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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>16.2 Ephemerons</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" 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data-pltdoc="x">Ephemerons</a></td></tr><tr><td align="right">16.3 </td><td><a href="willexecutor.html" class="tocviewlink" data-pltdoc="x">Wills and Executors</a></td></tr><tr><td align="right">16.4 </td><td><a href="garbagecollection.html" class="tocviewlink" data-pltdoc="x">Garbage Collection</a></td></tr><tr><td align="right">16.5 </td><td><a href="phantom-bytes.html" class="tocviewlink" data-pltdoc="x">Phantom Byte Strings</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._make-ephemeron%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">make-<wbr></wbr>ephemeron</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._ephemeron-value%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">ephemeron-<wbr></wbr>value</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._ephemeron~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">ephemeron?</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="weakbox.html" title="backward to "16.1 Weak Boxes"" data-pltdoc="x">← prev</a> <a href="memory.html" title="up to "16 Memory Management"" data-pltdoc="x">up</a> <a href="willexecutor.html" title="forward to "16.3 Wills and Executors"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-part-tag=""ephemerons"">16.2<tt> </tt><a name="(part._ephemerons)"></a>Ephemerons</h4><p>An <a name="(tech._ephemeron)"></a><span style="font-style: italic">ephemeron</span> [<a href="doc-bibliography.html#%28cite._.Hayes97%29" data-pltdoc="x">Hayes97</a>] is a generalization of a
<a href="weakbox.html#%28tech._weak._box%29" class="techoutside" data-pltdoc="x"><span class="techinside">weak box</span></a> (see <a href="weakbox.html" data-pltdoc="x">Weak Boxes</a>). Instead of just containing
one value, an ephemeron holds two values: one that is considered the
value of the ephemeron and another that is the ephemeron’s key. Like
the value in a weak box, the value in an ephemeron may be replaced by
<span class="RktVal">#f</span>, but when the <span style="font-style: italic">key</span> is no longer reachable (except
possibly via weak references) instead of when the value is no longer
reachable.</p><p>As long as an ephemeron’s value is retained, the reference is
considered a non-weak reference. References to the key via the value
are treated specially, however, in that the reference does not
necessarily count toward the key’s reachability. A <a href="weakbox.html#%28tech._weak._box%29" class="techoutside" data-pltdoc="x"><span class="techinside">weak box</span></a> can
be seen as a specialization of an ephemeron where the key and value
are the same.</p><p>One particularly common use of ephemerons is to combine them with a
weak hash table (see <a href="hashtables.html" data-pltdoc="x">Hash Tables</a>) to produce a mapping where
the memory manager can reclaim key–value pairs even when the value
refers to the key. A related use is to retain a reference to a value
as long as any value for which it is an <a href="chaperones.html#%28tech._impersonator%29" class="techoutside" data-pltdoc="x"><span class="techinside">impersonator</span></a> is
reachable; see <span class="RktSym"><a href="chaperones.html#%28def._%28%28quote._~23~25kernel%29._impersonator-ephemeron%29%29" class="RktValLink" data-pltdoc="x">impersonator-ephemeron</a></span>.</p><p><div class="SIntrapara">More precisely,
</div><div class="SIntrapara"><ul><li><p>the value in an ephemeron is replaced
by <span class="RktVal">#f</span> when the automatic memory manager can prove that
either the ephemeron or the key is reachable only through weak
references (see <a href="weakbox.html" data-pltdoc="x">Weak Boxes</a>); and</p></li><li><p>nothing reachable from the value in an ephemeron counts toward
the reachability of an ephemeron key (whether for the same ephemeron
or another), unless the same value is reachable through a non-weak
reference, or unless the value’s ephemeron key is reachable through a
non-weak reference (see <a href="weakbox.html" data-pltdoc="x">Weak Boxes</a> for information on weak
references).</p></li></ul></div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._make-ephemeron))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-ephemeron%29%29" class="RktValDef RktValLink" data-pltdoc="x">make-ephemeron</a></span></span><span class="hspace"> </span><span class="RktVar">key</span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._ephemeron~3f%29%29" class="RktValLink" data-pltdoc="x">ephemeron?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">key</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns a new <a href="#%28tech._ephemeron%29" class="techoutside" data-pltdoc="x"><span class="techinside">ephemeron</span></a> whose key is <span class="RktVar">key</span> and whose
value is initially <span class="RktVar">v</span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._ephemeron-value))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._ephemeron-value%29%29" class="RktValDef RktValLink" data-pltdoc="x">ephemeron-value</a></span></span><span class="hspace"> </span><span class="RktVar">ephemeron</span><span class="hspace"> </span>[<span class="RktVar">gced-v</span>]<span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">ephemeron</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._ephemeron~3f%29%29" class="RktValLink" data-pltdoc="x">ephemeron?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">gced-v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktVal">#f</span></td></tr></table></blockquote></div><div class="SIntrapara">Returns the value contained in <span class="RktVar">ephemeron</span>. If the garbage
collector has proven that the key for <span class="RktVar">ephemeron</span> is only
weakly reachable, then the result is <span class="RktVar">gced-v</span> (which defaults to <span class="RktVal">#f</span>).</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._ephemeron~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._ephemeron~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">ephemeron?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">v</span> is an <a href="#%28tech._ephemeron%29" class="techoutside" data-pltdoc="x"><span class="techinside">ephemeron</span></a>, <span class="RktVal">#f</span>
otherwise.</div></p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="weakbox.html" title="backward to "16.1 Weak Boxes"" data-pltdoc="x">← prev</a> <a href="memory.html" title="up to "16 Memory Management"" data-pltdoc="x">up</a> <a href="willexecutor.html" title="forward to "16.3 Wills and Executors"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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