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     Copyright (c) Jeremy Siek 2000
    
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<Head>
<Title>Boost Graph Library: Named Parameters</Title>
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<H1><A NAME="sec:bgl-named-params"></A>
<pre>
bgl_named_params&lt;Param, Type, Rest&gt;
</pre>
</H1>

<p>
Many of the Boost.Graph algorithms have a long list of parameters,
most of which have default values. This causes several problems.
First, C++ does not provide a mechanism for handling default
parameters of template functions. However, this can be overcome by
creating multiply version of an algorithm with different numbers of
parameters with each version providing defaults for some subset of
the parameters. This is the approach used in previous versions of
Boost.Graph. This solution is still unsatisfactory for several
reasons:

<ul>
  <li>The defaults for parameters can only been used in a particular
  order. If the ordering of the defaults does not fit the users situation
  he or she has to resort to providing all the parameters.

  <li>Since the list of parameters is long, it is easy to forget
   the ordering. 
</ul>

<p>
A better solution is provided by <tt>bgl_named_params</tt>. This class
allows users to provide parameters is any order, and matches
arguments to parameters based on parameter names.

<p>
The following code shows an example of calling
<tt>bellman_ford_shortest_paths</tt> using the named parameter
technique. Each of the arguments is passed to a function whose name
indicates which parameter the argument is for. Each of the named
parameters is separated by a <b>period</b>, not a comma.

<pre>
  bool r = boost::bellman_ford_shortest_paths(g, int(N), 
     boost::weight_map(weight).
     distance_map(&amp;distance[0]).
     predecessor_map(&amp;parent[0]));
</pre>

<p>The order in which the arguments are provided does not matter as
long as they are matched with the correct parameter function.  Here is
an call to <tt>bellman_ford_shortest_paths</tt> that is equivalent to
the one above.

<pre>
  bool r = boost::bellman_ford_shortest_paths(g, int(N), 
     boost::predecessor_map(&amp;parent[0]).
     distance_map(&amp;distance[0]).
     weight_map(weight));
</pre>

<p>Typically the user never needs to deal with the
<tt>bgl_named_params</tt> class directly, since there are functions
like <tt>boost::weight_map</tt> that create an instance of
<tt>bgl_named_params</tt>.


<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright &copy; 2000-2001</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>)<br>
<A HREF="http://www.boost.org/people/liequan_lee.htm">Lie-Quan Lee</A>, Indiana University (<A HREF="mailto:llee@cs.indiana.edu">llee@cs.indiana.edu</A>)<br>
<A HREF="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>,
Indiana University (<A
HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
</TD></TR></TABLE>

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