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| <Head> |
| <Title>Readable Property Map</Title> |
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| <H2><A NAME="concept:ReadablePropertyMap"></A> |
| Readable Property Map |
| </H2> |
| |
| A Readable Property Map provides read-access to the value object associated with a given key via a call to the <tt>get()</tt> function. |
| The <tt>get()</tt> function returns a copy of the value object. |
| |
| <h3>Refinement of</h3> |
| |
| <a href="../../utility/CopyConstructible.html">Copy Constructible</a> |
| |
| <h3>Notation</h3> |
| |
| <Table> |
| <TR> |
| <TD><tt>PMap</tt></TD> |
| <TD>A type that is a model of Readable Property Map.</TD> |
| </TR> |
| <TR> |
| <TD><tt>pmap</tt></TD> |
| <TD>An object of type <tt>PMap</tt>.</td> |
| </TR> |
| <TR> |
| <TD><tt>key</tt></TD> |
| <TD>An object of type <tt>boost::property_traits<PMap>::key_type</tt>.</td> |
| </TR> |
| |
| </table> |
| |
| <h3>Associated Types</h3> |
| |
| <table border> |
| |
| <tr> |
| <td>Value Type</td> |
| <td><TT>boost::property_traits<PMap>::value_type</TT></td> |
| <td>The type of the property.</td> |
| </tr> |
| |
| <tr> |
| <td>Reference Type</td> |
| <td><TT>boost::property_traits<PMap>::reference</TT></td> |
| <td> |
| A type that is convertible to the value type. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Key Type</td> |
| <td><TT>boost::property_traits<PMap>::key_type</TT></td> |
| <td> |
| The type of the key object used to look up the property. The property |
| map may be templated on the key type, in which case this |
| typedef can be <TT>void</TT>. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Property Map Category</td> |
| <td><TT>boost::property_traits<PMap>::category</TT></td> |
| <td> |
| The category of the property: a type convertible to |
| <TT>readable_property_map_tag</TT>. |
| </td> |
| </tr> |
| |
| </table> |
| |
| <h3>Valid Expressions</h3> |
| |
| <table border> |
| <tr> |
| <th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th> |
| </tr> |
| |
| <tr> |
| <td>Get Property Value </td> |
| <TD><TT>get(pmap, key)</TT></TD> |
| <TD><TT>reference</TT></TD> |
| <TD>Lookup the property of the object identified by <TT>key</TT>.</TD> |
| </TR> |
| |
| </TABLE> |
| |
| <h3>Concept Checking Class</h3> |
| |
| <pre> template <class PMap, class Key> |
| struct ReadablePropertyMapConcept |
| { |
| typedef typename property_traits<PMap>::key_type key_type; |
| typedef typename property_traits<PMap>::category Category; |
| typedef boost::readable_property_map_tag ReadableTag; |
| void constraints() { |
| function_requires< ConvertibleConcept<Category, ReadableTag> >(); |
| |
| val = get(pmap, k); |
| } |
| PMap pmap; |
| Key k; |
| typename property_traits<PMap>::value_type val; |
| };</pre> |
| |
| <h3>See Also</h3> |
| |
| <a href="./property_map.html">Property map concepts</a> |
| |
| <h3>Design Notes</h3> |
| |
| At various times the name "read-only" was considered for |
| this concept. However, that name is inappropriate because concepts are |
| inherently positive, not negative. This becomes obvious when we define |
| the Read Write Property Map, which refines both the Readable Property |
| Map and the Writable Property Map concept. It would not make much |
| sense to combine "read-only" and "write-only" |
| concepts! |
| |
| <br> |
| <HR> |
| <TABLE> |
| <TR valign=top> |
| <TD nowrap>Copyright © 2000</TD><TD> |
| <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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