
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which is a template parameterized on the enum
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__gnu_cxx::_Lock_policy. The entire family of classes is
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__gnu_cxx::_Lock_policy. The entire family of classes is
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parameterized on the lock policy, right up to
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parameterized on the lock policy, right up to
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__shared_ptr, __weak_ptr and
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__shared_ptr, __weak_ptr and
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__enable_shared_from_this. The actual
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__enable_shared_from_this. The actual
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std::shared_ptr class inherits from
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std::shared_ptr class inherits from
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__shared_ptr with the lock policy parameter
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__shared_ptr with the lock policy parameter
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selected automatically based on the thread model and platform that
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selected automatically based on the thread model and platform that
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libstdc++ is configured for, so that the best available template
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libstdc++ is configured for, so that the best available template
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specialization will be used. This design is necessary because it would
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specialization will be used. This design is necessary because it would
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not be conforming for shared_ptr to have an
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not be conforming for shared_ptr to have an
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extra template parameter, even if it had a default value. The
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extra template parameter, even if it had a default value. The
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available policies are:
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available policies are:
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_S_Atomic
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_S_Atomic
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Selected when GCC supports a builtin atomic compare-and-swap operation
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Selected when GCC supports a builtin atomic compare-and-swap operation
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on the target processor (see Atomic
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on the target processor (see Atomic
|
Builtins.) The reference counts are maintained using a lock-free
|
Builtins.) The reference counts are maintained using a lock-free
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algorithm and GCC's atomic builtins, which provide the required memory
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algorithm and GCC's atomic builtins, which provide the required memory
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synchronisation.
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synchronisation.
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_S_Mutex
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_S_Mutex
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The _Sp_counted_base specialization for this policy contains a mutex,
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The _Sp_counted_base specialization for this policy contains a mutex,
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which is locked in add_ref_lock(). This policy is used when GCC's atomic
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which is locked in add_ref_lock(). This policy is used when GCC's atomic
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builtins aren't available so explicit memory barriers are needed in places.
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builtins aren't available so explicit memory barriers are needed in places.
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_S_Single
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_S_Single
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This policy uses a non-reentrant add_ref_lock() with no locking. It is
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This policy uses a non-reentrant add_ref_lock() with no locking. It is
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used when libstdc++ is built without --enable-threads.
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used when libstdc++ is built without --enable-threads.
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For all three policies, reference count increments and
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For all three policies, reference count increments and
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decrements are done via the functions in
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decrements are done via the functions in
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ext/atomicity.h, which detect if the program
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ext/atomicity.h, which detect if the program
|
is multi-threaded. If only one thread of execution exists in
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is multi-threaded. If only one thread of execution exists in
|
the program then less expensive non-atomic operations are used.
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the program then less expensive non-atomic operations are used.
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Dual C++0x and TR1 Implementation
|
Dual C++0x and TR1 Implementation
|
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The classes derived from _Sp_counted_base (see Class Hierarchy
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The classes derived from _Sp_counted_base (see Class Hierarchy
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below) and __shared_count are implemented separately for C++0x
|
below) and __shared_count are implemented separately for C++0x
|
and TR1, in bits/boost_sp_shared_count.h and
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and TR1, in bits/boost_sp_shared_count.h and
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tr1/boost_sp_shared_count.h respectively. All other classes
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tr1/boost_sp_shared_count.h respectively. All other classes
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including _Sp_counted_base are shared by both implementations.
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including _Sp_counted_base are shared by both implementations.
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The TR1 implementation is considered relatively stable, so is unlikely to
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The TR1 implementation is considered relatively stable, so is unlikely to
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change unless bug fixes require it. If the code that is common to both
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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
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C++0x and TR1 modes needs to diverge further then it might be necessary to
|
duplicate additional classes and only make changes to the C++0x versions.
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duplicate additional classes and only make changes to the C++0x versions.
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Related functions and classes
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Related functions and classes
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dynamic_pointer_cast , static_pointer_cast ,
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dynamic_pointer_cast , static_pointer_cast ,
|
const_pointer_cast
|
const_pointer_cast
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As noted in N2351, these functions can be implemented non-intrusively using
|
As noted in N2351, these functions can be implemented non-intrusively using
|
the alias constructor. However the aliasing constructor is only available
|
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
|
in C++0x mode, so in TR1 mode these casts rely on three non-standard
|
constructors in shared_ptr and __shared_ptr.
|
constructors in shared_ptr and __shared_ptr.
|
In C++0x mode these constructors and the related tag types are not needed.
|
In C++0x mode these constructors and the related tag types are not needed.
|
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enable_shared_from_this
|
enable_shared_from_this
|
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The clever overload to detect a base class of type
|
The clever overload to detect a base class of type
|
enable_shared_from_this comes straight from Boost.
|
enable_shared_from_this comes straight from Boost.
|
There is an extra overload for __enable_shared_from_this to
|
There is an extra overload for __enable_shared_from_this to
|
work smoothly with __shared_ptr<Tp, Lp> using any lock
|
work smoothly with __shared_ptr<Tp, Lp> using any lock
|
policy.
|
policy.
|
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make_shared , allocate_shared
|
make_shared , allocate_shared
|
|
|
|
|
make_shared simply forwards to allocate_shared
|
make_shared simply forwards to allocate_shared
|
with std::allocator as the allocator.
|
with std::allocator as the allocator.
|
Although these functions can be implemented non-intrusively using the
|
Although these functions can be implemented non-intrusively using the
|
alias constructor, if they have access to the implementation then it is
|
alias constructor, if they have access to the implementation then it is
|
possible to save storage and reduce the number of heap allocations. The
|
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
|
newly constructed object and the _Sp_counted_* can be allocated in a single
|
block and the standard says implementations are "encouraged, but not required,"
|
block and the standard says implementations are "encouraged, but not required,"
|
to do so. This implementation provides additional non-standard constructors
|
to do so. This implementation provides additional non-standard constructors
|
(selected with the type _Sp_make_shared_tag ) which create an
|
(selected with the type _Sp_make_shared_tag ) which create an
|
object of type _Sp_counted_ptr_inplace to hold the new object.
|
object of type _Sp_counted_ptr_inplace to hold the new object.
|
The returned shared_ptr<A> needs to know the address of the
|
The returned shared_ptr<A> needs to know the address of the
|
new A object embedded in the _Sp_counted_ptr_inplace ,
|
new A object embedded in the _Sp_counted_ptr_inplace ,
|
but it has no way to access it.
|
but it has no way to access it.
|
This implementation uses a "covert channel" to return the address of the
|
This implementation uses a "covert channel" to return the address of the
|
embedded object when get_deleter<_Sp_make_shared_tag>()
|
embedded object when get_deleter<_Sp_make_shared_tag>()
|
is called. Users should not try to use this.
|
is called. Users should not try to use this.
|
As well as the extra constructors, this implementation also needs some
|
As well as the extra constructors, this implementation also needs some
|
members of _Sp_counted_deleter to be protected where they could otherwise
|
members of _Sp_counted_deleter to be protected where they could otherwise
|
be private.
|
be private.
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-->
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Use
|
Use
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Examples
|
Examples
|
|
|
Examples of use can be found in the testsuite, under
|
Examples of use can be found in the testsuite, under
|
testsuite/tr1/2_general_utilities/shared_ptr.
|
testsuite/tr1/2_general_utilities/shared_ptr.
|
|
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Unresolved Issues
|
Unresolved Issues
|
|
|
The resolution to C++ Standard Library issue 674,
|
The resolution to C++ Standard Library issue 674,
|
"shared_ptr interface changes for consistency with N1856" will
|
"shared_ptr interface changes for consistency with N1856" will
|
need to be implemented after it is accepted into the working
|
need to be implemented after it is accepted into the working
|
paper. Issue 743
|
paper. Issue 743
|
might also require changes.
|
might also require changes.
|
|
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|
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The _S_single policy uses atomics when used in MT
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The _S_single policy uses atomics when used in MT
|
code, because it uses the same dispatcher functions that check
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code, because it uses the same dispatcher functions that check
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__gthread_active_p(). This could be
|
__gthread_active_p(). This could be
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addressed by providing template specialisations for some members
|
addressed by providing template specialisations for some members
|
of _Sp_counted_base<_S_single>.
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of _Sp_counted_base<_S_single>.
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Unlike Boost, this implementation does not use separate classes
|
Unlike Boost, this implementation does not use separate classes
|
for the pointer+deleter and pointer+deleter+allocator cases in
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for the pointer+deleter and pointer+deleter+allocator cases in
|
C++0x mode, combining both into _Sp_counted_deleter and using
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C++0x mode, combining both into _Sp_counted_deleter and using
|
allocator when the user doesn't specify
|
allocator when the user doesn't specify
|
an allocator. If it was found to be beneficial an additional
|
an allocator. If it was found to be beneficial an additional
|
class could easily be added. With the current implementation,
|
class could easily be added. With the current implementation,
|
the _Sp_counted_deleter and __shared_count constructors taking a
|
the _Sp_counted_deleter and __shared_count constructors taking a
|
custom deleter but no allocator are technically redundant and
|
custom deleter but no allocator are technically redundant and
|
could be removed, changing callers to always specify an
|
could be removed, changing callers to always specify an
|
allocator. If a separate pointer+deleter class was added the
|
allocator. If a separate pointer+deleter class was added the
|
__shared_count constructor would be needed, so it has been kept
|
__shared_count constructor would be needed, so it has been kept
|
for now.
|
for now.
|
|
|
|
|
|
|
The hack used to get the address of the managed object from
|
The hack used to get the address of the managed object from
|
_Sp_counted_ptr_inplace::_M_get_deleter()
|
_Sp_counted_ptr_inplace::_M_get_deleter()
|
is accessible to users. This could be prevented if
|
is accessible to users. This could be prevented if
|
get_deleter<_Sp_make_shared_tag>()
|
get_deleter<_Sp_make_shared_tag>()
|
always returned NULL, since the hack only needs to work at a
|
always returned NULL, since the hack only needs to work at a
|
lower level, not in the public API. This wouldn't be difficult,
|
lower level, not in the public API. This wouldn't be difficult,
|
but hasn't been done since there is no danger of accidental
|
but hasn't been done since there is no danger of accidental
|
misuse: users already know they are relying on unsupported
|
misuse: users already know they are relying on unsupported
|
features if they refer to implementation details such as
|
features if they refer to implementation details such as
|
_Sp_make_shared_tag.
|
_Sp_make_shared_tag.
|
|
|
|
|
|
|
tr1::_Sp_deleter could be a private member of tr1::__shared_count but it
|
tr1::_Sp_deleter could be a private member of tr1::__shared_count but it
|
would alter the ABI.
|
would alter the ABI.
|
|
|
|
|
|
|
Exposing the alias constructor in TR1 mode could simplify the
|
Exposing the alias constructor in TR1 mode could simplify the
|
*_pointer_cast functions. Constructor could be private in TR1
|
*_pointer_cast functions. Constructor could be private in TR1
|
mode, with the cast functions as friends.
|
mode, with the cast functions as friends.
|
|
|
|
|
|
|
|
|
|
|
|
|
Acknowledgments
|
Acknowledgments
|
|
|
|
|
The original authors of the Boost shared_ptr, which is really nice
|
The original authors of the Boost shared_ptr, which is really nice
|
code to work with, Peter Dimov in particular for his help and
|
code to work with, Peter Dimov in particular for his help and
|
invaluable advice on thread safety. Phillip Jordan and Paolo
|
invaluable advice on thread safety. Phillip Jordan and Paolo
|
Carlini for the lock policy implementation.
|
Carlini for the lock policy implementation.
|
|
|
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|
|
|
|
Bibliography
|
Bibliography
|
|
|
|
|
|
|
|
|
|
|
Improving shared_ptr for C++0x, Revision 2
|
Improving shared_ptr for C++0x, Revision 2
|
|
|
|
|
|
|
|
|
N2351
|
N2351
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
C++ Standard Library Active Issues List
|
C++ Standard Library Active Issues List
|
|
|
|
|
|
|
|
|
N2456
|
N2456
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Working Draft, Standard for Programming Language C++
|
Working Draft, Standard for Programming Language C++
|
|
|
|
|
|
|
|
|
N2461
|
N2461
|
|
|
|
|
|
|
|
|
|
|
shared_ptr
|
shared_ptr
|
|
|
Boost C++ Libraries documentation, shared_ptr
|
Boost C++ Libraries documentation, shared_ptr
|
|
|
|
|
|
|
|
|
N2461
|
N2461
|
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