COIN-OR::LEMON - Graph Library

source: lemon-0.x/doc/named-param.dox @ 1660:93792a112fd5

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[953]3\page named-param Named Parameters
[1624]5\section named-func-param Named Function Parameters
[1536]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.
24class named_fn
26  int _id;
27  double _val;
28  int _dim;
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; }
36  run() {
37    printf("Here is the function itself.");
38  }
43The usage is the following.
[1624]45We have to define a class, let's call it \c named_fn.  Let us assume that
46we would like to use a parameter, called \c X. In the \c named_fn class we
47have to define an \c _X attribute, and a function \c X. The function
48expects a parameter with the type of \c _X, and sets the value of
49\c _X. After setting the value the function returns the class itself. The
50class also have to have a function, called for example <tt>run()</tt>, we have
[1536]51to implement here the original function itself. The constructor of the
[1624]52class have to give all the attributes like \c _X the default values of
[1536]55If we instantiate this class, the default values will be set for the
[1619]56attributes (originally the parameters), initially. If we call function
[1624]57\c X, we get a class with the modified parameter value of
58\c X. Therefore we can modify any parameter-value, independent from the
59order. To run the algorithm we have to call the <tt>run()</tt> function at the
[1536]60end of the row.
67\section traits-classes Traits Classes
[1536]69The procedure above can also be applied when defining classes. In this
70case the type of the attributes can be changed.  Initially we have to
71define a class with the default attribute types. This is the so called
72Traits Class. Later on the types of these attributes can be changed,
73as described below. In our software \ref lemon::DijkstraDefaultTraits is an
74example of how a traits class looks like.
[955]76\section named-templ-param Named Class Template Parameters
[1536]78If we would like to change the type of an attribute in a class that
79was instantiated by using a traits class as a template parameter, and
80the class contains named parameters, we do not have to reinstantiate
81the class with new traits class. Instead of that, adaptor classes can
82be used like in the following cases.
85Dijkstra<>::SetPredNodeMap<NullMap<Node,Node> >
[954]88It can also be used in conjunction with other named template
89parameters in arbitrary order.
92Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Edge> >
[1536]95The result will be an instantiated Dijkstra class, in which the
96DistMap and the PredMap is modified.
[1624]98\section named-templ-func-param Named Function Template Parameters
[1536]100If the class has so called wizard functions, the new class with the
101modified tpye of attributes can be returned by the appropriate wizard
102function. The usage of these wizard functions is the following:
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