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