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| <Head> |
| <Title>Boost Graph Library: Named Parameters</Title> |
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| |
| <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="http://www.osl.iu.edu/~lums">Andrew Lumsdaine</A>, |
| Indiana University (<A |
| HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>) |
| </TD></TR></TABLE> |
| |
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