COIN-OR::LEMON - Graph Library

source: lemon-0.x/doc/named-param.dox @ 1333:2640cf6547ff

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