COIN-OR::LEMON - Graph Library

source: lemon-0.x/doc/named-param.dox @ 1619:f0700b9e6418

Last change on this file since 1619:f0700b9e6418 was 1619:f0700b9e6418, checked in by Hegyi Péter, 14 years ago

X function -> function X

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1*!
2
3\page named-param Named Parameters
4
5\section named-func-param Named "Function" Parameters
6
7C++ makes it possible to use default parameter values when calling a
8function. In such a case we do not have to give value for parameters,
9the program will use the default ones.  Unfortunately sometimes this
10is not enough. If we do not want to give values for all the
11parameters, only for some of them we come across problems, because an
12arbitrary set of parameters cannot be omitted. On the other hand
13parameters have a fixed order in the head of the function.  C++ can
14apply the default values only in the back of the order, if we do not
15give other value for them.  So we can not give the function for
16example the value of the first, and the third parameter, expecting
17that the program will aplly the default value for the second
18parameter.  However sometimes we would like to use some functinos
19exactly in this way. With a crafty trick and with some little
20inconvenience this is possible. We have implemented this little trick
21as an example below.
22
23\code
24class named_fn
25{
26  int _id;
27  double _val;
28  int _dim;
29 
30  public:
31  named_fn() : _id(0), _val(1), _dim(2) {}
32  named_fn& id(int p)     { _id  = p ; return *this; }
33  named_fn& val(double p) { _val = p ; return *this; }
34  named_fn& dim(int p)    { _dim = p ; return *this; }
35
36  run() {
37    printf("Here is the function itself.");
38  }
39};
40\endcode
41
42
43The usage is the following.
44
45We have to define a class, let's call it named_fn.  Let us assume that
46we would like to use a parameter, called X. In the named_fn class we
47have to define an _X attribute, and a function X. The function
48expects a parameter with the type of _X, and sets the value of
49_X. After setting the value the function returns the class itself. The
50class also have to have a function, called for example run(), we have
51to implement here the original function itself. The constructor of the
52class have to give all the attributes like _X the default values of
53them.
54
55If we instantiate this class, the default values will be set for the
56attributes (originally the parameters), initially. If we call function
57X, we get a class with the modified parameter value of
58X. Therefore we can modify any parameter-value, independent from the
59order. To run the algorithm we have to call the run() function at the
60end of the row.
61
62Example: named_fn().id(3).val(2).run();
63
64\section traits-classes Traits Classes
65
66The procedure above can also be applied when defining classes. In this
67case the type of the attributes can be changed.  Initially we have to
68define a class with the default attribute types. This is the so called
69Traits Class. Later on the types of these attributes can be changed,
70as described below. In our software \ref lemon::DijkstraDefaultTraits is an
71example of how a traits class looks like.
72
73\section named-templ-param Named Class Template Parameters
74
75If we would like to change the type of an attribute in a class that
76was instantiated by using a traits class as a template parameter, and
77the class contains named parameters, we do not have to reinstantiate
78the class with new traits class. Instead of that, adaptor classes can
79be used like in the following cases.
80
81\code
82Dijkstra<>::SetPredNodeMap<NullMap<Node,Node> >
83\endcode
84
85It can also be used in conjunction with other named template
86parameters in arbitrary order.
87
88\code
89Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Edge> >
90\endcode
91
92The result will be an instantiated Dijkstra class, in which the
93DistMap and the PredMap is modified.
94
95\section named-templ-func-param Named "Function" Template Parameters
96
97If the class has so called wizard functions, the new class with the
98modified tpye of attributes can be returned by the appropriate wizard
99function. The usage of these wizard functions is the following:
100
101*/
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