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<title>C++11 Conformance</title>
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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 Conformance">C++11 Conformance</a>
</h2></div></div></div>
<div class="toc"><dl>
<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.initializer_lists">Initializer
lists</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_bool"><code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></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 (10) 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>.
</p>
<p>
If possible, a single allocator is hold to construct <code class="computeroutput"><span class="identifier">value_type</span></code>.
If the container needs an auxiliary allocator (e.g. a 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
constructed 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"><span class="identifier">scoped_allocator_adaptor</span></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_wit_id411670.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_wit_id411450.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.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> does not support initializer
lists when constructing or assigning containers but it will support it for
compilers with initialized-list support. This feature won't be backported
to C++03 compilers.
</p>
</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_bool"></a><a class="link" href="Cpp11_conformance.html#container.Cpp11_conformance.Vector_bool" title="vector<bool>"><code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code></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 papers <a href="http://www.gotw.ca/publications/N1211.pdf" target="_top">vector<bool>:
N1211: More Problems, Better Solutions</a>, <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>, <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>.
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<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>
functionalities, please use <a href="http://www.boost.org/libs/dynamic_bitset/" target="_top">Boost.DynamicBitset</a>.
</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 © 2009-2012 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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