1<HTML> 2<!-- 3 Copyright (c) Jeremy Siek 2000 4 5 Distributed under the Boost Software License, Version 1.0. 6 (See accompanying file LICENSE_1_0.txt or copy at 7 http://www.boost.org/LICENSE_1_0.txt) 8 --> 9<Head> 10<Title>Boost Graph Library: Named Parameters</Title> 11<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" 12 ALINK="#ff0000"> 13<IMG SRC="../../../boost.png" 14 ALT="C++ Boost" width="277" height="86"> 15 16<BR Clear> 17 18<H1><A NAME="sec:bgl-named-params"></A> 19<pre> 20bgl_named_params<Param, Type, Rest> 21</pre> 22</H1> 23 24<p> 25Many of the Boost.Graph algorithms have a long list of parameters, 26most of which have default values. This causes several problems. 27First, C++ does not provide a mechanism for handling default 28parameters of template functions. However, this can be overcome by 29creating multiply version of an algorithm with different numbers of 30parameters with each version providing defaults for some subset of 31the parameters. This is the approach used in previous versions of 32Boost.Graph. This solution is still unsatisfactory for several 33reasons: 34 35<ul> 36 <li>The defaults for parameters can only been used in a particular 37 order. If the ordering of the defaults does not fit the users situation 38 he or she has to resort to providing all the parameters. 39 40 <li>Since the list of parameters is long, it is easy to forget 41 the ordering. 42</ul> 43 44<p> 45A better solution is provided by <tt>bgl_named_params</tt>. This class 46allows users to provide parameters is any order, and matches 47arguments to parameters based on parameter names. 48 49<p> 50The following code shows an example of calling 51<tt>bellman_ford_shortest_paths</tt> using the named parameter 52technique. Each of the arguments is passed to a function whose name 53indicates which parameter the argument is for. Each of the named 54parameters is separated by a <b>period</b>, not a comma. 55 56<pre> 57 bool r = boost::bellman_ford_shortest_paths(g, int(N), 58 boost::weight_map(weight). 59 distance_map(&distance[0]). 60 predecessor_map(&parent[0])); 61</pre> 62 63<p>The order in which the arguments are provided does not matter as 64long as they are matched with the correct parameter function. Here is 65an call to <tt>bellman_ford_shortest_paths</tt> that is equivalent to 66the one above. 67 68<pre> 69 bool r = boost::bellman_ford_shortest_paths(g, int(N), 70 boost::predecessor_map(&parent[0]). 71 distance_map(&distance[0]). 72 weight_map(weight)); 73</pre> 74 75<p>Typically the user never needs to deal with the 76<tt>bgl_named_params</tt> class directly, since there are functions 77like <tt>boost::weight_map</tt> that create an instance of 78<tt>bgl_named_params</tt>. 79 80 81<br> 82<HR> 83<TABLE> 84<TR valign=top> 85<TD nowrap>Copyright © 2000-2001</TD><TD> 86<A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>, 87Indiana University (<A 88HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br> 89<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> 90<A HREF="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>, 91Indiana University (<A 92HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>) 93</TD></TR></TABLE> 94 95</BODY> 96</HTML> 97
