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diff --git a/gcc-4.8.1/libstdc++-v3/doc/html/manual/memory.html b/gcc-4.8.1/libstdc++-v3/doc/html/manual/memory.html deleted file mode 100644 index b31c49da9..000000000 --- a/gcc-4.8.1/libstdc++-v3/doc/html/manual/memory.html +++ /dev/null @@ -1,685 +0,0 @@ -<?xml version="1.0" encoding="UTF-8" standalone="no"?> -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Memory</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.77.1" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="utilities.html" title="Chapter 6. Utilities" /><link rel="prev" href="pairs.html" title="Pairs" /><link rel="next" href="traits.html" title="Traits" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Memory</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="pairs.html">Prev</a> </td><th width="60%" align="center">Chapter 6. - Utilities - -</th><td width="20%" align="right"> <a accesskey="n" href="traits.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.util.memory"></a>Memory</h2></div></div></div><p> - Memory contains three general areas. First, function and operator - calls via <code class="function">new</code> and <code class="function">delete</code> - operator or member function calls. Second, allocation via - <code class="classname">allocator</code>. And finally, smart pointer and - intelligent pointer abstractions. - </p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.allocator"></a>Allocators</h3></div></div></div><p> - Memory management for Standard Library entities is encapsulated in a - class template called <code class="classname">allocator</code>. The - <code class="classname">allocator</code> abstraction is used throughout the - library in <code class="classname">string</code>, container classes, - algorithms, and parts of iostreams. This class, and base classes of - it, are the superset of available free store (<span class="quote">“<span class="quote">heap</span>”</span>) - management classes. -</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.req"></a>Requirements</h4></div></div></div><p> - The C++ standard only gives a few directives in this area: - </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> - When you add elements to a container, and the container must - allocate more memory to hold them, the container makes the - request via its <span class="type">Allocator</span> template - parameter, which is usually aliased to - <span class="type">allocator_type</span>. This includes adding chars - to the string class, which acts as a regular STL container in - this respect. - </p></li><li class="listitem"><p> - The default <span class="type">Allocator</span> argument of every - container-of-T is <code class="classname">allocator<T></code>. - </p></li><li class="listitem"><p> - The interface of the <code class="classname">allocator<T></code> class is - extremely simple. It has about 20 public declarations (nested - typedefs, member functions, etc), but the two which concern us most - are: - </p><pre class="programlisting"> - T* allocate (size_type n, const void* hint = 0); - void deallocate (T* p, size_type n); - </pre><p> - The <code class="varname">n</code> arguments in both those - functions is a <span class="emphasis"><em>count</em></span> of the number of - <span class="type">T</span>'s to allocate space for, <span class="emphasis"><em>not their - total size</em></span>. - (This is a simplification; the real signatures use nested typedefs.) - </p></li><li class="listitem"><p> - The storage is obtained by calling <code class="function">::operator - new</code>, but it is unspecified when or how - often this function is called. The use of the - <code class="varname">hint</code> is unspecified, but intended as an - aid to locality if an implementation so - desires. <code class="constant">[20.4.1.1]/6</code> - </p></li></ul></div><p> - Complete details can be found in the C++ standard, look in - <code class="constant">[20.4 Memory]</code>. - </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.design_issues"></a>Design Issues</h4></div></div></div><p> - The easiest way of fulfilling the requirements is to call - <code class="function">operator new</code> each time a container needs - memory, and to call <code class="function">operator delete</code> each time - the container releases memory. This method may be <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00105.html" target="_top">slower</a> - than caching the allocations and re-using previously-allocated - memory, but has the advantage of working correctly across a wide - variety of hardware and operating systems, including large - clusters. The <code class="classname">__gnu_cxx::new_allocator</code> - implements the simple operator new and operator delete semantics, - while <code class="classname">__gnu_cxx::malloc_allocator</code> - implements much the same thing, only with the C language functions - <code class="function">std::malloc</code> and <code class="function">std::free</code>. - </p><p> - Another approach is to use intelligence within the allocator - class to cache allocations. This extra machinery can take a variety - of forms: a bitmap index, an index into an exponentially increasing - power-of-two-sized buckets, or simpler fixed-size pooling cache. - The cache is shared among all the containers in the program: when - your program's <code class="classname">std::vector<int></code> gets - cut in half and frees a bunch of its storage, that memory can be - reused by the private - <code class="classname">std::list<WonkyWidget></code> brought in from - a KDE library that you linked against. And operators - <code class="function">new</code> and <code class="function">delete</code> are not - always called to pass the memory on, either, which is a speed - bonus. Examples of allocators that use these techniques are - <code class="classname">__gnu_cxx::bitmap_allocator</code>, - <code class="classname">__gnu_cxx::pool_allocator</code>, and - <code class="classname">__gnu_cxx::__mt_alloc</code>. - </p><p> - Depending on the implementation techniques used, the underlying - operating system, and compilation environment, scaling caching - allocators can be tricky. In particular, order-of-destruction and - order-of-creation for memory pools may be difficult to pin down - with certainty, which may create problems when used with plugins - or loading and unloading shared objects in memory. As such, using - caching allocators on systems that do not support - <code class="function">abi::__cxa_atexit</code> is not recommended. - </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.impl"></a>Implementation</h4></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp13920976"></a>Interface Design</h5></div></div></div><p> - The only allocator interface that - is supported is the standard C++ interface. As such, all STL - containers have been adjusted, and all external allocators have - been modified to support this change. - </p><p> - The class <code class="classname">allocator</code> just has typedef, - constructor, and rebind members. It inherits from one of the - high-speed extension allocators, covered below. Thus, all - allocation and deallocation depends on the base class. - </p><p> - The base class that <code class="classname">allocator</code> is derived from - may not be user-configurable. -</p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp13924528"></a>Selecting Default Allocation Policy</h5></div></div></div><p> - It's difficult to pick an allocation strategy that will provide - maximum utility, without excessively penalizing some behavior. In - fact, it's difficult just deciding which typical actions to measure - for speed. - </p><p> - Three synthetic benchmarks have been created that provide data - that is used to compare different C++ allocators. These tests are: - </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p> - Insertion. - </p><p> - Over multiple iterations, various STL container - objects have elements inserted to some maximum amount. A variety - of allocators are tested. - Test source for <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert/sequence.cc?view=markup" target="_top">sequence</a> - and <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert/associative.cc?view=markup" target="_top">associative</a> - containers. - </p></li><li class="listitem"><p> - Insertion and erasure in a multi-threaded environment. - </p><p> - This test shows the ability of the allocator to reclaim memory - on a per-thread basis, as well as measuring thread contention - for memory resources. - Test source - <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert_erase/associative.cc?view=markup" target="_top">here</a>. - </p></li><li class="listitem"><p> - A threaded producer/consumer model. - </p><p> - Test source for - <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc++-v3/testsuite/performance/23_containers/producer_consumer/sequence.cc?view=markup" target="_top">sequence</a> - and - <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc++-v3/testsuite/performance/23_containers/producer_consumer/associative.cc?view=markup" target="_top">associative</a> - containers. - </p></li></ol></div><p> - The current default choice for - <code class="classname">allocator</code> is - <code class="classname">__gnu_cxx::new_allocator</code>. - </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp13937824"></a>Disabling Memory Caching</h5></div></div></div><p> - In use, <code class="classname">allocator</code> may allocate and - deallocate using implementation-specific strategies and - heuristics. Because of this, a given call to an allocator object's - <code class="function">allocate</code> member function may not actually - call the global <code class="code">operator new</code> and a given call to - to the <code class="function">deallocate</code> member function may not - call <code class="code">operator delete</code>. - </p><p> - This can be confusing. - </p><p> - In particular, this can make debugging memory errors more - difficult, especially when using third-party tools like valgrind or - debug versions of <code class="function">new</code>. - </p><p> - There are various ways to solve this problem. One would be to use - a custom allocator that just called operators - <code class="function">new</code> and <code class="function">delete</code> - directly, for every allocation. (See the default allocator, - <code class="filename">include/ext/new_allocator.h</code>, for instance.) - However, that option may involve changing source code to use - a non-default allocator. Another option is to force the - default allocator to remove caching and pools, and to directly - allocate with every call of <code class="function">allocate</code> and - directly deallocate with every call of - <code class="function">deallocate</code>, regardless of efficiency. As it - turns out, this last option is also available. - </p><p> - To globally disable memory caching within the library for some of - the optional non-default allocators, merely set - <code class="constant">GLIBCXX_FORCE_NEW</code> (with any value) in the - system's environment before running the program. If your program - crashes with <code class="constant">GLIBCXX_FORCE_NEW</code> in the - environment, it likely means that you linked against objects - built against the older library (objects which might still using the - cached allocations...). - </p></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.using"></a>Using a Specific Allocator</h4></div></div></div><p> - You can specify different memory management schemes on a - per-container basis, by overriding the default - <span class="type">Allocator</span> template parameter. For example, an easy - (but non-portable) method of specifying that only <code class="function">malloc</code> or <code class="function">free</code> - should be used instead of the default node allocator is: - </p><pre class="programlisting"> - std::list <int, __gnu_cxx::malloc_allocator<int> > malloc_list;</pre><p> - Likewise, a debugging form of whichever allocator is currently in use: - </p><pre class="programlisting"> - std::deque <int, __gnu_cxx::debug_allocator<std::allocator<int> > > debug_deque; - </pre></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.custom"></a>Custom Allocators</h4></div></div></div><p> - Writing a portable C++ allocator would dictate that the interface - would look much like the one specified for - <code class="classname">allocator</code>. Additional member functions, but - not subtractions, would be permissible. - </p><p> - Probably the best place to start would be to copy one of the - extension allocators: say a simple one like - <code class="classname">new_allocator</code>. - </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.ext"></a>Extension Allocators</h4></div></div></div><p> - Several other allocators are provided as part of this - implementation. The location of the extension allocators and their - names have changed, but in all cases, functionality is - equivalent. Starting with gcc-3.4, all extension allocators are - standard style. Before this point, SGI style was the norm. Because of - this, the number of template arguments also changed. Here's a simple - chart to track the changes. - </p><p> - More details on each of these extension allocators follows. - </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p> - <code class="classname">new_allocator</code> - </p><p> - Simply wraps <code class="function">::operator new</code> - and <code class="function">::operator delete</code>. - </p></li><li class="listitem"><p> - <code class="classname">malloc_allocator</code> - </p><p> - Simply wraps <code class="function">malloc</code> and - <code class="function">free</code>. There is also a hook for an - out-of-memory handler (for - <code class="function">new</code>/<code class="function">delete</code> this is - taken care of elsewhere). - </p></li><li class="listitem"><p> - <code class="classname">array_allocator</code> - </p><p> - Allows allocations of known and fixed sizes using existing - global or external storage allocated via construction of - <code class="classname">std::tr1::array</code> objects. By using this - allocator, fixed size containers (including - <code class="classname">std::string</code>) can be used without - instances calling <code class="function">::operator new</code> and - <code class="function">::operator delete</code>. This capability - allows the use of STL abstractions without runtime - complications or overhead, even in situations such as program - startup. For usage examples, please consult the testsuite. - </p></li><li class="listitem"><p> - <code class="classname">debug_allocator</code> - </p><p> - A wrapper around an arbitrary allocator A. It passes on - slightly increased size requests to A, and uses the extra - memory to store size information. When a pointer is passed - to <code class="function">deallocate()</code>, the stored size is - checked, and <code class="function">assert()</code> is used to - guarantee they match. - </p></li><li class="listitem"><p> - <code class="classname">throw_allocator</code> - </p><p> - Includes memory tracking and marking abilities as well as hooks for - throwing exceptions at configurable intervals (including random, - all, none). - </p></li><li class="listitem"><p> - <code class="classname">__pool_alloc</code> - </p><p> - A high-performance, single pool allocator. The reusable - memory is shared among identical instantiations of this type. - It calls through <code class="function">::operator new</code> to - obtain new memory when its lists run out. If a client - container requests a block larger than a certain threshold - size, then the pool is bypassed, and the allocate/deallocate - request is passed to <code class="function">::operator new</code> - directly. - </p><p> - Older versions of this class take a boolean template - parameter, called <code class="varname">thr</code>, and an integer template - parameter, called <code class="varname">inst</code>. - </p><p> - The <code class="varname">inst</code> number is used to track additional memory - pools. The point of the number is to allow multiple - instantiations of the classes without changing the semantics at - all. All three of - </p><pre class="programlisting"> - typedef __pool_alloc<true,0> normal; - typedef __pool_alloc<true,1> private; - typedef __pool_alloc<true,42> also_private; - </pre><p> - behave exactly the same way. However, the memory pool for each type - (and remember that different instantiations result in different types) - remains separate. - </p><p> - The library uses <span class="emphasis"><em>0</em></span> in all its instantiations. If you - wish to keep separate free lists for a particular purpose, use a - different number. - </p><p>The <code class="varname">thr</code> boolean determines whether the - pool should be manipulated atomically or not. When - <code class="varname">thr</code> = <code class="constant">true</code>, the allocator - is thread-safe, while <code class="varname">thr</code> = - <code class="constant">false</code>, is slightly faster but unsafe for - multiple threads. - </p><p> - For thread-enabled configurations, the pool is locked with a - single big lock. In some situations, this implementation detail - may result in severe performance degradation. - </p><p> - (Note that the GCC thread abstraction layer allows us to provide - safe zero-overhead stubs for the threading routines, if threads - were disabled at configuration time.) - </p></li><li class="listitem"><p> - <code class="classname">__mt_alloc</code> - </p><p> - A high-performance fixed-size allocator with - exponentially-increasing allocations. It has its own - <a class="link" href="mt_allocator.html" title="Chapter 20. The mt_allocator">chapter</a> - in the documentation. - </p></li><li class="listitem"><p> - <code class="classname">bitmap_allocator</code> - </p><p> - A high-performance allocator that uses a bit-map to keep track - of the used and unused memory locations. It has its own - <a class="link" href="bitmap_allocator.html" title="Chapter 21. The bitmap_allocator">chapter</a> - in the documentation. - </p></li></ol></div></div><div class="bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.biblio"></a>Bibliography</h4></div></div></div><div class="biblioentry"><a id="idp13992848"></a><p><span class="citetitle"><em class="citetitle"> - ISO/IEC 14882:1998 Programming languages - C++ - </em>. </span> - isoc++_1998 - <span class="pagenums">20.4 Memory. </span></p></div><div class="biblioentry"><a id="idp13994688"></a><p><span class="title"><em> - <a class="link" href="http://www.drdobbs.com/cpp/184403759" target="_top"> - The Standard Librarian: What Are Allocators Good For? - </a> - </em>. </span><span class="author"><span class="firstname">Matt</span> <span class="surname">Austern</span>. </span><span class="publisher"><span class="publishername"> - C/C++ Users Journal - . </span></span></p></div><div class="biblioentry"><a id="idp13998416"></a><p><span class="title"><em> - <a class="link" href="http://www.hoard.org/" target="_top"> - The Hoard Memory Allocator - </a> - </em>. </span><span class="author"><span class="firstname">Emery</span> <span class="surname">Berger</span>. </span></p></div><div class="biblioentry"><a id="idp14001184"></a><p><span class="title"><em> - <a class="link" href="http://people.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf" target="_top"> - Reconsidering Custom Memory Allocation - </a> - </em>. </span><span class="author"><span class="firstname">Emery</span> <span class="surname">Berger</span>. </span><span class="author"><span class="firstname">Ben</span> <span class="surname">Zorn</span>. </span><span class="author"><span class="firstname">Kathryn</span> <span class="surname">McKinley</span>. </span><span class="copyright">Copyright © 2002 OOPSLA. </span></p></div><div class="biblioentry"><a id="idp14007344"></a><p><span class="title"><em> - <a class="link" href="http://www.angelikalanger.com/Articles/C++Report/Allocators/Allocators.html" target="_top"> - Allocator Types - </a> - </em>. </span><span class="author"><span class="firstname">Klaus</span> <span class="surname">Kreft</span>. </span><span class="author"><span class="firstname">Angelika</span> <span class="surname">Langer</span>. </span><span class="publisher"><span class="publishername"> - C/C++ Users Journal - . </span></span></p></div><div class="biblioentry"><a id="idp14012080"></a><p><span class="citetitle"><em class="citetitle">The C++ Programming Language</em>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span><span class="copyright">Copyright © 2000 . </span><span class="pagenums">19.4 Allocators. </span><span class="publisher"><span class="publishername"> - Addison Wesley - . </span></span></p></div><div class="biblioentry"><a id="idp14016512"></a><p><span class="citetitle"><em class="citetitle">Yalloc: A Recycling C++ Allocator</em>. </span><span class="author"><span class="firstname">Felix</span> <span class="surname">Yen</span>. </span></p></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.auto_ptr"></a>auto_ptr</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="auto_ptr.limitations"></a>Limitations</h4></div></div></div><p>Explaining all of the fun and delicious things that can - happen with misuse of the <code class="classname">auto_ptr</code> class - template (called <acronym class="acronym">AP</acronym> here) would take some - time. Suffice it to say that the use of <acronym class="acronym">AP</acronym> - safely in the presence of copying has some subtleties. - </p><p> - The AP class is a really - nifty idea for a smart pointer, but it is one of the dumbest of - all the smart pointers -- and that's fine. - </p><p> - AP is not meant to be a supersmart solution to all resource - leaks everywhere. Neither is it meant to be an effective form - of garbage collection (although it can help, a little bit). - And it can <span class="emphasis"><em>not</em></span>be used for arrays! - </p><p> - <acronym class="acronym">AP</acronym> is meant to prevent nasty leaks in the - presence of exceptions. That's <span class="emphasis"><em>all</em></span>. This - code is AP-friendly: - </p><pre class="programlisting"> - // Not a recommend naming scheme, but good for web-based FAQs. - typedef std::auto_ptr<MyClass> APMC; - - extern function_taking_MyClass_pointer (MyClass*); - extern some_throwable_function (); - - void func (int data) - { - APMC ap (new MyClass(data)); - - some_throwable_function(); // this will throw an exception - - function_taking_MyClass_pointer (ap.get()); - } - </pre><p>When an exception gets thrown, the instance of MyClass that's - been created on the heap will be <code class="function">delete</code>'d as the stack is - unwound past <code class="function">func()</code>. - </p><p>Changing that code as follows is not <acronym class="acronym">AP</acronym>-friendly: - </p><pre class="programlisting"> - APMC ap (new MyClass[22]); - </pre><p>You will get the same problems as you would without the use - of <acronym class="acronym">AP</acronym>: - </p><pre class="programlisting"> - char* array = new char[10]; // array new... - ... - delete array; // ...but single-object delete - </pre><p> - AP cannot tell whether the pointer you've passed at creation points - to one or many things. If it points to many things, you are about - to die. AP is trivial to write, however, so you could write your - own <code class="code">auto_array_ptr</code> for that situation (in fact, this has - been done many times; check the mailing lists, Usenet, Boost, etc). - </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="auto_ptr.using"></a>Use in Containers</h4></div></div></div><p> - </p><p>All of the <a class="link" href="containers.html" title="Chapter 9. Containers">containers</a> - described in the standard library require their contained types - to have, among other things, a copy constructor like this: - </p><pre class="programlisting"> - struct My_Type - { - My_Type (My_Type const&); - }; - </pre><p> - Note the const keyword; the object being copied shouldn't change. - The template class <code class="code">auto_ptr</code> (called AP here) does not - meet this requirement. Creating a new AP by copying an existing - one transfers ownership of the pointed-to object, which means that - the AP being copied must change, which in turn means that the - copy ctors of AP do not take const objects. - </p><p> - The resulting rule is simple: <span class="emphasis"><em>Never ever use a - container of auto_ptr objects</em></span>. The standard says that - <span class="quote">“<span class="quote">undefined</span>”</span> behavior is the result, but it is - guaranteed to be messy. - </p><p> - To prevent you from doing this to yourself, the - <a class="link" href="ext_compile_checks.html" title="Chapter 16. Compile Time Checks">concept checks</a> built - in to this implementation will issue an error if you try to - compile code like this: - </p><pre class="programlisting"> - #include <vector> - #include <memory> - - void f() - { - std::vector< std::auto_ptr<int> > vec_ap_int; - } - </pre><p> -Should you try this with the checks enabled, you will see an error. - </p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.shared_ptr"></a>shared_ptr</h3></div></div></div><p> -The shared_ptr class template stores a pointer, usually obtained via new, -and implements shared ownership semantics. -</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.req"></a>Requirements</h4></div></div></div><p> - </p><p> - The standard deliberately doesn't require a reference-counted - implementation, allowing other techniques such as a - circular-linked-list. - </p><p> - </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.design_issues"></a>Design Issues</h4></div></div></div><p> -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 -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. - </p><p> -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 -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. - </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.impl"></a>Implementation</h4></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp15512272"></a>Class Hierarchy</h5></div></div></div><p> -A <code class="classname">shared_ptr<T></code> contains a pointer of -type <span class="type">T*</span> and an object of type -<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 -object that maintains the reference-counts and destroys the managed -resource. - </p><div class="variablelist"><dl class="variablelist"><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 (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. - </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 <code class="code">Ptr</code> -and a deleter of type <code class="code">Deleter</code>. <code class="classname">_Sp_deleter</code> 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 -<code class="function">delete</code> is used with an incomplete type. -This is the only derived type used by <code class="classname">tr1::shared_ptr<Ptr></code> -and it is never used by <code class="classname">std::shared_ptr</code>, which uses one of -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> -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. -This is the simplest form and is used when there is no custom deleter or -allocator. - </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 -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 -<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> -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>, -which is constructed in-place with placement <code class="function">new</code>. -Has a variadic template constructor allowing any number of arguments to -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 -type of object, not the type of pointer; this is purely a convenience -that simplifies the implementation slightly. - </p></dd></dl></div><p> -C++11-only features are: rvalue-ref/move support, allocator support, -aliasing constructor, make_shared & allocate_shared. Additionally, -the constructors taking <code class="classname">auto_ptr</code> parameters are -deprecated in C++11 mode. - </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp15534784"></a>Thread Safety</h5></div></div></div><p> -The -<a class="link" 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 -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. -</p><pre class="programlisting"> -shared_ptr<A> a(new A); -shared_ptr<A> b(a); - -// Thread 1 // Thread 2 - a.reset(); b.reset(); -</pre><p> -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. -</p><p> -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 -resource still exists and either increment the reference count or throw -<code class="classname">bad_weak_ptr</code>. -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. -</p><p> -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 <span class="emphasis"><em>Compare-And-Swap</em></span> -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. -</p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp15542080"></a>Selecting Lock Policy</h5></div></div></div><p> - </p><p> -There is a single <code class="classname">_Sp_counted_base</code> class, -which is a template parameterized on the enum -<span class="type">__gnu_cxx::_Lock_policy</span>. The entire family of classes is -parameterized on the lock policy, right up to -<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">std::shared_ptr</code> class inherits from -<code class="classname">__shared_ptr</code> 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 <code class="classname">shared_ptr</code> to have an -extra template parameter, even if it had a default value. The -available policies are: - </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p> - <code class="constant">_S_Atomic</code> - </p><p> -Selected when GCC supports a builtin atomic compare-and-swap operation -on the target processor (see <a class="link" href="http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html" target="_top">Atomic -Builtins</a>.) The reference counts are maintained using a lock-free -algorithm and GCC's atomic builtins, which provide the required memory -synchronisation. - </p></li><li class="listitem"><p> - <code class="constant">_S_Mutex</code> - </p><p> -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. - </p></li><li class="listitem"><p> - <code class="constant">_S_Single</code> - </p><p> -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>. - </p></li></ol></div><p> - For all three policies, reference count increments and - decrements are done via the functions in - <code class="filename">ext/atomicity.h</code>, 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. - </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp15556704"></a>Related functions and classes</h5></div></div></div><div class="variablelist"><dl class="variablelist"><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> -As noted in N2351, these functions can be implemented non-intrusively using -the alias constructor. However the aliasing constructor is only available -in C++11 mode, so in TR1 mode these casts rely on three non-standard -constructors in shared_ptr and __shared_ptr. -In C++11 mode these constructors and the related tag types are not needed. - </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 -<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 -work smoothly with <code class="code">__shared_ptr<Tp, Lp></code> using any lock -policy. - </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> -with <code class="code">std::allocator</code> 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 <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. -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>, -but it has no way to access it. -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> -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. - </p></dd></dl></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.using"></a>Use</h4></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp15573056"></a>Examples</h5></div></div></div><p> - 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/20_util/shared_ptr</code> - and - <code class="filename">testsuite/20_util/weak_ptr</code>. - </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idp15576640"></a>Unresolved Issues</h5></div></div></div><p> - The <span class="emphasis"><em><code class="classname">shared_ptr</code> atomic access</em></span> - clause in the C++11 standard is not implemented in GCC. - </p><p> - 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 class="function">__gthread_active_p()</code>. This could be - addressed by providing template specialisations for some members - of <code class="classname">_Sp_counted_base<_S_single></code>. - </p><p> - Unlike Boost, this implementation does not use separate classes - for the pointer+deleter and pointer+deleter+allocator cases in - C++11 mode, combining both into _Sp_counted_deleter and using - <code class="classname">allocator</code> 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. - </p><p> - The hack used to get the address of the managed object from - <code class="function">_Sp_counted_ptr_inplace::_M_get_deleter()</code> - is accessible to users. This could be prevented if - <code class="function">get_deleter<_Sp_make_shared_tag>()</code> - 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. - </p><p> - tr1::_Sp_deleter could be a private member of tr1::__shared_count but it - would alter the ABI. - </p></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.ack"></a>Acknowledgments</h4></div></div></div><p> - 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. - </p></div><div class="bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.biblio"></a>Bibliography</h4></div></div></div><div class="biblioentry"><a id="idp15587936"></a><p><span class="title"><em> - <a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2351.htm" target="_top"> - Improving shared_ptr for C++0x, Revision 2 - </a> - </em>. </span><span class="subtitle"> - N2351 - . </span></p></div><div class="biblioentry"><a id="idp15590224"></a><p><span class="title"><em> - <a class="link" href="http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2456.html" target="_top"> - C++ Standard Library Active Issues List - </a> - </em>. </span><span class="subtitle"> - N2456 - . </span></p></div><div class="biblioentry"><a id="idp15592512"></a><p><span class="title"><em> - <a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2461.pdf" target="_top"> - Working Draft, Standard for Programming Language C++ - </a> - </em>. </span><span class="subtitle"> - N2461 - . </span></p></div><div class="biblioentry"><a id="idp15594816"></a><p><span class="title"><em> - <a class="link" href="http://boost.org/libs/smart_ptr/shared_ptr.htm" target="_top"> - Boost C++ Libraries documentation, shared_ptr - </a> - </em>. </span><span class="subtitle"> - N2461 - . </span></p></div></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pairs.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="utilities.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">Pairs </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Traits</td></tr></table></div></body></html>
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