projects / ceph.git / blame
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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> | |
| 20 | bgl_named_params<Param, Type, Rest> | |
| 21 | </pre> | |
| 22 | </H1> | |
| 23 | ||
| 24 | <p> | |
| 25 | Many of the Boost.Graph algorithms have a long list of parameters, | |
| 26 | most of which have default values. This causes several problems. | |
| 27 | First, C++ does not provide a mechanism for handling default | |
| 28 | parameters of template functions. However, this can be overcome by | |
| 29 | creating multiply version of an algorithm with different numbers of | |
| 30 | parameters with each version providing defaults for some subset of | |
| 31 | the parameters. This is the approach used in previous versions of | |
| 32 | Boost.Graph. This solution is still unsatisfactory for several | |
| 33 | reasons: | |
| 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> | |
| 45 | A better solution is provided by <tt>bgl_named_params</tt>. This class | |
| 46 | allows users to provide parameters is any order, and matches | |
| 47 | arguments to parameters based on parameter names. | |
| 48 | ||
| 49 | <p> | |
| 50 | The following code shows an example of calling | |
| 51 | <tt>bellman_ford_shortest_paths</tt> using the named parameter | |
| 52 | technique. Each of the arguments is passed to a function whose name | |
| 53 | indicates which parameter the argument is for. Each of the named | |
| 54 | parameters 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 | |
| 64 | long as they are matched with the correct parameter function. Here is | |
| 65 | an call to <tt>bellman_ford_shortest_paths</tt> that is equivalent to | |
| 66 | the 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 | |
| 77 | like <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>, | |
| 87 | Indiana University (<A | |
| 88 | HREF="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="http://www.osl.iu.edu/~lums">Andrew Lumsdaine</A>, | |
| 91 | Indiana University (<A | |
| 92 | HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>) | |
| 93 | </TD></TR></TABLE> | |
| 94 | ||
| 95 | </BODY> | |
| 96 | </HTML> |