123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596 |
- <HTML>
- <!--
- Copyright (c) Jeremy Siek 2000
-
- Distributed under the Boost Software License, Version 1.0.
- (See accompanying file LICENSE_1_0.txt or copy at
- http://www.boost.org/LICENSE_1_0.txt)
- -->
- <Head>
- <Title>Boost Graph Library: Named Parameters</Title>
- <BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
- ALINK="#ff0000">
- <IMG SRC="../../../boost.png"
- ALT="C++ Boost" width="277" height="86">
- <BR Clear>
- <H1><A NAME="sec:bgl-named-params"></A>
- <pre>
- bgl_named_params<Param, Type, Rest>
- </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(&distance[0]).
- predecessor_map(&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(&parent[0]).
- distance_map(&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 © 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>
- </BODY>
- </HTML>
|