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101 lines
3.5 KiB
HTML
<HTML>
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<!--
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-- Copyright (c) Jeremy Siek 2000
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--
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-- Permission to use, copy, modify, distribute and sell this software
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-- and its documentation for any purpose is hereby granted without fee,
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-- provided that the above copyright notice appears in all copies and
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-- that both that copyright notice and this permission notice appear
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-- in supporting documentation. Jeremy Siek makes no
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-- representations about the suitability of this software for any
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-- purpose. It is provided "as is" without express or implied warranty.
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-->
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<Head>
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<Title>Boost Graph Library: Named Parameters</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="../../../c++boost.gif"
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ALT="C++ Boost" width="277" height="86">
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<BR Clear>
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<H1><A NAME="sec:bgl-named-params"></A>
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<pre>
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bgl_named_params<Param, Type, Rest>
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</pre>
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</H1>
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<p>
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Many of the Boost.Graph algorithms have a long list of parameters,
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most of which have default values. This causes several problems.
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First, C++ does not provide a mechanism for handling default
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parameters of template functions. However, this can be overcome by
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creating multiply version of an algorithm with different numbers of
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parameters with each version providing defaults for some subset of
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the parameters. This is the approach used in previous versions of
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Boost.Graph. This solution is still unsatisfactory for several
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reasons:
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<ul>
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<li>The defaults for parameters can only been used in a particular
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order. If the ordering of the defaults does not fit the users situation
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he or she has to resort to providing all the parameters.
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<li>Since the list of parameters is long, it is easy to forget
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the ordering.
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</ul>
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<p>
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A better solution is provided by <tt>bgl_named_params</tt>. This class
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allows users to provide parameters is any order, and matches
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arguments to parameters based on parameter names.
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<p>
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The following code shows an example of calling
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<tt>bellman_ford_shortest_paths</tt> using the named parameter
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technique. Each of the arguments is passed to a function whose name
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indicates which parameter the argument is for. Each of the named
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parameters is separated by a <b>period</b>, not a comma.
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<pre>
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bool r = boost::bellman_ford_shortest_paths(g, int(N),
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boost::weight_map(weight).
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distance_map(&distance[0]).
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predecessor_map(&parent[0]));
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</pre>
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<p>The order in which the arguments are provided does not matter as
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long as they are matched with the correct parameter function. Here is
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an call to <tt>bellman_ford_shortest_paths</tt> that is equivalent to
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the one above.
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<pre>
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bool r = boost::bellman_ford_shortest_paths(g, int(N),
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boost::predecessor_map(&parent[0]).
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distance_map(&distance[0]).
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weight_map(weight));
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</pre>
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<p>Typically the user never needs to deal with the
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<tt>bgl_named_params</tt> class directly, since there are functions
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like <tt>boost::weight_map</tt> that create an instance of
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<tt>bgl_named_params</tt>.
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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 © 2000-2001</TD><TD>
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<A HREF="../../../people/jeremy_siek.htm">Jeremy Siek</A>,
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Indiana University (<A
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HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
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<A HREF="../../../people/liequan_lee.htm">Lie-Quan Lee</A>, Indiana University (<A HREF="mailto:llee@cs.indiana.edu">llee@cs.indiana.edu</A>)<br>
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<A HREF=http://www.osl.iu.edu/~lums>Andrew Lumsdaine</A>,
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Indiana University (<A
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HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
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</TD></TR></TABLE>
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</BODY>
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</HTML>
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