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<a name="container.cpp_conformance"></a><a class="link" href="cpp_conformance.html" title="C++11/C++14/C++17 Conformance">C++11/C++14/C++17 Conformance</a>
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<div class="toc"><dl class="toc">
<dt><span class="section"><a href="cpp_conformance.html#container.cpp_conformance.move_emplace">Move and Emplace</a></span></dt>
<dt><span class="section"><a href="cpp_conformance.html#container.cpp_conformance.alloc_traits_move_traits">Stateful
allocators</a></span></dt>
<dt><span class="section"><a href="cpp_conformance.html#container.cpp_conformance.scoped_allocator">Scoped allocators</a></span></dt>
<dt><span class="section"><a href="cpp_conformance.html#container.cpp_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="cpp_conformance.html#container.cpp_conformance.initializer_lists">Initializer
lists</a></span></dt>
<dt><span class="section"><a href="cpp_conformance.html#container.cpp_conformance.null_iterators">Null Forward
Iterators</a></span></dt>
<dt><span class="section"><a href="cpp_conformance.html#container.cpp_conformance.polymorphic_memory_resources">Polymorphic
Memory Resources </a></span></dt>
<dt><span class="section"><a href="cpp_conformance.html#container.cpp_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="cpp_conformance.html#container.cpp_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="cpp_conformance.html#container.cpp_conformance.container_const_reference_parameters">Parameter
taken by const reference that can be changed</a></span></dt>
<dt><span class="section"><a href="cpp_conformance.html#container.cpp_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="cpp_conformance.html#container.cpp_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/C++14/C++17
features are implemented and which of them have been backported to earlier
standard conformig compilers.
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="container.cpp_conformance.move_emplace"></a><a class="link" href="cpp_conformance.html#container.cpp_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.cpp_conformance.alloc_traits_move_traits"></a><a class="link" href="cpp_conformance.html#container.cpp_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.cpp_conformance.scoped_allocator"></a><a class="link" href="cpp_conformance.html#container.cpp_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/construc_idm46123637584144.html" title="Struct template constructible_with_allocator_suffix">constructible_with_allocator_suffix</a></code>
and <code class="computeroutput"><a class="link" href="../boost/container/construc_idm46123637594512.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.cpp_conformance.insertion_hints"></a><a class="link" href="cpp_conformance.html#container.cpp_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.cpp_conformance.initializer_lists"></a><a class="link" href="cpp_conformance.html#container.cpp_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.cpp_conformance.null_iterators"></a><a class="link" href="cpp_conformance.html#container.cpp_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.cpp_conformance.polymorphic_memory_resources"></a><a class="link" href="cpp_conformance.html#container.cpp_conformance.polymorphic_memory_resources" title="Polymorphic Memory Resources">Polymorphic
Memory Resources </a>
</h3></div></div></div>
<p>
The document <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4480.html" target="_top">C++
Extensions for Library Fundamentals (final draft)</a> includes classes
that provide allocator type erasure and runtime polymorphism. As Pablo Halpern,
the author of the proposal, explains in the paper (<a href="https://isocpp.org/files/papers/N3916.pdf" target="_top">N3916
Polymorphic Memory Resources (r2)</a>):
</p>
<p>
<span class="quote">“<span class="quote"><span class="emphasis"><em>A significant impediment to effective memory management
in C++ has been the inability to use allocators in non-generic contexts.
In large software systems, most of the application program consists of non-generic
procedural or object-oriented code that is compiled once and linked many
times.</em></span></span>”</span>
</p>
<p>
<span class="quote">“<span class="quote"><span class="emphasis"><em>Allocators in C++, however, have historically relied solely
on compile-time polymorphism, and therefore have not been suitable for use
in vocabulary types, which are passed through interfaces between separately-compiled
modules, because the allocator type necessarily affects the type of the object
that uses it. This proposal builds upon the improvements made to allocators
in C++11 and describes a set of facilities for runtime polymorphic memory
resources that interoperate with the existing compile-time polymorphic allocators.</em></span></span>”</span>
</p>
<p>
Most utilities from the Fundamentals TS were merged into C++17, but <span class="bold"><strong>Boost.Container</strong></span> offers them for C++03, C++11 and C++14
compilers.
</p>
<p>
<span class="bold"><strong>Boost.Container</strong></span> implements nearly all classes
of the proposal under the namespace <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">::</span><span class="identifier">pmr</span></code>.
There are two groups,
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Header only utilities (these don't require the separately compiled library):
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem">
<code class="computeroutput"><a class="link" href="../boost/container/pmr/memory_resource.html" title="Class memory_resource">memory_resource</a></code>.
</li>
<li class="listitem">
<code class="computeroutput"><a class="link" href="../boost/container/pmr/resource_adaptor.html" title="Type definition resource_adaptor">resource_adaptor</a></code>.
</li>
</ul></div>
</li>
<li class="listitem">
Utilities that require the the separately compiled library:
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem">
<code class="computeroutput"><a class="link" href="../boost/container/pmr/polymorphic_allocator.html" title="Class template polymorphic_allocator">polymorphic_allocator</a></code>.
</li>
<li class="listitem">
<code class="computeroutput"><a class="link" href="../boost/container/pmr/monotonic_buffer_resource.html" title="Class monotonic_buffer_resource">monotonic_buffer_resource</a></code>.
</li>
<li class="listitem">
<code class="computeroutput"><a class="link" href="../boost/container/pmr/unsynchr_idm46123641784000.html" title="Class unsynchronized_pool_resource">unsynchronized_pool_resource</a></code>.
</li>
<li class="listitem">
<code class="computeroutput"><a class="link" href="../boost/container/pmr/synchronized_pool_resource.html" title="Class synchronized_pool_resource">synchronized_pool_resource</a></code>.
</li>
<li class="listitem">
Global resource functions: <code class="computeroutput"><a class="link" href="../boost/container/pmr/get_default_resource.html" title="Function get_default_resource">get_default_resource</a></code>/
<code class="computeroutput"><a class="link" href="../boost/container/pmr/set_default_resource.html" title="Function set_default_resource">set_default_resource</a></code>/
<code class="computeroutput"><a class="link" href="../boost/container/pmr/new_delete_resource.html" title="Function new_delete_resource">new_delete_resource</a></code>/
<code class="computeroutput"><a class="link" href="../boost/container/pmr/null_memory_resource.html" title="Function null_memory_resource">null_memory_resource</a></code>
</li>
<li class="listitem">
Aliases for boost containers using the polymorphic allocator (like
<code class="computeroutput"><a class="link" href="../boost_container_header_reference.html#boost.container.pmr.vector">pmr::vector</a></code>,
etc.)
</li>
</ul></div>
</li>
</ul></div>
<p>
<span class="bold"><strong>Boost.Container</strong></span>'s polymorphic resource library
is usable from C++03 containers, and offers some alternative utilities if
the required C++11 features of the <span class="emphasis"><em>Library Fundamentals</em></span>
specification are not available.
</p>
<p>
Let's review the usage example given in <a href="https://isocpp.org/files/papers/N3916.pdf" target="_top">N3916</a>
and see how it can be implemented using <span class="bold"><strong>Boost.Container</strong></span>:
<span class="emphasis"><em>Suppose we are processing a series of shopping lists, where a shopping
list is a container of strings, and storing them in a collection (a list)
of shopping lists. Each shopping list being processed uses a bounded amount
of memory that is needed for a short period of time, while the collection
of shopping lists uses an unbounded amount of memory and will exist for a
longer period of time. For efficiency, we can use a more time-efficient memory
allocator based on a finite buffer for the temporary shopping lists.</em></span>
</p>
<p>
Let's see how <code class="computeroutput"><span class="identifier">ShoppingList</span></code>
can be defined to support an polymorphic memory resource that can allocate
memory from different underlying mechanisms. The most important details are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
It should declare that supports an allocator defining an <code class="computeroutput"><span class="identifier">allocator_type</span></code> typedef. This <code class="computeroutput"><span class="identifier">allocator_type</span></code> will be of type <code class="computeroutput"><a class="link" href="../boost/container/pmr/memory_resource.html" title="Class memory_resource">memory_resource *</a></code>,
which is a base class for polymorphic resources.
</li>
<li class="listitem">
It must define constructors that take the the allocator as argument.
It can be implemented in two ways:
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem">
<code class="computeroutput"><span class="identifier">ShoppingList</span></code> has
constructors taking <code class="computeroutput"><a class="link" href="../boost/container/pmr/memory_resource.html" title="Class memory_resource">memory_resource*</a></code>
as the last argument.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">ShoppingList</span></code> has
constructors taking <code class="computeroutput"><a class="link" href="../boost/container/allocator_arg_t.html" title="Type definition allocator_arg_t">allocator_arg_t</a></code>
as the first argument and <code class="computeroutput"><a class="link" href="../boost/container/pmr/memory_resource.html" title="Class memory_resource">memory_resource*</a></code>
as the second argument.
</li>
</ul></div>
</li>
</ul></div>
<p>
<span class="bold"><strong>Note:</strong></span> <span class="emphasis"><em>In C++03 compilers, it is
required that the programmer specializes as <code class="computeroutput"><span class="keyword">true</span></code>
<code class="computeroutput"><a class="link" href="../boost/container/construc_idm46123637584144.html" title="Struct template constructible_with_allocator_suffix">constructible_with_allocator_suffix</a></code>
or <code class="computeroutput"><a class="link" href="../boost/container/construc_idm46123637594512.html" title="Struct template constructible_with_allocator_prefix">constructible_with_allocator_prefix</a></code>
as in C++03 there is no way to automatically detect the chosen option at
compile time. If no specialization is done, <span class="bold"><strong>Boost.Container</strong></span>
assumes the suffix option</em></span>.
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//ShoppingList.hpp</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">pmr</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">pmr</span><span class="special">/</span><span class="identifier">string</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">ShoppingList</span>
<span class="special">{</span>
<span class="comment">// A vector of strings using polymorphic allocators. Every element</span>
<span class="comment">// of the vector will use the same allocator as the vector itself.</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">::</span><span class="identifier">pmr</span><span class="special">::</span><span class="identifier">vector_of</span>
<span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">::</span><span class="identifier">pmr</span><span class="special">::</span><span class="identifier">string</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">m_strvec</span><span class="special">;</span>
<span class="comment">//Alternatively in compilers that support template aliases:</span>
<span class="comment">// boost::container::pmr::vector<boost::container::pmr::string> m_strvec;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">// This makes uses_allocator<ShoppingList, memory_resource*>::value true</span>
<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">::</span><span class="identifier">pmr</span><span class="special">::</span><span class="identifier">memory_resource</span><span class="special">*</span> <span class="identifier">allocator_type</span><span class="special">;</span>
<span class="comment">// If the allocator is not specified, "m_strvec" uses pmr::get_default_resource().</span>
<span class="keyword">explicit</span> <span class="identifier">ShoppingList</span><span class="special">(</span><span class="identifier">allocator_type</span> <span class="identifier">alloc</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">m_strvec</span><span class="special">(</span><span class="identifier">alloc</span><span class="special">)</span> <span class="special">{}</span>
<span class="comment">// Copy constructor. As allocator is not specified,</span>
<span class="comment">// "m_strvec" uses pmr::get_default_resource().</span>
<span class="identifier">ShoppingList</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">ShoppingList</span><span class="special">&</span> <span class="identifier">other</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">m_strvec</span><span class="special">(</span><span class="identifier">other</span><span class="special">.</span><span class="identifier">m_strvec</span><span class="special">)</span> <span class="special">{}</span>
<span class="comment">// Copy construct using the given memory_resource.</span>
<span class="identifier">ShoppingList</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">ShoppingList</span><span class="special">&</span> <span class="identifier">other</span><span class="special">,</span> <span class="identifier">allocator_type</span> <span class="identifier">a</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">m_strvec</span><span class="special">(</span><span class="identifier">other</span><span class="special">.</span><span class="identifier">m_strvec</span><span class="special">,</span> <span class="identifier">a</span><span class="special">)</span> <span class="special">{}</span>
<span class="identifier">allocator_type</span> <span class="identifier">get_allocator</span><span class="special">()</span> <span class="keyword">const</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">m_strvec</span><span class="special">.</span><span class="identifier">get_allocator</span><span class="special">().</span><span class="identifier">resource</span><span class="special">();</span> <span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">add_item</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">item</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">m_strvec</span><span class="special">.</span><span class="identifier">emplace_back</span><span class="special">(</span><span class="identifier">item</span><span class="special">);</span> <span class="special">}</span>
<span class="comment">//...</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
<span class="emphasis"><em>However, this time-efficient allocator is not appropriate for the
longer lived collection of shopping lists. This example shows how those temporary
shopping lists, using a time-efficient allocator, can be used to populate
the long lived collection of shopping lists, using a general purpose allocator,
something that would be annoyingly difficult without the polymorphic allocators.</em></span>
</p>
<p>
In <span class="bold"><strong>Boost.Container</strong></span> for the time-efficient
allocation we can use <code class="computeroutput"><a class="link" href="../boost/container/pmr/monotonic_buffer_resource.html" title="Class monotonic_buffer_resource">monotonic_buffer_resource</a></code>,
providing an external buffer that will be used until it's exhausted. In the
default configuration, when the buffer is exhausted, the default memory resource
will be used instead.
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="string">"ShoppingList.hpp"</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">pmr</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">pmr</span><span class="special">/</span><span class="identifier">monotonic_buffer_resource</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">void</span> <span class="identifier">processShoppingList</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">ShoppingList</span><span class="special">&)</span>
<span class="special">{</span> <span class="comment">/**/</span> <span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">;</span>
<span class="comment">//All memory needed by folder and its contained objects will</span>
<span class="comment">//be allocated from the default memory resource (usually new/delete) </span>
<span class="identifier">pmr</span><span class="special">::</span><span class="identifier">list_of</span><span class="special"><</span><span class="identifier">ShoppingList</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">folder</span><span class="special">;</span> <span class="comment">// Default allocator resource</span>
<span class="comment">//Alternatively in compilers that support template aliases:</span>
<span class="comment">// boost::container::pmr::list<ShoppingList> folder;</span>
<span class="special">{</span>
<span class="keyword">char</span> <span class="identifier">buffer</span><span class="special">[</span><span class="number">1024</span><span class="special">];</span>
<span class="identifier">pmr</span><span class="special">::</span><span class="identifier">monotonic_buffer_resource</span> <span class="identifier">buf_rsrc</span><span class="special">(&</span><span class="identifier">buffer</span><span class="special">,</span> <span class="number">1024</span><span class="special">);</span>
<span class="comment">//All memory needed by temporaryShoppingList will be allocated</span>
<span class="comment">//from the local buffer (speeds up "processShoppingList")</span>
<span class="identifier">ShoppingList</span> <span class="identifier">temporaryShoppingList</span><span class="special">(&</span><span class="identifier">buf_rsrc</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">buf_rsrc</span> <span class="special">==</span> <span class="identifier">temporaryShoppingList</span><span class="special">.</span><span class="identifier">get_allocator</span><span class="special">());</span>
<span class="comment">//list nodes, and strings "salt" and "pepper" will be allocated</span>
<span class="comment">//in the stack thanks to "monotonic_buffer_resource".</span>
<span class="identifier">temporaryShoppingList</span><span class="special">.</span><span class="identifier">add_item</span><span class="special">(</span><span class="string">"salt"</span><span class="special">);</span>
<span class="identifier">temporaryShoppingList</span><span class="special">.</span><span class="identifier">add_item</span><span class="special">(</span><span class="string">"pepper"</span><span class="special">);</span>
<span class="comment">//...</span>
<span class="comment">//All modifications and additions to "temporaryShoppingList"</span>
<span class="comment">//will use memory from "buffer" until it's exhausted.</span>
<span class="identifier">processShoppingList</span><span class="special">(</span><span class="identifier">temporaryShoppingList</span><span class="special">);</span>
<span class="comment">//Processing done, now insert it in "folder",</span>
<span class="comment">//which uses the default memory resource</span>
<span class="identifier">folder</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">temporaryShoppingList</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">pmr</span><span class="special">::</span><span class="identifier">get_default_resource</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">folder</span><span class="special">.</span><span class="identifier">back</span><span class="special">().</span><span class="identifier">get_allocator</span><span class="special">());</span>
<span class="comment">//temporaryShoppingList, buf_rsrc, and buffer go out of scope</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
<span class="emphasis"><em>Notice that the shopping lists within <code class="computeroutput"><span class="identifier">folder</span></code>
use the default allocator resource whereas the shopping list <code class="computeroutput"><span class="identifier">temporaryShoppingList</span></code> uses the short-lived
but very fast <code class="computeroutput"><span class="identifier">buf_rsrc</span></code>. Despite
using different allocators, you can insert <code class="computeroutput"><span class="identifier">temporaryShoppingList</span></code>
into folder because they have the same <code class="computeroutput"><span class="identifier">ShoppingList</span></code>
type. Also, while <code class="computeroutput"><span class="identifier">ShoppingList</span></code>
uses memory_resource directly, <code class="computeroutput"><a class="link" href="../boost_container_header_reference.html#boost.container.pmr.list">pmr::list</a></code>,
<code class="computeroutput"><a class="link" href="../boost_container_header_reference.html#boost.container.pmr.vector">pmr::vector</a></code> and
<code class="computeroutput"><a class="link" href="../boost_container_header_reference.html#boost.container.pmr.string">pmr::string</a></code> all
use <code class="computeroutput"><a class="link" href="../boost/container/pmr/polymorphic_allocator.html" title="Class template polymorphic_allocator">polymorphic_allocator</a></code>.</em></span>
</p>
<p>
<span class="emphasis"><em>The resource passed to the <code class="computeroutput"><span class="identifier">ShoppingList</span></code>
constructor is propagated to the vector and each string within that <code class="computeroutput"><span class="identifier">ShoppingList</span></code>. Similarly, the resource used
to construct <code class="computeroutput"><span class="identifier">folder</span></code> is propagated
to the constructors of the ShoppingLists that are inserted into the list
(and to the strings within those <code class="computeroutput"><span class="identifier">ShoppingLists</span></code>).
The <code class="computeroutput"><a class="link" href="../boost/container/pmr/polymorphic_allocator.html" title="Class template polymorphic_allocator">polymorphic_allocator</a></code>
template is designed to be almost interchangeable with a pointer to <code class="computeroutput"><a class="link" href="../boost/container/pmr/memory_resource.html" title="Class memory_resource">memory_resource</a></code>,
thus producing a <span class="emphasis"><em>bridge</em></span> between the template-policy
style of allocator and the polymorphic-base-class style of allocator.</em></span>
</p>
<p>
This example actually shows how easy is to use <span class="bold"><strong>Boost.Container</strong></span>
to write type-erasured allocator-capable classes even in C++03 compilers.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="container.cpp_conformance.forward_list"></a><a class="link" href="cpp_conformance.html#container.cpp_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.cpp_conformance.vector_exception_guarantees"></a><a class="link" href="cpp_conformance.html#container.cpp_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_allocators.html" title="Extended functionality: 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.cpp_conformance.container_const_reference_parameters"></a><a class="link" href="cpp_conformance.html#container.cpp_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.cpp_conformance.Vector_bool"></a><a class="link" href="cpp_conformance.html#container.cpp_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.cpp_conformance.non_standard_memset_initialization"></a><a class="link" href="cpp_conformance.html#container.cpp_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 © 2009-2018 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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