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<Title>Property Map Library</Title>
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<H1><A NAME="sec:property-maps"></A>
Boost Property Map Library
</H1>

<p>The Boost Property Map Library consists mainly of interface
specifications in the form of concepts (similar to the iterator
concepts in the STL <a
href="http://www.sgi.com/tech/stl/stl_introduction.html">[2]</a>).
These interface specifications are intended for use by implementors of
generic libraries in communicating requirements on template parameters
to their users.  In particular, the Boost Property Map concepts define
a general purpose interface for mapping key objects to corresponding
value objects, thereby hiding the details of how the mapping is
implemented from algorithms. The implementation of types fulfilling
the property map interface is up to the client of the algorithm to
provide. The property map requirements are purposefully vague on the
type of the key and value objects to allow for the utmost genericity
in the function templates of the generic library.
</p>

<p>
The need for the property map interface came from the <a
href="../../graph/doc/index.html">Boost Graph Library</a> (BGL), which
contains many examples of algorithms that use the property map
concepts to specify their interface. For an example, note the
<tt>ColorMap</tt> template parameter of the <a
href="../../graph/doc/breadth_first_search.html">
<tt>breadth_first_search</tt></a>. In addition, the BGL contains many
examples of concrete types that implement the property map interface.
The <a href="../../graph/doc/adjacency_list.html">
<tt>adjacency_list</tt></a> class implements property maps for
accessing objects (properties) that are attached to vertices and edges
of the graph.
</p>

<p>
The Boost Property Map Library also contains a <a
href="#sec:property-map-types"> few adaptors</a> that convert commonly
used data-structures that implement a mapping operation, such as
builtin arrays (pointers), iterators, and <a
href="http://www.sgi.com/tech/stl/Map.html"> <tt>std::map</tt></a>, to
have the property map interface. These adaptors are not meant to
fulfill all mapping needs, but are to serve as an example of how to
implement the interface as well as covering a few common cases. See
the header files for details.
</p>

<p>Property maps are statically-typed entities. If you need to access
property maps in a more dynamic setting (e.g., because you're reading
an unknown set of attributes from a file), you can use the <a
href="dynamic_property_map.html"><code>dynamic_properties</code></a>
class to access a set of property maps through a dynamically-typed
interface. </p>

<h2><A NAME="sec:property-map-concepts"></A>
Property Map Concepts
</h2>

The property map interface consists of a set of concepts (see
definition of &quot;concept&quot; in <a
href="../../concept_check/concept_check.htm#introduction">[1]</a> and <a
href="http://www.sgi.com/tech/stl/stl_introduction.html">[2]</a>) that
define a syntax for mapping key objects to corresponding value
objects. Since the property map operations are global functions
(actually they don't have to be global, but they are always called
unqualified and may be found via argument dependent lookup), it is
possible to overload the map functions such that nearly arbitrary
property map types and key types can be used. The interface for
property maps consists of three functions: <tt>get()</tt>,
<tt>put()</tt>, and <tt>operator[]</tt>. The following concrete
example from <a href="../example/example1.cpp">example1.cpp</a> shows how the
three functions could be used to access the addresses associated with
various people.  We use a separate function template here to highlight
the parts of the program that use the property map concept
interface. In the <tt>main()</tt> function we use <tt>std::map</tt>
and <tt>boost::associative_property_map</tt>, but it would have been
OK to use any type (including a custom type that you create) that
fulfills the property map requirements.

<pre>#include &lt;iostream&gt;
#include &lt;map&gt;
#include &lt;string&gt;
#include &lt;boost/property_map/property_map.hpp&gt;


template &lt;typename AddressMap&gt;
void foo(AddressMap address)
{
  typedef typename boost::property_traits&lt;AddressMap&gt;::value_type value_type;
  typedef typename boost::property_traits&lt;AddressMap&gt;::key_type key_type;

  value_type old_address, new_address;
  key_type fred = &quot;Fred&quot;;
  old_address = get(address, fred);
  new_address = &quot;384 Fitzpatrick Street&quot;;
  put(address, fred, new_address);

  key_type joe = &quot;Joe&quot;;
  value_type&amp; joes_address = address[joe];
  joes_address = &quot;325 Cushing Avenue&quot;;
}

int
main()
{
  std::map&lt;std::string, std::string&gt; name2address;
  boost::associative_property_map&lt; std::map&lt;std::string, std::string&gt; &gt;
    address_map(name2address);

  name2address.insert(make_pair(std::string(&quot;Fred&quot;), 
				std::string(&quot;710 West 13th Street&quot;)));
  name2address.insert(make_pair(std::string(&quot;Joe&quot;), 
				std::string(&quot;710 West 13th Street&quot;)));

  foo(address_map);
  
  for (std::map&lt;std::string, std::string&gt;::iterator i = name2address.begin();
       i != name2address.end(); ++i)
    std::cout &lt;&lt; i-&gt;first &lt;&lt; &quot;: &quot; &lt;&lt; i-&gt;second &lt;&lt; &quot;\n&quot;;

  return EXIT_SUCCESS;
}</pre>

<p>
For each property map object there is a set of <i>valid keys</i>
for which the mapping to value objects is defined.  Invoking a
property map function on an <i>invalid</i> key results in
undefined behavior. The property map concepts do not specify how
this set of valid keys is created or modified. A function that uses a
property map must specify the expected set of valid keys in its
preconditions.

<p>
The need for property maps came out of the design of the Boost
Graph Library, whose algorithms needed an interface for accessing
properties attached to vertices and edges in a graph. In this context
the vertex and edge descriptors are the key type of the property
maps.

<!-- historical note about Decorators and Data Maps -->

<P>
Several categories of property maps provide
different access capabilities:
<DL>
<DT><STRONG>readable</STRONG></DT>
<DD>The associated property data can only be read. 
  The data is returned by-value. Many property maps defining the
  problem input (such as edge weight) can be defined as readable
  property maps.

<P>
</DD>
<DT><STRONG>writeable</STRONG></DT>
<DD>The associated property can only be written to.
  The parent array used to record the paths in a bread-first search tree
  is an example of a property map that would be defined writeable.

<P>
</DD>
<DT><STRONG>read/write</STRONG></DT>
<DD>The associated property can both be written and read.
  The distance property use in Dijkstra's shortest paths algorithm
  would need to provide both read and write capabilities.

<P>
</DD>
<DT><STRONG>lvalue</STRONG></DT>
<DD>The associated property is actually represented in 
  memory and it is possible to get a reference to it. 
  The property maps in the lvalue
  category also support the requirements for read/write property
  maps.

<P>
</DD>
</DL>

<P>
There is a separate concept defined for each of the four property
map categories.  These property map concepts are listed
below, with links to the documentation for each of them.

<ul>
<li><a href="./ReadablePropertyMap.html">ReadablePropertyMap</a></li>
<li><a href="./WritablePropertyMap.html">WritablePropertyMap</a></li>
<li><a href="./ReadWritePropertyMap.html">ReadWritePropertyMap</a></li>
<li><a href="./LvaluePropertyMap.html">LvaluePropertyMap</a></li>
</ul>

<h2><a name="sec:property-map-tags">Property Map Category Tags</a></h2>

<P>
There is a tag struct for each of the categories of property
maps, which is defined in the header
<tt>&lt;boost/property_map/property_map.hpp&gt;</tt>.

<PRE>namespace boost {

  struct readable_property_map_tag { };

  struct writable_property_map_tag { };

  struct read_write_property_map_tag :
    public readable_property_map_tag,
    public writable_property_map_tag { };

  struct lvalue_property_map_tag : 
    public read_write_property_map_tag { };

}</PRE>

<h2><a name="sec:property-map-traits">Property Map Traits</a></h2>

<P>
Similar to the <TT>std::iterator_traits</TT> class of the STL, there
is a <TT>boost::property_traits</TT> class that can be used to deduce
the types associated with a property map type: the key and value
types, and the property map category. There is a specialization
of <TT>boost::property_traits</TT> so that pointers can be used as
property map objects. In addition, the property map
functions are overloaded for pointers. These traits classes and
functions are defined in <tt>&lt;boost/property_map/property_map.hpp&gt;</tt>.

<PRE>namespace boost {

  template &lt;typename PropertyMap&gt;
  struct property_traits {
     typedef typename PropertyMap::key_type key_type;
     typedef typename PropertyMap::value_type value_type;
     typedef typename PropertyMap::category category;
  };

}</PRE>

<h2><a name="sec:property-map-types">Property Map Types</a></h2>

<ul>
  <li>Builtin C++ pointer types.<br>

    The following specialization of the <tt>property_traits</tt> class
    and the overloads of <tt>put()</tt> and <tt>get()</tt> make it
    possible to use builtin C++ pointer types as property maps. These
    are defined in <tt>boost/property_map/property_map.hpp</tt>. More specifically,
    it means that <tt>T*</tt> is a model of <a
    href="./LvaluePropertyMap.html">LvaluePropertyMap</a>, given a key
    type that is at least convertible <tt>std::ptrdiff_t</tt>.

<PRE>namespace boost {
  // specialization for using pointers as property maps
  template &lt;typename T&gt;
  struct property_traits&lt;T*&gt; {
    typedef T value_type;
    typedef std::ptrdiff_t key_type;
    typedef random_access_iterator_pa_tag category;
  };

  // overloads of the property map functions for pointers
  template&lt;&gt;
  void put(T* pmap, std::ptrdiff_t k, const T&amp; val) { pmap[k] = val;  }

  template&lt;&gt;
  const T&amp; get(const T* pmap, std::ptrdiff_t k) { return pmap[k]; }

}</PRE>
  </li>
  <li><a href="./identity_property_map.html">identity_property_map</a> </li>
  <li><a href="./iterator_property_map.html">iterator_property_map</a></li>
  <li><a href="./shared_array_property_map.html">shared_array_property_map</a></li>
  <li><a href="./associative_property_map.html">associative_property_map</a></li>
  <li><a href="./const_assoc_property_map.html">const_associative_property_map</a></li>
  <li><a href="./vector_property_map.html">vector_property_map</a></li>
  <li><a href="./ref_property_map.html">ref_property_map</a> </li>
</ul>

<h3>History</h3>

The property map interface originated as <i>data accessors</i> in
Dietmar K&uuml;hl's Masters Thesis on generic graph algorithms.  The
property map idea also appeared under the guise of <i>decorators</i>
in early versions of the Generic Graph Component Library (GGCL), which
is now the Boost Graph Library (BGL).  The main motivation for the
property map interface was to support the access of data associated
with vertices and edges in a graph, though the applicability of
property maps goes beyond this.

<h3>Acknowledgments</h3>

Thanks go to Dietmar K&uuml;hl for coming up with this mechanism, and
thanks go to the Boost members who helped refine and improve the
property map interface. Thanks to Dave Abrahams for managing the
formal review of the BGL which included the property map library.

<h3>Notes to Implementors</h3>

Copying a property map should be inexpensive since they are often
passed by value.

<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright &copy 2000-2002</TD><TD>
<a HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</a>, Indiana University (<A HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)
</TD></TR></TABLE>

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