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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>Using non-POD-classes as template parameter to <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
results in undefined behavior</strong></span>: This means that any class containing
a constructor, destructor, virtual methods or access control specifications
is not a valid argument in C++98. C++11 relaxes this slightly by allowing
"trivial" classes containing only empty constructors. <span class="bold"><strong>Advise</strong></span>: Use only POD types.
</li>
<li class="listitem">
<span class="bold"><strong>C++98 compilers may transform computation- 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 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 there is no
way to express this constraint to the compiler, these act as "full
compiler barriers" in this implementation. In corner cases this may
result in a less efficient code than a C++11 compiler could generate.
</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"><</span><span class="identifier">T</span><span class="special">></span></code>
in shared memory only works correctly, if <code class="computeroutput"><span class="identifier">atomic</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">is_lock_free</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">true</span></code>.
</li>
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<td align="right"><div class="copyright-footer">Copyright © 2011 Helge Bahmann<br>Copyright © 2012 Tim Blechmann<br>Copyright © 2013, 2017 Andrey 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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