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<HTML>
<Head>
<Title>OptionalPointee Concept</Title>
</HEAD>
<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
ALINK="#ff0000">
<IMG SRC="../../c++boost.gif"
ALT="C++ Boost" width="277" height="86">
<!--end header-->
<BR Clear>
<H1>Concept: OptionalPointee</H1>
<h3>Description</h3>
A type is a model of <i>OptionalPointee</i> if it points to (or refers to) a value
that may not exist. That is, if it has a <b>pointee</b> which might be <b>valid</b>
(existent) or <b>invalid</b> (inexistent); and it is possible to test whether the
pointee is valid or not.
This model does <u>not</u> imply pointer semantics: i.e., it does not imply shallow copy nor
aliasing.
<h3>Notation</h3>
<Table>
<TR>
<TD VAlign=top> <tt>T</tt> </TD>
<TD VAlign=top> is a type that is a model of OptionalPointee</TD>
</TR>
<TR>
<TD VAlign=top> <tt>t</tt> </TD>
<TD VAlign=top> is an object of type <tt>T</tt> or possibly <tt>const T</tt></TD>
</tr>
</table>
<h3>Definitions</h3>
<h3>Valid expressions</h3>
<Table border>
<TR>
<TH> Name </TH>
<TH> Expression </TH>
<TH> Return type </TH>
<TH> Semantics </TH>
</TR>
<TR>
<TD VAlign=top>Value Access</TD>
<TD VAlign=top>&nbsp;<tt>*t</tt></TD>
<TD VAlign=top>&nbsp;<tt>T&amp;</tt></TD>
<TD VAlign=top>If the pointee is valid returns a reference to
the pointee.<br>
If the pointee is invalid the result is <i>undefined</i>.</TD>
<TD VAlign=top> </TD>
</TR>
<TR>
<TD VAlign=top>Value Access</TD>
<TD VAlign=top>&nbsp;<tt>t-><i>xyz</i></tt></TD>
<TD VAlign=top>&nbsp;<tt>T*</tt></TD>
<TD VAlign=top>If the pointee is valid returns a builtin pointer to the pointee.<br>
If the pointee is invalid the result is <i>undefined</i> (It might not even return NULL).<br>
</TD>
<TD VAlign=top> </TD>
</TR>
<TR>
<TD VAlign=top>Validity Test</TD>
<TD VAlign=top>&nbsp;<tt>t</tt><br>
&nbsp;<tt>t != 0</tt><br>
&nbsp;<tt>!!t</tt>
</TD>
<TD VAlign=top>&nbsp;bool </TD>
<TD VAlign=top>If the pointee is valid returns true.<br>
If the pointee is invalid returns false.</TD>
<TD VAlign=top></TD>
</TR>
<TR>
<TD VAlign=top>Invalidity Test</TD>
<TD VAlign=top>&nbsp;<tt>t == 0</tt><br>
&nbsp;<tt>!t</tt>
</TD>
<TD VAlign=top>&nbsp;bool </TD>
<TD VAlign=top>If the pointee is valid returns false.<br>
If the pointee is invalid returns true.</TD>
<TD VAlign=top></TD>
</TR>
</table>
<h3>Models</h3>
<UL>
<LI><tt>pointers, both builtin and smart.</tt>
<LI><tt>boost::optional&lt;&gt;</tt>
</UL>
<HR>
<h3>OptionalPointee and relational operations</h3>
<p>This concept does not define any particular semantic for relational operations, therefore,
a type which models this concept might have either shallow or deep relational semantics.<br>
For instance, pointers, which are models of OptionalPointee, have shallow relational operators:
comparisons of pointers do not involve comparisons of pointees.
This makes sense for pointers because they have shallow copy semantics.<br>
But boost::optional&lt;T&gt;, on the other hand, which is also a model of OptionalPointee, has
deep-copy and deep-relational semantics.<br>
If generic code is written for this concept, it is important not to use relational
operators directly because the semantics might be different depending on the actual type.<br>
Still, the concept itsef can be used to define a <i>deep</i> equality-test that can
be used in generic code with any type which models OptionalPointee:</p>
<a name="equal"></a>
<pre>
template&lt;class OptionalPointee&gt;
inline
bool equal_pointees ( OptionalPointee const&amp; x, OptionalPointee const&amp; y )
{
return (!x) != (!y) ? false : ( !x ? true : (*x) == (*y) ) ;
}
</pre>
<p>The preceding generic function has the following semantics:<br>
If both x and y have valid pointees, it compares pointee's values via (*x == *y).<br>
If only one has a valid pointee, returns false.<br>
If both have invalid pointees, returns true.</p>
<p><code>equal_pointees()</code> is implemented in <a href="../../boost/optional.hpp">optional.hpp</a></p>
<p>Notice that OptionalPointee does not imply aliasing (and optional&lt;&gt; for instance does not alias);
so direct usage of relational operators with the implied aliasing of shallow semantics
-as with pointers- should not be used with generic code written for this concept.</p>
<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright &copy 2003</TD><TD>
<A HREF="mailto:fernando_cacciola@hotmail.com">Fernando Cacciola</A>,
based on the original concept developed by Augustus Saunders.
</TD></TR></TABLE>
</BODY>
</HTML>