145 lines
3.6 KiB
HTML
145 lines
3.6 KiB
HTML
<HTML>
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<!--
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Copyright (c) Jeremy Siek 2000
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)
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-->
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<Head>
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<Title>Readable Property Map</Title>
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<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
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ALINK="#ff0000">
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<IMG SRC="../../../boost.png"
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ALT="C++ Boost" width="277" height="86">
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<BR Clear>
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<H2><A NAME="concept:ReadablePropertyMap"></A>
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Readable Property Map
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</H2>
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A Readable Property Map provides read-access to the value associated with a given key via a call to the <tt>get()</tt> function.
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The return type of the <tt>get()</tt>function is either the <tt>value_type</tt> of the property map or a (const or non-const) reference to that type.
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<h3>Refinement of</h3>
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<a href="../../utility/CopyConstructible.html">Copy Constructible</a>
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<h3>Notation</h3>
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<Table>
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<TR>
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<TD><tt>PMap</tt></TD>
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<TD>A type that is a model of Readable Property Map.</TD>
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</TR>
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<TR>
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<TD><tt>pmap</tt></TD>
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<TD>An object of type <tt>PMap</tt>.</td>
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</TR>
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<TR>
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<TD><tt>key</tt></TD>
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<TD>An object of type <tt>boost::property_traits<PMap>::key_type</tt>.</td>
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</TR>
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</table>
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<h3>Associated Types</h3>
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<table border>
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<tr>
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<td>Value Type</td>
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<td><TT>boost::property_traits<PMap>::value_type</TT></td>
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<td>The type of the property.</td>
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</tr>
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<tr>
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<td>Reference Type</td>
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<td><TT>boost::property_traits<PMap>::reference</TT></td>
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<td>
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A type that is convertible to the value type.
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</td>
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</tr>
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<tr>
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<td>Key Type</td>
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<td><TT>boost::property_traits<PMap>::key_type</TT></td>
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<td>
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The type of the key object used to look up the property. The property
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map may be templated on the key type, in which case this
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typedef can be <TT>void</TT>.
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</td>
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</tr>
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<tr>
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<td>Property Map Category</td>
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<td><TT>boost::property_traits<PMap>::category</TT></td>
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<td>
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The category of the property: a type convertible to
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<TT>readable_property_map_tag</TT>.
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</td>
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</tr>
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</table>
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<h3>Valid Expressions</h3>
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<table border>
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<tr>
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<th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th>
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</tr>
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<tr>
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<td>Get Property Value </td>
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<TD><TT>get(pmap, key)</TT></TD>
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<TD><TT>reference</TT></TD>
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<TD>Lookup the value associated with <TT>key</TT>.</TD>
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</TR>
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</TABLE>
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<h3>Concept Checking Class</h3>
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<pre> template <class PMap, class Key>
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struct ReadablePropertyMapConcept
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{
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typedef typename property_traits<PMap>::key_type key_type;
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typedef typename property_traits<PMap>::category Category;
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typedef boost::readable_property_map_tag ReadableTag;
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void constraints() {
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function_requires< ConvertibleConcept<Category, ReadableTag> >();
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val = get(pmap, k);
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}
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PMap pmap;
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Key k;
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typename property_traits<PMap>::value_type val;
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};</pre>
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<h3>See Also</h3>
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<a href="./property_map.html">Property map concepts</a>
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<h3>Design Notes</h3>
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At various times the name "read-only" was considered for
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this concept. However, that name is inappropriate because concepts are
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inherently positive, not negative. This becomes obvious when we define
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the Read Write Property Map, which refines both the Readable Property
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Map and the Writable Property Map concept. It would not make much
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sense to combine "read-only" and "write-only"
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concepts!
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<br>
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<HR>
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<TABLE>
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<TR valign=top>
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<TD nowrap>Copyright © 2000</TD><TD>
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<a HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</a>, Univ.of Notre Dame (<A HREF="mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</A>)
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</TD></TR></TABLE>
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