COIN-OR::LEMON - Graph Library

source: lemon-0.x/doc/named-param.dox @ 2553:bfced05fa852

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1/* -*- C++ -*-
2 *
3 * This file is a part of LEMON, a generic C++ optimization library
4 *
5 * Copyright (C) 2003-2008
6 * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
7 * (Egervary Research Group on Combinatorial Optimization, EGRES).
8 *
9 * Permission to use, modify and distribute this software is granted
10 * provided that this copyright notice appears in all copies. For
11 * precise terms see the accompanying LICENSE file.
12 *
13 * This software is provided "AS IS" with no warranty of any kind,
14 * express or implied, and with no claim as to its suitability for any
15 * purpose.
16 *
17 */
18
19/*!
20
21\page named-param Named Parameters
22
23\section named-func-param Named Function Parameters
24
25C++ makes it possible to use default parameter values when calling a
26function. In such a case we do not have to give value for parameters,
27the program will use the default ones.  Unfortunately sometimes this
28is not enough. If we do not want to give values for all the
29parameters, only for some of them we come across problems, because an
30arbitrary set of parameters cannot be omitted. On the other hand
31parameters have a fixed order in the head of the function.  C++ can
32apply the default values only in the back of the order, if we do not
33give other value for them.  So we can not give the function for
34example the value of the first, and the third parameter, expecting
35that the program will aplly the default value for the second
36parameter.  However sometimes we would like to use some functinos
37exactly in this way. With a crafty trick and with some little
38inconvenience this is possible. We have implemented this little trick
39as an example below.
40
41\code
42class namedFn
43{
44  int _id;
45  double _val;
46  int _dim;
47 
48  public:
49  namedFn() : _id(0), _val(1), _dim(2) {}
50  namedFn& id(int p)     { _id  = p ; return *this; }
51  namedFn& val(double p) { _val = p ; return *this; }
52  namedFn& dim(int p)    { _dim = p ; return *this; }
53
54  run() {
55    printf("Here is the function itself.");
56  }
57};
58\endcode
59
60
61The usage is the following.
62
63We have to define a class, let's call it \c namedFn.  Let us assume that
64we would like to use a parameter, called \c X. In the \c namedFn class we
65have to define an \c _X attribute, and a function \c X. The function
66expects a parameter with the type of \c _X, and sets the value of
67\c _X. After setting the value the function returns the class itself. The
68class also have to have a function, called for example <tt>run()</tt>, we have
69to implement here the original function itself. The constructor of the
70class have to give all the attributes like \c _X the default values of
71them.
72
73If we instantiate this class, the default values will be set for the
74attributes (originally the parameters), initially. If we call function
75\c X, we get a class with the modified parameter value of
76\c X. Therefore we can modify any parameter-value, independently from the
77order. To run the algorithm we have to call the <tt>run()</tt> function at the
78end of the row.
79
80Example:
81\code
82namedFn().id(3).val(2).run();
83\endcode
84
85\note Although it is a class, namedFn is used pretty much like as it were
86a function. That it why it is called namedFn and not \c NamedFn.
87
88\note In fact, the final <tt>.run()</tt> could be made unnecessary if the
89actual function code were put in the destructor instead. This however would make
90hard to implement functions with return values, and would also make the
91implementation of \ref named-templ-func-param "named template parameters"
92very problematic. <b>Therefore, by convention, <tt>.run()</tt> must be used
93to explicitly execute function having named parameters in Lemon.</b>
94
95
96\section traits-classes Traits Classes
97
98The procedure above can also be applied when defining classes. In this
99case the type of the attributes can be changed.  Initially we have to
100define a class with the default attribute types. This is the so called
101Traits Class. Later on the types of these attributes can be changed,
102as described below. In our software \ref lemon::DijkstraDefaultTraits is an
103example of how a traits class looks like.
104
105\section named-templ-param Named Class Template Parameters
106
107If we would like to change the type of an attribute in a class that
108was instantiated by using a traits class as a template parameter, and
109the class contains named parameters, we do not have to reinstantiate
110the class with new traits class. Instead of that, adaptor classes can
111be used like in the following cases.
112
113\code
114Dijkstra<>::SetPredNodeMap<NullMap<Node,Node> >::Create
115\endcode
116
117It can also be used in conjunction with other named template
118parameters in arbitrary order.
119
120\code
121Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Edge> >::Create
122\endcode
123
124The result will be an instantiated Dijkstra class, in which the
125DistMap and the PredMap is modified.
126
127\section named-templ-func-param Named Function Template Parameters
128
129If the class has so called wizard functions, the new class with the
130modified tpye of attributes can be returned by the appropriate wizard
131function. The usage of these wizard functions is the following:
132
133*/
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