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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>shared_ptr</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><meta name="keywords" content=" ISO C++ , shared_ptr " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="memory.html" title="Chapter 11. Memory" /><link rel="prev" href="auto_ptr.html" title="auto_ptr" /><link rel="next" href="traits.html" title="Chapter 12. Traits" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">shared_ptr</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="auto_ptr.html">Prev</a> </td><th width="60%" align="center">Chapter 11. Memory</th><td width="20%" align="right"> <a accesskey="n" href="traits.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="shared_ptr"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.util.memory.shared_ptr"></a>shared_ptr</h2></div></div></div><p>
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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>shared_ptr</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><meta name="keywords" content=" ISO C++ , shared_ptr " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="memory.html" title="Chapter 11. Memory" /><link rel="prev" href="auto_ptr.html" title="auto_ptr" /><link rel="next" href="traits.html" title="Chapter 12. Traits" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">shared_ptr</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="auto_ptr.html">Prev</a> </td><th width="60%" align="center">Chapter 11. Memory</th><td width="20%" align="right"> <a accesskey="n" href="traits.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="shared_ptr"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.util.memory.shared_ptr"></a>shared_ptr</h2></div></div></div><p>
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The shared_ptr class template stores a pointer, usually obtained via new,
|
The shared_ptr class template stores a pointer, usually obtained via new,
|
and implements shared ownership semantics.
|
and implements shared ownership semantics.
|
</p><div class="sect2" title="Requirements"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.req"></a>Requirements</h3></div></div></div><p>
|
</p><div class="sect2" title="Requirements"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.req"></a>Requirements</h3></div></div></div><p>
|
</p><p>
|
</p><p>
|
The standard deliberately doesn't require a reference-counted
|
The standard deliberately doesn't require a reference-counted
|
implementation, allowing other techniques such as a
|
implementation, allowing other techniques such as a
|
circular-linked-list.
|
circular-linked-list.
|
</p><p>
|
</p><p>
|
At the time of writing the C++0x working paper doesn't mention how
|
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
|
threads affect shared_ptr, but it is likely to follow the existing
|
practice set by <code class="classname">boost::shared_ptr</code>. The
|
practice set by <code class="classname">boost::shared_ptr</code>. The
|
shared_ptr in libstdc++ is derived from Boost's, so the same rules
|
shared_ptr in libstdc++ is derived from Boost's, so the same rules
|
apply.
|
apply.
|
</p><p>
|
</p><p>
|
</p></div><div class="sect2" title="Design Issues"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.design_issues"></a>Design Issues</h3></div></div></div><p>
|
</p></div><div class="sect2" title="Design Issues"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.design_issues"></a>Design Issues</h3></div></div></div><p>
|
The <code class="classname">shared_ptr</code> code is kindly donated to GCC by the Boost
|
The <code class="classname">shared_ptr</code> code is kindly donated to GCC by the Boost
|
project and the original authors of the code. The basic design and
|
project and the original authors of the code. The basic design and
|
algorithms are from Boost, the notes below describe details specific to
|
algorithms are from Boost, the notes below describe details specific to
|
the GCC implementation. Names have been uglified in this implementation,
|
the GCC implementation. Names have been uglified in this implementation,
|
but the design should be recognisable to anyone familiar with the Boost
|
but the design should be recognisable to anyone familiar with the Boost
|
1.32 shared_ptr.
|
1.32 shared_ptr.
|
</p><p>
|
</p><p>
|
The basic design is an abstract base class, <code class="code">_Sp_counted_base</code> that
|
The basic design is an abstract base class, <code class="code">_Sp_counted_base</code> that
|
does the reference-counting and calls virtual functions when the count
|
does the reference-counting and calls virtual functions when the count
|
drops to zero.
|
drops to zero.
|
Derived classes override those functions to destroy resources in a context
|
Derived classes override those functions to destroy resources in a context
|
where the correct dynamic type is known. This is an application of the
|
where the correct dynamic type is known. This is an application of the
|
technique known as type erasure.
|
technique known as type erasure.
|
</p></div><div class="sect2" title="Implementation"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.impl"></a>Implementation</h3></div></div></div><div class="sect3" title="Class Hierarchy"><div class="titlepage"><div><div><h4 class="title"><a id="id628890"></a>Class Hierarchy</h4></div></div></div><p>
|
</p></div><div class="sect2" title="Implementation"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.impl"></a>Implementation</h3></div></div></div><div class="sect3" title="Class Hierarchy"><div class="titlepage"><div><div><h4 class="title"><a id="id628890"></a>Class Hierarchy</h4></div></div></div><p>
|
A <code class="classname">shared_ptr<T></code> contains a pointer of
|
A <code class="classname">shared_ptr<T></code> contains a pointer of
|
type <span class="type">T*</span> and an object of type
|
type <span class="type">T*</span> and an object of type
|
<code class="classname">__shared_count</code>. The shared_count contains a
|
<code class="classname">__shared_count</code>. The shared_count contains a
|
pointer of type <span class="type">_Sp_counted_base*</span> which points to the
|
pointer of type <span class="type">_Sp_counted_base*</span> which points to the
|
object that maintains the reference-counts and destroys the managed
|
object that maintains the reference-counts and destroys the managed
|
resource.
|
resource.
|
</p><div class="variablelist"><dl><dt><span class="term"><code class="classname">_Sp_counted_base<Lp></code></span></dt><dd><p>
|
</p><div class="variablelist"><dl><dt><span class="term"><code class="classname">_Sp_counted_base<Lp></code></span></dt><dd><p>
|
The base of the hierarchy is parameterized on the lock policy alone.
|
The base of the hierarchy is parameterized on the lock policy alone.
|
_Sp_counted_base doesn't depend on the type of pointer being managed,
|
_Sp_counted_base doesn't depend on the type of pointer being managed,
|
it only maintains the reference counts and calls virtual functions when
|
it only maintains the reference counts and calls virtual functions when
|
the counts drop to zero. The managed object is destroyed when the last
|
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
|
strong reference is dropped, but the _Sp_counted_base itself must exist
|
until the last weak reference is dropped.
|
until the last weak reference is dropped.
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_base_impl<Ptr, Deleter, Lp></code></span></dt><dd><p>
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_base_impl<Ptr, Deleter, Lp></code></span></dt><dd><p>
|
Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>
|
Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>
|
and a deleter of type <code class="code">Deleter</code>. <code class="code">_Sp_deleter</code> is
|
and a deleter of type <code class="code">Deleter</code>. <code class="code">_Sp_deleter</code> is
|
used when the user doesn't supply a custom deleter. Unlike Boost's, this
|
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
|
default deleter is not "checked" because GCC already issues a warning if
|
<code class="function">delete</code> is used with an incomplete type.
|
<code class="function">delete</code> is used with an incomplete type.
|
This is the only derived type used by <code class="classname">shared_ptr<Ptr></code>
|
This is the only derived type used by <code class="classname">shared_ptr<Ptr></code>
|
and it is never used by <code class="classname">shared_ptr</code>, which uses one of
|
and it is never used by <code class="classname">shared_ptr</code>, which uses one of
|
the following types, depending on how the shared_ptr is constructed.
|
the following types, depending on how the shared_ptr is constructed.
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr<Ptr, Lp></code></span></dt><dd><p>
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr<Ptr, Lp></code></span></dt><dd><p>
|
Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>,
|
Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>,
|
which is passed to <code class="function">delete</code> when the last reference is dropped.
|
which is passed to <code class="function">delete</code> when the last reference is dropped.
|
This is the simplest form and is used when there is no custom deleter or
|
This is the simplest form and is used when there is no custom deleter or
|
allocator.
|
allocator.
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_deleter<Ptr, Deleter, Alloc></code></span></dt><dd><p>
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_deleter<Ptr, Deleter, Alloc></code></span></dt><dd><p>
|
Inherits from _Sp_counted_ptr and adds support for custom deleter and
|
Inherits from _Sp_counted_ptr and adds support for custom deleter and
|
allocator. Empty Base Optimization is used for the allocator. This class
|
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
|
is used even when the user only provides a custom deleter, in which case
|
<code class="classname">allocator</code> is used as the allocator.
|
<code class="classname">allocator</code> is used as the allocator.
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr_inplace<Tp, Alloc, Lp></code></span></dt><dd><p>
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr_inplace<Tp, Alloc, Lp></code></span></dt><dd><p>
|
Used by <code class="code">allocate_shared</code> and <code class="code">make_shared</code>.
|
Used by <code class="code">allocate_shared</code> and <code class="code">make_shared</code>.
|
Contains aligned storage to hold an object of type <span class="type">Tp</span>,
|
Contains aligned storage to hold an object of type <span class="type">Tp</span>,
|
which is constructed in-place with placement <code class="function">new</code>.
|
which is constructed in-place with placement <code class="function">new</code>.
|
Has a variadic template constructor allowing any number of arguments to
|
Has a variadic template constructor allowing any number of arguments to
|
be forwarded to <span class="type">Tp</span>'s constructor.
|
be forwarded to <span class="type">Tp</span>'s constructor.
|
Unlike the other <code class="classname">_Sp_counted_*</code> classes, this one is parameterized on the
|
Unlike the other <code class="classname">_Sp_counted_*</code> classes, this one is parameterized on the
|
type of object, not the type of pointer; this is purely a convenience
|
type of object, not the type of pointer; this is purely a convenience
|
that simplifies the implementation slightly.
|
that simplifies the implementation slightly.
|
</p></dd></dl></div></div><div class="sect3" title="Thread Safety"><div class="titlepage"><div><div><h4 class="title"><a id="id612958"></a>Thread Safety</h4></div></div></div><p>
|
</p></dd></dl></div></div><div class="sect3" title="Thread Safety"><div class="titlepage"><div><div><h4 class="title"><a id="id612958"></a>Thread Safety</h4></div></div></div><p>
|
The interface of <code class="classname">tr1::shared_ptr</code> was extended for C++0x
|
The interface of <code class="classname">tr1::shared_ptr</code> was extended for C++0x
|
with support for rvalue-references and the other features from
|
with support for rvalue-references and the other features from
|
N2351. As with other libstdc++ headers shared by TR1 and C++0x,
|
N2351. As with other libstdc++ headers shared by TR1 and C++0x,
|
boost_shared_ptr.h uses conditional compilation, based on the macros
|
boost_shared_ptr.h uses conditional compilation, based on the macros
|
<code class="constant">_GLIBCXX_INCLUDE_AS_CXX0X</code> and
|
<code class="constant">_GLIBCXX_INCLUDE_AS_CXX0X</code> and
|
<code class="constant">_GLIBCXX_INCLUDE_AS_TR1</code>, to enable and disable
|
<code class="constant">_GLIBCXX_INCLUDE_AS_TR1</code>, to enable and disable
|
features.
|
features.
|
</p><p>
|
</p><p>
|
C++0x-only features are: rvalue-ref/move support, allocator support,
|
C++0x-only features are: rvalue-ref/move support, allocator support,
|
aliasing constructor, make_shared & allocate_shared. Additionally,
|
aliasing constructor, make_shared & allocate_shared. Additionally,
|
the constructors taking <code class="classname">auto_ptr</code> parameters are
|
the constructors taking <code class="classname">auto_ptr</code> parameters are
|
deprecated in C++0x mode.
|
deprecated in C++0x mode.
|
</p><p>
|
</p><p>
|
The
|
The
|
<a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm#ThreadSafety" target="_top">Thread
|
<a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm#ThreadSafety" target="_top">Thread
|
Safety</a> section of the Boost shared_ptr documentation says "shared_ptr
|
Safety</a> section of the Boost shared_ptr documentation says "shared_ptr
|
objects offer the same level of thread safety as built-in types."
|
objects offer the same level of thread safety as built-in types."
|
The implementation must ensure that concurrent updates to separate shared_ptr
|
The implementation must ensure that concurrent updates to separate shared_ptr
|
instances are correct even when those instances share a reference count e.g.
|
instances are correct even when those instances share a reference count e.g.
|
</p><pre class="programlisting">
|
</p><pre class="programlisting">
|
shared_ptr<A> a(new A);
|
shared_ptr<A> a(new A);
|
shared_ptr<A> b(a);
|
shared_ptr<A> b(a);
|
|
|
// Thread 1 // Thread 2
|
// Thread 1 // Thread 2
|
a.reset(); b.reset();
|
a.reset(); b.reset();
|
</pre><p>
|
</pre><p>
|
The dynamically-allocated object must be destroyed by exactly one of the
|
The dynamically-allocated object must be destroyed by exactly one of the
|
threads. Weak references make things even more interesting.
|
threads. Weak references make things even more interesting.
|
The shared state used to implement shared_ptr must be transparent to the
|
The shared state used to implement shared_ptr must be transparent to the
|
user and invariants must be preserved at all times.
|
user and invariants must be preserved at all times.
|
The key pieces of shared state are the strong and weak reference counts.
|
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
|
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
|
correct cleanup of the managed resource (which is, after all, shared_ptr's
|
job!)
|
job!)
|
On multi-processor systems memory synchronisation may be needed so that
|
On multi-processor systems memory synchronisation may be needed so that
|
reference-count updates and the destruction of the managed resource are
|
reference-count updates and the destruction of the managed resource are
|
race-free.
|
race-free.
|
</p><p>
|
</p><p>
|
The function <code class="function">_Sp_counted_base::_M_add_ref_lock()</code>, called when
|
The function <code class="function">_Sp_counted_base::_M_add_ref_lock()</code>, called when
|
obtaining a shared_ptr from a weak_ptr, has to test if the managed
|
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
|
resource still exists and either increment the reference count or throw
|
<code class="classname">bad_weak_ptr</code>.
|
<code class="classname">bad_weak_ptr</code>.
|
In a multi-threaded program there is a potential race condition if the last
|
In a multi-threaded program there is a potential race condition if the last
|
reference is dropped (and the managed resource destroyed) between testing
|
reference is dropped (and the managed resource destroyed) between testing
|
the reference count and incrementing it, which could result in a shared_ptr
|
the reference count and incrementing it, which could result in a shared_ptr
|
pointing to invalid memory.
|
pointing to invalid memory.
|
</p><p>
|
</p><p>
|
The Boost shared_ptr (as used in GCC) features a clever lock-free
|
The Boost shared_ptr (as used in GCC) features a clever lock-free
|
algorithm to avoid the race condition, but this relies on the
|
algorithm to avoid the race condition, but this relies on the
|
processor supporting an atomic <span class="emphasis"><em>Compare-And-Swap</em></span>
|
processor supporting an atomic <span class="emphasis"><em>Compare-And-Swap</em></span>
|
instruction. For other platforms there are fall-backs using mutex
|
instruction. For other platforms there are fall-backs using mutex
|
locks. Boost (as of version 1.35) includes several different
|
locks. Boost (as of version 1.35) includes several different
|
implementations and the preprocessor selects one based on the
|
implementations and the preprocessor selects one based on the
|
compiler, standard library, platform etc. For the version of
|
compiler, standard library, platform etc. For the version of
|
shared_ptr in libstdc++ the compiler and library are fixed, which
|
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
|
makes things much simpler: we have an atomic CAS or we don't, see Lock
|
Policy below for details.
|
Policy below for details.
|
</p></div><div class="sect3" title="Selecting Lock Policy"><div class="titlepage"><div><div><h4 class="title"><a id="id637767"></a>Selecting Lock Policy</h4></div></div></div><p>
|
</p></div><div class="sect3" title="Selecting Lock Policy"><div class="titlepage"><div><div><h4 class="title"><a id="id637767"></a>Selecting Lock Policy</h4></div></div></div><p>
|
</p><p>
|
</p><p>
|
There is a single <code class="classname">_Sp_counted_base</code> class,
|
There is a single <code class="classname">_Sp_counted_base</code> class,
|
which is a template parameterized on the enum
|
which is a template parameterized on the enum
|
<span class="type">__gnu_cxx::_Lock_policy</span>. The entire family of classes is
|
<span class="type">__gnu_cxx::_Lock_policy</span>. The entire family of classes is
|
parameterized on the lock policy, right up to
|
parameterized on the lock policy, right up to
|
<code class="classname">__shared_ptr</code>, <code class="classname">__weak_ptr</code> and
|
<code class="classname">__shared_ptr</code>, <code class="classname">__weak_ptr</code> and
|
<code class="classname">__enable_shared_from_this</code>. The actual
|
<code class="classname">__enable_shared_from_this</code>. The actual
|
<code class="classname">std::shared_ptr</code> class inherits from
|
<code class="classname">std::shared_ptr</code> class inherits from
|
<code class="classname">__shared_ptr</code> with the lock policy parameter
|
<code class="classname">__shared_ptr</code> with the lock policy parameter
|
selected automatically based on the thread model and platform that
|
selected automatically based on the thread model and platform that
|
libstdc++ is configured for, so that the best available template
|
libstdc++ is configured for, so that the best available template
|
specialization will be used. This design is necessary because it would
|
specialization will be used. This design is necessary because it would
|
not be conforming for <code class="classname">shared_ptr</code> to have an
|
not be conforming for <code class="classname">shared_ptr</code> to have an
|
extra template parameter, even if it had a default value. The
|
extra template parameter, even if it had a default value. The
|
available policies are:
|
available policies are:
|
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
|
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
|
<span class="type">_S_Atomic</span>
|
<span class="type">_S_Atomic</span>
|
</p><p>
|
</p><p>
|
Selected when GCC supports a builtin atomic compare-and-swap operation
|
Selected when GCC supports a builtin atomic compare-and-swap operation
|
on the target processor (see <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html" target="_top">Atomic
|
on the target processor (see <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html" target="_top">Atomic
|
Builtins</a>.) The reference counts are maintained using a lock-free
|
Builtins</a>.) The reference counts are maintained using a lock-free
|
algorithm and GCC's atomic builtins, which provide the required memory
|
algorithm and GCC's atomic builtins, which provide the required memory
|
synchronisation.
|
synchronisation.
|
</p></li><li class="listitem"><p>
|
</p></li><li class="listitem"><p>
|
<span class="type">_S_Mutex</span>
|
<span class="type">_S_Mutex</span>
|
</p><p>
|
</p><p>
|
The _Sp_counted_base specialization for this policy contains a 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
|
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.
|
builtins aren't available so explicit memory barriers are needed in places.
|
</p></li><li class="listitem"><p>
|
</p></li><li class="listitem"><p>
|
<span class="type">_S_Single</span>
|
<span class="type">_S_Single</span>
|
</p><p>
|
</p><p>
|
This policy uses a non-reentrant add_ref_lock() with no locking. It is
|
This policy uses a non-reentrant add_ref_lock() with no locking. It is
|
used when libstdc++ is built without <code class="literal">--enable-threads</code>.
|
used when libstdc++ is built without <code class="literal">--enable-threads</code>.
|
</p></li></ol></div><p>
|
</p></li></ol></div><p>
|
For all three policies, reference count increments and
|
For all three policies, reference count increments and
|
decrements are done via the functions in
|
decrements are done via the functions in
|
<code class="filename">ext/atomicity.h</code>, which detect if the program
|
<code class="filename">ext/atomicity.h</code>, which detect if the program
|
is multi-threaded. If only one thread of execution exists in
|
is multi-threaded. If only one thread of execution exists in
|
the program then less expensive non-atomic operations are used.
|
the program then less expensive non-atomic operations are used.
|
</p></div><div class="sect3" title="Dual C++0x and TR1 Implementation"><div class="titlepage"><div><div><h4 class="title"><a id="id618991"></a>Dual C++0x and TR1 Implementation</h4></div></div></div><p>
|
</p></div><div class="sect3" title="Dual C++0x and TR1 Implementation"><div class="titlepage"><div><div><h4 class="title"><a id="id618991"></a>Dual C++0x and TR1 Implementation</h4></div></div></div><p>
|
The classes derived from <code class="classname">_Sp_counted_base</code> (see Class Hierarchy
|
The classes derived from <code class="classname">_Sp_counted_base</code> (see Class Hierarchy
|
below) and <code class="classname">__shared_count</code> are implemented separately for C++0x
|
below) and <code class="classname">__shared_count</code> are implemented separately for C++0x
|
and TR1, in <code class="filename">bits/boost_sp_shared_count.h</code> and
|
and TR1, in <code class="filename">bits/boost_sp_shared_count.h</code> and
|
<code class="filename">tr1/boost_sp_shared_count.h</code> respectively. All other classes
|
<code class="filename">tr1/boost_sp_shared_count.h</code> respectively. All other classes
|
including <code class="classname">_Sp_counted_base</code> are shared by both implementations.
|
including <code class="classname">_Sp_counted_base</code> are shared by both implementations.
|
</p><p>
|
</p><p>
|
The TR1 implementation is considered relatively stable, so is unlikely to
|
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
|
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
|
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.
|
duplicate additional classes and only make changes to the C++0x versions.
|
</p></div><div class="sect3" title="Related functions and classes"><div class="titlepage"><div><div><h4 class="title"><a id="id589355"></a>Related functions and classes</h4></div></div></div><div class="variablelist"><dl><dt><span class="term"><code class="code">dynamic_pointer_cast</code>, <code class="code">static_pointer_cast</code>,
|
</p></div><div class="sect3" title="Related functions and classes"><div class="titlepage"><div><div><h4 class="title"><a id="id589355"></a>Related functions and classes</h4></div></div></div><div class="variablelist"><dl><dt><span class="term"><code class="code">dynamic_pointer_cast</code>, <code class="code">static_pointer_cast</code>,
|
<code class="code">const_pointer_cast</code></span></dt><dd><p>
|
<code class="code">const_pointer_cast</code></span></dt><dd><p>
|
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.
|
</p></dd><dt><span class="term"><code class="code">enable_shared_from_this</code></span></dt><dd><p>
|
</p></dd><dt><span class="term"><code class="code">enable_shared_from_this</code></span></dt><dd><p>
|
The clever overload to detect a base class of type
|
The clever overload to detect a base class of type
|
<code class="code">enable_shared_from_this</code> comes straight from Boost.
|
<code class="code">enable_shared_from_this</code> comes straight from Boost.
|
There is an extra overload for <code class="code">__enable_shared_from_this</code> to
|
There is an extra overload for <code class="code">__enable_shared_from_this</code> to
|
work smoothly with <code class="code">__shared_ptr<Tp, Lp></code> using any lock
|
work smoothly with <code class="code">__shared_ptr<Tp, Lp></code> using any lock
|
policy.
|
policy.
|
</p></dd><dt><span class="term"><code class="code">make_shared</code>, <code class="code">allocate_shared</code></span></dt><dd><p>
|
</p></dd><dt><span class="term"><code class="code">make_shared</code>, <code class="code">allocate_shared</code></span></dt><dd><p>
|
<code class="code">make_shared</code> simply forwards to <code class="code">allocate_shared</code>
|
<code class="code">make_shared</code> simply forwards to <code class="code">allocate_shared</code>
|
with <code class="code">std::allocator</code> as the allocator.
|
with <code class="code">std::allocator</code> 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 <code class="code">_Sp_make_shared_tag</code>) which create an
|
(selected with the type <code class="code">_Sp_make_shared_tag</code>) which create an
|
object of type <code class="code">_Sp_counted_ptr_inplace</code> to hold the new object.
|
object of type <code class="code">_Sp_counted_ptr_inplace</code> to hold the new object.
|
The returned <code class="code">shared_ptr<A></code> needs to know the address of the
|
The returned <code class="code">shared_ptr<A></code> needs to know the address of the
|
new <code class="code">A</code> object embedded in the <code class="code">_Sp_counted_ptr_inplace</code>,
|
new <code class="code">A</code> object embedded in the <code class="code">_Sp_counted_ptr_inplace</code>,
|
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 <code class="code">get_deleter<_Sp_make_shared_tag>()</code>
|
embedded object when <code class="code">get_deleter<_Sp_make_shared_tag>()</code>
|
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.
|
</p></dd></dl></div></div></div><div class="sect2" title="Use"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.using"></a>Use</h3></div></div></div><div class="sect3" title="Examples"><div class="titlepage"><div><div><h4 class="title"><a id="id619880"></a>Examples</h4></div></div></div><p>
|
</p></dd></dl></div></div></div><div class="sect2" title="Use"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.using"></a>Use</h3></div></div></div><div class="sect3" title="Examples"><div class="titlepage"><div><div><h4 class="title"><a id="id619880"></a>Examples</h4></div></div></div><p>
|
Examples of use can be found in the testsuite, under
|
Examples of use can be found in the testsuite, under
|
<code class="filename">testsuite/tr1/2_general_utilities/shared_ptr</code>.
|
<code class="filename">testsuite/tr1/2_general_utilities/shared_ptr</code>.
|
</p></div><div class="sect3" title="Unresolved Issues"><div class="titlepage"><div><div><h4 class="title"><a id="id653780"></a>Unresolved Issues</h4></div></div></div><p>
|
</p></div><div class="sect3" title="Unresolved Issues"><div class="titlepage"><div><div><h4 class="title"><a id="id653780"></a>Unresolved Issues</h4></div></div></div><p>
|
The resolution to C++ Standard Library issue <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#674" target="_top">674</a>,
|
The resolution to C++ Standard Library issue <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#674" target="_top">674</a>,
|
"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 <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#743" target="_top">743</a>
|
paper. Issue <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#743" target="_top">743</a>
|
might also require changes.
|
might also require changes.
|
</p><p>
|
</p><p>
|
The <span class="type">_S_single</span> policy uses atomics when used in MT
|
The <span class="type">_S_single</span> policy uses atomics when used in MT
|
code, because it uses the same dispatcher functions that check
|
code, because it uses the same dispatcher functions that check
|
<code class="function">__gthread_active_p()</code>. This could be
|
<code class="function">__gthread_active_p()</code>. This could be
|
addressed by providing template specialisations for some members
|
addressed by providing template specialisations for some members
|
of <code class="classname">_Sp_counted_base<_S_single></code>.
|
of <code class="classname">_Sp_counted_base<_S_single></code>.
|
</p><p>
|
</p><p>
|
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
|
for the pointer+deleter and pointer+deleter+allocator cases in
|
C++0x mode, combining both into _Sp_counted_deleter and using
|
C++0x mode, combining both into _Sp_counted_deleter and using
|
<code class="classname">allocator</code> when the user doesn't specify
|
<code class="classname">allocator</code> 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.
|
</p><p>
|
</p><p>
|
The hack used to get the address of the managed object from
|
The hack used to get the address of the managed object from
|
<code class="function">_Sp_counted_ptr_inplace::_M_get_deleter()</code>
|
<code class="function">_Sp_counted_ptr_inplace::_M_get_deleter()</code>
|
is accessible to users. This could be prevented if
|
is accessible to users. This could be prevented if
|
<code class="function">get_deleter<_Sp_make_shared_tag>()</code>
|
<code class="function">get_deleter<_Sp_make_shared_tag>()</code>
|
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.
|
</p><p>
|
</p><p>
|
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.
|
</p><p>
|
</p><p>
|
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.
|
</p></div></div><div class="sect2" title="Acknowledgments"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.ack"></a>Acknowledgments</h3></div></div></div><p>
|
</p></div></div><div class="sect2" title="Acknowledgments"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.ack"></a>Acknowledgments</h3></div></div></div><p>
|
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.
|
</p></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry" title="Improving shared_ptr for C++0x, Revision 2"><a id="id678655"></a><p>[<abbr class="abbrev">
|
</p></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry" title="Improving shared_ptr for C++0x, Revision 2"><a id="id678655"></a><p>[<abbr class="abbrev">
|
n2351
|
n2351
|
</abbr>] <span class="title"><i>
|
</abbr>] <span class="title"><i>
|
Improving shared_ptr for C++0x, Revision 2
|
Improving shared_ptr for C++0x, Revision 2
|
</i>. </span><span class="subtitle">
|
</i>. </span><span class="subtitle">
|
N2351
|
N2351
|
. </span><span class="biblioid">
|
. </span><span class="biblioid">
|
<a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2351.htm" target="_top">
|
<a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2351.htm" target="_top">
|
</a>
|
</a>
|
. </span></p></div><div class="biblioentry" title="C++ Standard Library Active Issues List (Revision R52)"><a id="id678679"></a><p>[<abbr class="abbrev">
|
. </span></p></div><div class="biblioentry" title="C++ Standard Library Active Issues List (Revision R52)"><a id="id678679"></a><p>[<abbr class="abbrev">
|
n2456
|
n2456
|
</abbr>] <span class="title"><i>
|
</abbr>] <span class="title"><i>
|
C++ Standard Library Active Issues List (Revision R52)
|
C++ Standard Library Active Issues List (Revision R52)
|
</i>. </span><span class="subtitle">
|
</i>. </span><span class="subtitle">
|
N2456
|
N2456
|
. </span><span class="biblioid">
|
. </span><span class="biblioid">
|
<a class="ulink" href="http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2456.html" target="_top">
|
<a class="ulink" href="http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2456.html" target="_top">
|
</a>
|
</a>
|
. </span></p></div><div class="biblioentry" title="Working Draft, Standard for Programming Language C++"><a id="id661793"></a><p>[<abbr class="abbrev">
|
. </span></p></div><div class="biblioentry" title="Working Draft, Standard for Programming Language C++"><a id="id661793"></a><p>[<abbr class="abbrev">
|
n2461
|
n2461
|
</abbr>] <span class="title"><i>
|
</abbr>] <span class="title"><i>
|
Working Draft, Standard for Programming Language C++
|
Working Draft, Standard for Programming Language C++
|
</i>. </span><span class="subtitle">
|
</i>. </span><span class="subtitle">
|
N2461
|
N2461
|
. </span><span class="biblioid">
|
. </span><span class="biblioid">
|
<a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2461.pdf" target="_top">
|
<a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2461.pdf" target="_top">
|
</a>
|
</a>
|
. </span></p></div><div class="biblioentry" title="Boost C++ Libraries documentation - shared_ptr class template"><a id="id612415"></a><p>[<abbr class="abbrev">
|
. </span></p></div><div class="biblioentry" title="Boost C++ Libraries documentation - shared_ptr class template"><a id="id612415"></a><p>[<abbr class="abbrev">
|
boostshared_ptr
|
boostshared_ptr
|
</abbr>] <span class="title"><i>
|
</abbr>] <span class="title"><i>
|
Boost C++ Libraries documentation - shared_ptr class template
|
Boost C++ Libraries documentation - shared_ptr class template
|
</i>. </span><span class="subtitle">
|
</i>. </span><span class="subtitle">
|
N2461
|
N2461
|
. </span><span class="biblioid">
|
. </span><span class="biblioid">
|
<a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">shared_ptr
|
<a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">shared_ptr
|
</a>
|
</a>
|
. </span></p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="auto_ptr.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="memory.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="traits.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">auto_ptr </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 12. Traits</td></tr></table></div></body></html>
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