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<title>20. Weak references &mdash; Memory Pool System 1.111.0 documentation</title>
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<div class="section" id="weak-references">
<span id="topic-weak"></span><span id="index-0"></span><h1>20. Weak references<a class="headerlink" href="#weak-references" title="Permalink to this headline"></a></h1>
<p>A <em class="dfn">weak reference</em> is a <a class="reference internal" href="../glossary/r.html#term-reference"><em class="xref std std-term">reference</em></a> that does not keep the
block it refers to <a class="reference internal" href="../glossary/l.html#term-live"><em class="xref std std-term">alive</em></a>.</p>
<p>The open source MPS supports weak references only:</p>
<ol class="arabic simple">
<li>in <a class="reference internal" href="../glossary/r.html#term-root"><em class="xref std std-term">roots</em></a> that are registered with <a class="reference internal" href="../glossary/r.html#term-rank"><em class="xref std std-term">rank</em></a>
<a class="reference internal" href="root.html#mps_rank_weak" title="mps_rank_weak"><tt class="xref c c-func docutils literal"><span class="pre">mps_rank_weak()</span></tt></a>;</li>
<li>in objects allocated on an <a class="reference internal" href="../glossary/a.html#term-allocation-point"><em class="xref std std-term">allocation point</em></a> in a pool of
class <a class="reference internal" href="../pool/awl.html#pool-awl"><em>AWL (Automatic Weak Linked)</em></a> that was created with <a class="reference internal" href="../glossary/r.html#term-rank"><em class="xref std std-term">rank</em></a>
<a class="reference internal" href="root.html#mps_rank_weak" title="mps_rank_weak"><tt class="xref c c-func docutils literal"><span class="pre">mps_rank_weak()</span></tt></a>.</li>
</ol>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">If you need more general handling of weak references,
<a class="reference internal" href="../contact.html#contact"><em>contact us</em></a>.</p>
</div>
<p>When the MPS determines that a block is only kept alive by one or more
weak references, it may choose to <a class="reference internal" href="../glossary/s.html#term-splat"><em class="xref std std-term">splat</em></a> those references by
replacing them with null pointers when they are <a class="reference internal" href="../glossary/f.html#term-fix"><em class="xref std std-term">fixed</em></a>. When
all weak references to the block have been splatted, the block may be
reclaimed.</p>
<p>For example, a <a class="reference internal" href="../glossary/s.html#term-scan-method"><em class="xref std std-term">scan method</em></a> for objects in an AWL pool might
look like this:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">mps_res_t</span> <span class="nf">obj_scan</span><span class="p">(</span><span class="n">mps_ss_t</span> <span class="n">ss</span><span class="p">,</span> <span class="n">mps_addr_t</span> <span class="n">base</span><span class="p">,</span> <span class="n">mps_addr_t</span> <span class="n">limit</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">MPS_SCAN_BEGIN</span><span class="p">(</span><span class="n">ss</span><span class="p">)</span> <span class="p">{</span>
<span class="k">while</span> <span class="p">(</span><span class="n">base</span> <span class="o">&lt;</span> <span class="n">limit</span><span class="p">)</span> <span class="p">{</span>
<span class="n">obj_t</span> <span class="n">obj</span> <span class="o">=</span> <span class="n">base</span><span class="p">;</span>
<span class="n">mps_addr_t</span> <span class="n">p</span> <span class="o">=</span> <span class="n">obj</span><span class="o">-&gt;</span><span class="n">ref</span><span class="p">;</span>
<span class="k">if</span> <span class="p">(</span><span class="n">MPS_FIX1</span><span class="p">(</span><span class="n">ss</span><span class="p">,</span> <span class="n">p</span><span class="p">))</span> <span class="p">{</span>
<span class="n">mps_res_t</span> <span class="n">res</span> <span class="o">=</span> <span class="n">MPS_FIX2</span><span class="p">(</span><span class="n">ss</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">p</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">res</span> <span class="o">!=</span> <span class="n">MPS_RES_OK</span><span class="p">)</span> <span class="k">return</span> <span class="n">res</span><span class="p">;</span>
<span class="k">if</span> <span class="p">(</span><span class="n">p</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span>
<span class="cm">/* reference was splatted */</span>
<span class="p">}</span>
<span class="n">obj</span><span class="o">-&gt;</span><span class="n">ref</span> <span class="o">=</span> <span class="n">p</span><span class="p">;</span>
<span class="p">}</span>
<span class="n">base</span> <span class="o">+=</span> <span class="k">sizeof</span><span class="p">(</span><span class="o">*</span><span class="n">obj</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span> <span class="n">MPS_SCAN_END</span><span class="p">(</span><span class="n">ss</span><span class="p">);</span>
<span class="k">return</span> <span class="n">MPS_RES_OK</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>A reference that passes the &#8220;interesting&#8221; test <a class="reference internal" href="scanning.html#MPS_FIX1" title="MPS_FIX1"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX1()</span></tt></a>
can&#8217;t be a null pointer, so if the reference is discovered to be null
after calling <a class="reference internal" href="scanning.html#MPS_FIX2" title="MPS_FIX2"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX2()</span></tt></a> then it must have just been splatted.</p>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">Because weak references are splatted when they are fixed, not all
weak references to a block are splatted at the same time.
Depending on the decisions the MPS makes about which objects to
scan, a weak reference may live on for some time after other weak
references to the same block have been splatted.</p>
</div>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">A common way in which weak references are used in programming
languages is in <a class="reference internal" href="../glossary/w.html#term-weak-key-hash-table"><em class="xref std std-term">weak-key</em></a> and
<a class="reference internal" href="../glossary/w.html#term-weak-value-hash-table"><em class="xref std std-term">weak-value hash tables</em></a>. A weak-key hash table contains
weak references to its keys: when it detects that a key has been
splatted, it deletes the corresponding value. The <a class="reference internal" href="../pool/awl.html#pool-awl"><em>AWL (Automatic Weak Linked)</em></a>
pool class supports this by allowing you to specify for each
object, a <a class="reference internal" href="../glossary/d.html#term-dependent-object"><em class="xref std std-term">dependent object</em></a> which may be written to by the
<a class="reference internal" href="../glossary/s.html#term-scan-method"><em class="xref std std-term">scan method</em></a>. See <a class="reference internal" href="../pool/awl.html#pool-awl-dependent"><em>Dependent objects</em></a>.</p>
</div>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">Weak references do not prevent blocks from being <a class="reference internal" href="../glossary/f.html#term-finalization"><em class="xref std std-term">finalized</em></a>. At the point that a block is finalized, weak
references will still validly refer to the block. The fact that a
block is registered for finalization prevents weak references to
that block from being splatted. See <a class="reference internal" href="finalization.html#topic-finalization"><em>Finalization</em></a>.</p>
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