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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="atomic.limitations"></a><a class="link" href="limitations.html" title="Limitations">Limitations</a>
</h2></div></div></div>
<p>
      While <span class="bold"><strong>Boost.Atomic</strong></span> strives to implement the
      atomic operations from C++11 and later as faithfully as possible, there are
      a few limitations that cannot be lifted without compiler support:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          <span class="bold"><strong>Aggregate initialization syntax is not supported</strong></span>:
          Since <span class="bold"><strong>Boost.Atomic</strong></span> sometimes uses storage
          type that is different from the value type, the <code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;&gt;</span></code> template needs an initialization
          constructor that performs the necessary conversion. This makes <code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;&gt;</span></code>
          a non-aggregate type and prohibits aggregate initialization syntax (<code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">a</span> <span class="special">=</span> <span class="special">{</span><span class="number">10</span><span class="special">}</span></code>).
          <span class="bold"><strong>Boost.Atomic</strong></span> does support direct and unified
          initialization syntax though. <span class="bold"><strong>Advice</strong></span>:
          Always use direct initialization (<code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">(</span><span class="number">10</span><span class="special">)</span></code>)
          or unified initialization (<code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">{</span><span class="number">10</span><span class="special">}</span></code>)
          syntax.
        </li>
<li class="listitem">
          <span class="bold"><strong>Initializing constructor is not <code class="computeroutput"><span class="keyword">constexpr</span></code>
          for some types</strong></span>: For value types other than integral types and
          <code class="computeroutput"><span class="keyword">bool</span></code>, <code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;&gt;</span></code> initializing constructor needs
          to perform runtime conversion to the storage type. This limitation may
          be lifted for more categories of types in the future.
        </li>
<li class="listitem">
          <span class="bold"><strong>Default constructor is not trivial in C++03</strong></span>:
          Because the initializing constructor has to be defined in <code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;&gt;</span></code>,
          the default constructor must also be defined. In C++03 the constructor
          cannot be defined as defaulted and therefore it is not trivial. In C++11
          the constructor is defaulted (and trivial, if the default constructor of
          the value type is). In any case, the default constructor of <code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;&gt;</span></code>
          performs default initialization of the atomic value, as required in C++11.
          <span class="bold"><strong>Advice</strong></span>: In C++03, do not use <span class="bold"><strong>Boost.Atomic</strong></span> in contexts where trivial default constructor
          is important (e.g. as a global variable which is required to be statically
          initialized).
        </li>
<li class="listitem">
          <span class="bold"><strong>C++03 compilers may transform computation dependency
          to control dependency</strong></span>: Crucially, <code class="computeroutput"><span class="identifier">memory_order_consume</span></code>
          only affects computationally-dependent operations, but in general there
          is nothing preventing a compiler from transforming a computation dependency
          into a control dependency. A fully compliant C++11 compiler would be forbidden
          from such a transformation, but in practice most if not all compilers have
          chosen to promote <code class="computeroutput"><span class="identifier">memory_order_consume</span></code>
          to <code class="computeroutput"><span class="identifier">memory_order_acquire</span></code>
          instead (see <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448" target="_top">this</a>
          gcc bug for example). In the current implementation <span class="bold"><strong>Boost.Atomic</strong></span>
          follows that trend, but this may change in the future. <span class="bold"><strong>Advice</strong></span>:
          In general, avoid <code class="computeroutput"><span class="identifier">memory_order_consume</span></code>
          and use <code class="computeroutput"><span class="identifier">memory_order_acquire</span></code>
          instead. Use <code class="computeroutput"><span class="identifier">memory_order_consume</span></code>
          only in conjunction with pointer values, and only if you can ensure that
          the compiler cannot speculate and transform these into control dependencies.
        </li>
<li class="listitem">
          <span class="bold"><strong>Fence operations may enforce "too strong"
          compiler ordering</strong></span>: Semantically, <code class="computeroutput"><span class="identifier">memory_order_acquire</span></code>/<code class="computeroutput"><span class="identifier">memory_order_consume</span></code> and <code class="computeroutput"><span class="identifier">memory_order_release</span></code> need to restrain
          reordering of memory operations only in one direction. Since in C++03 there
          is no way to express this constraint to the compiler, these act as "full
          compiler barriers" in C++03 implementation. In corner cases this may
          result in a slightly less efficient code than a C++11 compiler could generate.
          <span class="bold"><strong>Boost.Atomic</strong></span> will use compiler intrinsics,
          if possible, to express the proper ordering constraints.
        </li>
<li class="listitem">
          <span class="bold"><strong>Atomic operations may enforce "too strong"
          memory ordering in debug mode</strong></span>: On some compilers, disabling
          optimizations makes it impossible to provide memory ordering constraints
          as compile-time constants to the compiler intrinsics. This causes the compiler
          to silently ignore the provided constraints and choose the "strongest"
          memory order (<code class="computeroutput"><span class="identifier">memory_order_seq_cst</span></code>)
          to generate code. Not only this reduces performance, this may hide bugs
          in the user's code (e.g. if the user used a wrong memory order constraint,
          which caused a data race). <span class="bold"><strong>Advice</strong></span>: Always
          test your code with optimizations enabled.
        </li>
<li class="listitem">
          <span class="bold"><strong>No interprocess fallback</strong></span>: using <code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
          in shared memory only works correctly, if <code class="computeroutput"><span class="identifier">atomic</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">is_lock_free</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">true</span></code>.
          Same with <code class="computeroutput"><span class="identifier">atomic_ref</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>.
        </li>
<li class="listitem">
          <span class="bold"><strong>Signed integers must use <a href="https://en.wikipedia.org/wiki/Two%27s_complement" target="_top">two's
          complement</a> representation</strong></span>: <span class="bold"><strong>Boost.Atomic</strong></span>
          makes this requirement in order to implement conversions between signed
          and unsigned integers internally. C++11 requires all atomic arithmetic
          operations on integers to be well defined according to two's complement
          arithmetics, which means that <span class="bold"><strong>Boost.Atomic</strong></span>
          has to operate on unsigned integers internally to avoid undefined behavior
          that results from signed integer overflows. Platforms with other signed
          integer representations are not supported. Note that C++20 makes two's
          complement representation of signed integers mandatory.
        </li>
<li class="listitem">
          <span class="bold"><strong>Types with padding bits are not supported</strong></span>:
          As discussed in <a class="link" href="interface.html#atomic.interface.interface_atomic_ref.caveats" title="Caveats">this
          section</a>, <span class="bold"><strong>Boost.Atomic</strong></span> cannot support
          types with padding bits because their content is undefined, and there is
          no portable way to initialize them to a predefined value. This makes operations
          like <code class="computeroutput"><span class="identifier">compare_exchange_strong</span></code>/<code class="computeroutput"><span class="identifier">compare_exchange_weak</span></code> fail, and given
          that in some cases other operations are built upon these, potentially all
          operations become unreliable. <span class="bold"><strong>Boost.Atomic</strong></span>
          does support padding bits for floating point types on platforms where the
          location of the padding bits is known at compile time.
        </li>
</ul></div>
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      Semashev<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>)
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