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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="container.Cpp11_conformance"></a><a class="link" href="Cpp11_conformance.html" title="C++11/C++14 Conformance">C++11/C++14 Conformance</a> |
| </h2></div></div></div> |
| <div class="toc"><dl class="toc"> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.move_emplace">Move and Emplace</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.alloc_traits_move_traits">Stateful |
| allocators</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.scoped_allocator">Scoped |
| allocators</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.insertion_hints">Insertion |
| hints in associative containers and preserving insertion ordering for elements |
| with equivalent keys</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.initializer_lists">Initializer |
| lists</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.null_iterators">Null Forward |
| Iterators</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.forward_list"><code class="computeroutput"><span class="identifier">forward_list</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code></a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.vector_exception_guarantees"><code class="computeroutput"><span class="identifier">vector</span></code> vs. <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code> |
| exception guarantees</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.container_const_reference_parameters">Parameter |
| taken by const reference that can be changed</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.Vector_bool"><code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> specialization</a></span></dt> |
| <dt><span class="section"><a href="Cpp11_conformance.html#container.Cpp11_conformance.non_standard_memset_initialization">Non-standard |
| value initialization using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span></code></a></span></dt> |
| </dl></div> |
| <p> |
| <span class="bold"><strong>Boost.Container</strong></span> aims for full C++11 conformance |
| except reasoned deviations, backporting as much as possible for C++03. Obviously, |
| this conformance is a work in progress so this section explains what C++11 |
| features are implemented and which of them have been backported to C++03 compilers. |
| </p> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.move_emplace"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.move_emplace" title="Move and Emplace">Move and Emplace</a> |
| </h3></div></div></div> |
| <p> |
| For compilers with rvalue references and for those C++03 types that use |
| <a href="http://www.boost.org/libs/move" target="_top">Boost.Move</a> rvalue reference |
| emulation <span class="bold"><strong>Boost.Container</strong></span> supports all C++11 |
| features related to move semantics: containers are movable, requirements |
| for <code class="computeroutput"><span class="identifier">value_type</span></code> are those |
| specified for C++11 containers. |
| </p> |
| <p> |
| For compilers with variadic templates, <span class="bold"><strong>Boost.Container</strong></span> |
| supports placement insertion (<code class="computeroutput"><span class="identifier">emplace</span></code>, |
| ...) functions from C++11. For those compilers without variadic templates |
| support <span class="bold"><strong>Boost.Container</strong></span> uses the preprocessor |
| to create a set of overloads up to a finite number of parameters. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.alloc_traits_move_traits"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.alloc_traits_move_traits" title="Stateful allocators">Stateful |
| allocators</a> |
| </h3></div></div></div> |
| <p> |
| C++03 was not stateful-allocator friendly. For compactness of container objects |
| and for simplicity, it did not require containers to support allocators with |
| state: Allocator objects need not be stored in container objects. It was |
| not possible to store an allocator with state, say an allocator that holds |
| a pointer to an arena from which to allocate. C++03 allowed implementors |
| to suppose two allocators of the same type always compare equal (that means |
| that memory allocated by one allocator object could be deallocated by another |
| instance of the same type) and allocators were not swapped when the container |
| was swapped. |
| </p> |
| <p> |
| C++11 further improves stateful allocator support through <a href="http://en.cppreference.com/w/cpp/memory/allocator_traits" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator_traits</span></code></a>. |
| <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator_traits</span></code> is the protocol between |
| a container and an allocator, and an allocator writer can customize its behaviour |
| (should the container propagate it in move constructor, swap, etc.?) following |
| <code class="computeroutput"><span class="identifier">allocator_traits</span></code> requirements. |
| <span class="bold"><strong>Boost.Container</strong></span> not only supports this model |
| with C++11 but also <span class="bold"><strong>backports it to C++03</strong></span> |
| via <code class="computeroutput"><a class="link" href="../boost/container/allocator_traits.html" title="Struct template allocator_traits">boost::container::allocator_traits</a></code> |
| including some C++17 changes. This class offers some workarounds for C++03 |
| compilers to achieve the same allocator guarantees as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator_traits</span></code>. |
| </p> |
| <p> |
| In [Boost.Container] containers, if possible, a single allocator is hold |
| to construct <code class="computeroutput"><span class="identifier">value_type</span></code>s. |
| If the container needs an auxiliary allocator (e.g. an array allocator used |
| by <code class="computeroutput"><span class="identifier">deque</span></code> or <code class="computeroutput"><span class="identifier">stable_vector</span></code>), that allocator is also |
| stored in the container and initialized from the user-supplied allocator |
| when the container is constructed (i.e. it's not constructed on the fly when |
| auxiliary memory is needed). |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.scoped_allocator"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.scoped_allocator" title="Scoped allocators">Scoped |
| allocators</a> |
| </h3></div></div></div> |
| <p> |
| C++11 improves stateful allocators with the introduction of <a href="http://en.cppreference.com/w/cpp/memory/scoped_allocator_adaptor" target="_top"><code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">scoped_allocator_adaptor</span></code></a> |
| class template. <code class="computeroutput"><span class="identifier">scoped_allocator_adaptor</span></code> |
| is instantiated with one outer allocator and zero or more inner allocators. |
| </p> |
| <p> |
| A scoped allocator is a mechanism to automatically propagate the state of |
| the allocator to the subobjects of a container in a controlled way. If instantiated |
| with only one allocator type, the inner allocator becomes the <code class="computeroutput"><span class="identifier">scoped_allocator_adaptor</span></code> itself, thus using |
| the same allocator resource for the container and every element within the |
| container and, if the elements themselves are containers, each of their elements |
| recursively. If instantiated with more than one allocator, the first allocator |
| is the outer allocator for use by the container, the second allocator is |
| passed to the constructors of the container's elements, and, if the elements |
| themselves are containers, the third allocator is passed to the elements' |
| elements, and so on. |
| </p> |
| <p> |
| <span class="bold"><strong>Boost.Container</strong></span> implements its own <code class="computeroutput"><a class="link" href="../boost/container/scoped_allocator_adaptor.html" title="Class template scoped_allocator_adaptor">scoped_allocator_adaptor</a></code> |
| class and <span class="bold"><strong>backports this feature also to C++03 compilers</strong></span>. |
| Due to C++03 limitations, in those compilers the allocator propagation implemented |
| by <code class="computeroutput"><span class="identifier">scoped_allocator_adaptor</span><span class="special">::</span><span class="identifier">construct</span></code> |
| functions will be based on traits (<code class="computeroutput"><a class="link" href="../boost/container/constructible__idp29792032.html" title="Struct template constructible_with_allocator_suffix">constructible_with_allocator_suffix</a></code> |
| and <code class="computeroutput"><a class="link" href="../boost/container/constructible__idp29781664.html" title="Struct template constructible_with_allocator_prefix">constructible_with_allocator_prefix</a></code>) |
| proposed in <a href="http://www.open-std.org/jtc1/sc22/WG21/docs/papers/2008/n2554.pdf" target="_top">N2554: |
| The Scoped Allocator Model (Rev 2) proposal</a>. In conforming C++11 |
| compilers or compilers supporting SFINAE expressions (when <code class="computeroutput"><span class="identifier">BOOST_NO_SFINAE_EXPR</span></code> is NOT defined), traits |
| are ignored and C++11 rules (<code class="computeroutput"><span class="identifier">is_constructible</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> |
| <span class="identifier">Args</span><span class="special">...,</span> |
| <span class="identifier">inner_allocator_type</span><span class="special">>::</span><span class="identifier">value</span></code> and <code class="computeroutput"><span class="identifier">is_constructible</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> |
| <span class="identifier">allocator_arg_t</span><span class="special">,</span> |
| <span class="identifier">inner_allocator_type</span><span class="special">,</span> |
| <span class="identifier">Args</span><span class="special">...>::</span><span class="identifier">value</span></code>) will be used to detect if the allocator |
| must be propagated with suffix or prefix allocator arguments. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.insertion_hints"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.insertion_hints" title="Insertion hints in associative containers and preserving insertion ordering for elements with equivalent keys">Insertion |
| hints in associative containers and preserving insertion ordering for elements |
| with equivalent keys</a> |
| </h3></div></div></div> |
| <p> |
| <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#233" target="_top">LWG |
| Issue #233</a> corrected a defect in C++98 and specified how equivalent |
| keys were to be inserted in associative containers. <span class="bold"><strong>Boost.Container</strong></span> |
| implements the C++11 changes that were specified in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1780.html" target="_top">N1780 |
| <span class="emphasis"><em>Comments on LWG issue 233: Insertion hints in associative containers</em></span></a>: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">a_eq</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">t</span><span class="special">)</span></code>: |
| If a range containing elements equivalent to t exists in a_eq, t is inserted |
| at the end of that range. |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">a_eq</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">p</span><span class="special">,</span><span class="identifier">t</span><span class="special">)</span></code>: |
| t is inserted as close as possible to the position just prior to p. |
| </li> |
| </ul></div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.initializer_lists"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.initializer_lists" title="Initializer lists">Initializer |
| lists</a> |
| </h3></div></div></div> |
| <p> |
| <span class="bold"><strong>Boost.Container</strong></span> supports initialization, |
| assignments and insertions from initializer lists in compilers that implement |
| this feature. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.null_iterators"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.null_iterators" title="Null Forward Iterators">Null Forward |
| Iterators</a> |
| </h3></div></div></div> |
| <p> |
| <span class="bold"><strong>Boost.Container</strong></span> implements <a href="http://www.open-std.org/JTC1/sc22/WG21/docs/papers/2013/n3644.pdf" target="_top">C++14 |
| Null Forward Iterators</a>, which means that value-initialized iterators |
| may be compared and compare equal to other value-initialized iterators of |
| the same type. Value initialized iterators behave as if they refer past the |
| end of the same empty sequence (example taken from N3644): |
| </p> |
| <pre class="programlisting"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="special">{</span> <span class="special">...</span> <span class="special">};</span> |
| <span class="keyword">auto</span> <span class="identifier">ni</span> <span class="special">=</span> <span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">iterator</span><span class="special">();</span> |
| <span class="keyword">auto</span> <span class="identifier">nd</span> <span class="special">=</span> <span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">iterator</span><span class="special">();</span> |
| <span class="identifier">ni</span> <span class="special">==</span> <span class="identifier">ni</span><span class="special">;</span> <span class="comment">// True.</span> |
| <span class="identifier">nd</span> <span class="special">!=</span> <span class="identifier">nd</span><span class="special">;</span> <span class="comment">// False.</span> |
| <span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">ni</span><span class="special">;</span> <span class="comment">// ??? (likely false in practice).</span> |
| <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">ni</span><span class="special">;</span> <span class="comment">// ??? (likely false in practice).</span> |
| <span class="identifier">ni</span> <span class="special">==</span> <span class="identifier">nd</span><span class="special">;</span> <span class="comment">// Won't compile.</span> |
| </pre> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.forward_list"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.forward_list" title="forward_list<T>"><code class="computeroutput"><span class="identifier">forward_list</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code></a> |
| </h3></div></div></div> |
| <p> |
| <span class="bold"><strong>Boost.Container</strong></span> does not offer C++11 <code class="computeroutput"><span class="identifier">forward_list</span></code> container yet, but it will |
| be available in future versions. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.vector_exception_guarantees"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.vector_exception_guarantees" title="vector vs. std::vector exception guarantees"><code class="computeroutput"><span class="identifier">vector</span></code> vs. <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code> |
| exception guarantees</a> |
| </h3></div></div></div> |
| <p> |
| <code class="computeroutput"><a class="link" href="../boost/container/vector.html" title="Class template vector">vector</a></code> does not support |
| the strong exception guarantees given by <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code> |
| in functions like <code class="computeroutput"><span class="identifier">insert</span></code>, |
| <code class="computeroutput"><span class="identifier">push_back</span></code>, <code class="computeroutput"><span class="identifier">emplace</span></code>, <code class="computeroutput"><span class="identifier">emplace_back</span></code>, |
| <code class="computeroutput"><span class="identifier">resize</span></code>, <code class="computeroutput"><span class="identifier">reserve</span></code> |
| or <code class="computeroutput"><span class="identifier">shrink_to_fit</span></code> for either |
| copyable or no-throw moveable classes. In C++11 <a href="http://en.cppreference.com/w/cpp/utility/move_if_noexcept" target="_top">move_if_noexcept</a> |
| is used to maintain C++03 exception safety guarantees combined with C++11 |
| move semantics. This strong exception guarantee degrades the insertion performance |
| of copyable and throwing-moveable types, degrading moves to copies when such |
| types are inserted in the vector using the aforementioned members. |
| </p> |
| <p> |
| This strong exception guarantee also precludes the possibility of using some |
| type of in-place reallocations that can further improve the insertion performance |
| of <code class="computeroutput"><span class="identifier">vector</span></code> See <a class="link" href="extended_functionality.html#container.extended_functionality.extended_allocators" title="Extended allocators">Extended |
| Allocators</a> to know more about these optimizations. |
| </p> |
| <p> |
| <code class="computeroutput"><a class="link" href="../boost/container/vector.html" title="Class template vector">vector</a></code> always uses |
| move constructors/assignments to rearrange elements in the vector and uses |
| memory expansion mechanisms if the allocator supports them, while offering |
| only basic safety guarantees. It trades off exception guarantees for an improved |
| performance. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.container_const_reference_parameters"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.container_const_reference_parameters" title="Parameter taken by const reference that can be changed">Parameter |
| taken by const reference that can be changed</a> |
| </h3></div></div></div> |
| <p> |
| Several container operations use a parameter taken by const reference that |
| can be changed during execution of the function. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-closed.html#526" target="_top">LWG |
| Issue 526 (<span class="emphasis"><em>Is it undefined if a function in the standard changes |
| in parameters?</em></span>)</a> discusses them: |
| </p> |
| <pre class="programlisting"><span class="comment">//Given std::vector<int> v</span> |
| <span class="identifier">v</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">[</span><span class="number">2</span><span class="special">]);</span> |
| <span class="comment">//v[2] can be changed by moving elements of vector</span> |
| |
| <span class="comment">//Given std::list<int> l:</span> |
| <span class="identifier">l</span><span class="special">.</span><span class="identifier">remove</span><span class="special">(*</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">begin</span><span class="special">())</span> |
| <span class="comment">//The operation could delete the first element, and then continue trying to access it.</span> |
| </pre> |
| <p> |
| The adopted resolution, NAD (Not A Defect), implies that previous operations |
| must be well-defined. This requires code to detect a reference to an inserted |
| element and an additional copy in that case, impacting performance even when |
| references to already inserted objects are not used. Note that equivalent |
| functions taking rvalue references or iterator ranges require elements not |
| already inserted in the container. |
| </p> |
| <p> |
| <span class="bold"><strong>Boost.Container</strong></span> prioritizes performance |
| and has not implemented the NAD resolution: in functions that might modify |
| the argument, the library requires references to elements not stored in the |
| container. Using references to inserted elements yields to undefined behaviour |
| (although in debug mode, this precondition violation could be notified via |
| BOOST_ASSERT). |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.Vector_bool"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.Vector_bool" title="vector<bool> specialization"><code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> specialization</a> |
| </h3></div></div></div> |
| <p> |
| <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> specialization |
| has been quite problematic, and there have been several unsuccessful tries |
| to deprecate or remove it from the standard. <span class="bold"><strong>Boost.Container</strong></span> |
| does not implement it as there is a superior <a href="http://www.boost.org/libs/dynamic_bitset/" target="_top">Boost.DynamicBitset</a> |
| solution. For issues with <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> |
| see the following papers: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| <a href="http://howardhinnant.github.io/onvectorbool.html" target="_top">On <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code></a> |
| </li> |
| <li class="listitem"> |
| <a href="http://www.gotw.ca/publications/N1211.pdf" target="_top">vector<bool>: |
| N1211: More Problems, Better Solutions</a>, |
| </li> |
| <li class="listitem"> |
| <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2160.html" target="_top">N2160: |
| Library Issue 96: Fixing vector<bool></a>, |
| </li> |
| <li class="listitem"> |
| <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2204.html" target="_top">N2204 |
| A Specification to deprecate vector<bool></a>. |
| </li> |
| </ul></div> |
| <p> |
| Quotes: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| <span class="quote">“<span class="quote"><span class="emphasis"><em>But it is a shame that the C++ committee gave this excellent |
| data structure the name vector<bool> and that it gives no guidance |
| nor encouragement on the critical generic algorithms that need to be |
| optimized for this data structure. Consequently, few std::lib implementations |
| go to this trouble.</em></span></span>”</span> |
| </li> |
| <li class="listitem"> |
| <span class="quote">“<span class="quote"><span class="emphasis"><em>In 1998, admitting that the committee made a mistake |
| was controversial. Since then Java has had to deprecate such significant |
| portions of their libraries that the idea C++ would be ridiculed for |
| deprecating a single minor template specialization seems quaint.</em></span></span>”</span> |
| </li> |
| <li class="listitem"> |
| <span class="quote">“<span class="quote"><span class="emphasis"><em><code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> is not a container and <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">>::</span><span class="identifier">iterator</span></code> is not a random-access iterator |
| (or even a forward or bidirectional iterator either, for that matter). |
| This has already broken user code in the field in mysterious ways.</em></span></span>”</span> |
| </li> |
| <li class="listitem"> |
| <span class="quote">“<span class="quote"><span class="emphasis"><em><code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> forces a specific (and potentially |
| bad) optimization choice on all users by enshrining it in the standard. |
| The optimization is premature; different users have different requirements. |
| This too has already hurt users who have been forced to implement workarounds |
| to disable the 'optimization' (e.g., by using a vector<char> and |
| manually casting to/from bool).</em></span></span>”</span> |
| </li> |
| </ul></div> |
| <p> |
| So <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">>::</span><span class="identifier">iterator</span></code> returns real <code class="computeroutput"><span class="keyword">bool</span></code> |
| references and works as a fully compliant container. If you need a memory |
| optimized version of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>, |
| please use <a href="http://www.boost.org/libs/dynamic_bitset/" target="_top">Boost.DynamicBitset</a>. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="container.Cpp11_conformance.non_standard_memset_initialization"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.non_standard_memset_initialization" title="Non-standard value initialization using std::memset">Non-standard |
| value initialization using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span></code></a> |
| </h3></div></div></div> |
| <p> |
| <span class="bold"><strong>Boost.Container</strong></span> uses <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span></code> |
| with a zero value to initialize some types as in most platforms this initialization |
| yields to the desired value initialization with improved performance. |
| </p> |
| <p> |
| Following the C11 standard, <span class="bold"><strong>Boost.Container</strong></span> |
| assumes that <span class="emphasis"><em>for any integer type, the object representation where |
| all the bits are zero shall be a representation of the value zero in that |
| type</em></span>. Since <code class="computeroutput"><span class="identifier">_Bool</span></code>/<code class="computeroutput"><span class="keyword">wchar_t</span></code>/<code class="computeroutput"><span class="keyword">char16_t</span></code>/<code class="computeroutput"><span class="keyword">char32_t</span></code> are also integer types in C, it considers |
| all C++ integral types as initializable via <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span></code>. |
| </p> |
| <p> |
| By default, <span class="bold"><strong>Boost.Container</strong></span> also considers |
| floating point types to be initializable using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span></code>. |
| Most platforms are compatible with this initialization, but in case this |
| initialization is not desirable the user can <code class="computeroutput"><span class="preprocessor">#define</span> |
| <span class="identifier">BOOST_CONTAINER_MEMZEROED_FLOATING_POINT_IS_NOT_ZERO</span></code> |
| before including library headers. |
| </p> |
| <p> |
| By default, it also considers pointer types (pointer and pointer to function |
| types, excluding member object and member function pointers) to be initializable |
| using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span></code>. Most platforms are compatible with |
| this initialization, but in case this initialization is not desired the user |
| can <code class="computeroutput"><span class="preprocessor">#define</span> <span class="identifier">BOOST_CONTAINER_MEMZEROED_POINTER_IS_NOT_ZERO</span></code> |
| before including library headers. |
| </p> |
| <p> |
| If neither <code class="computeroutput"><span class="identifier">BOOST_CONTAINER_MEMZEROED_FLOATING_POINT_IS_NOT_ZERO</span></code> |
| nor <code class="computeroutput"><span class="identifier">BOOST_CONTAINER_MEMZEROED_POINTER_IS_NOT_ZERO</span></code> |
| is defined <span class="bold"><strong>Boost.Container</strong></span> also considers |
| POD types to be value initializable via <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span></code> |
| with value zero. |
| </p> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2009-2013 Ion Gaztanaga<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
| </div></td> |
| </tr></table> |
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