3 \page named-param Named Parameters
5 \section named-func-param Named Function Parameters
7 C++ makes it possible to use default parameter values when calling a
8 function. In such a case we do not have to give value for parameters,
9 the program will use the default ones. Unfortunately sometimes this
10 is not enough. If we do not want to give values for all the
11 parameters, only for some of them we come across problems, because an
12 arbitrary set of parameters cannot be omitted. On the other hand
13 parameters have a fixed order in the head of the function. C++ can
14 apply the default values only in the back of the order, if we do not
15 give other value for them. So we can not give the function for
16 example the value of the first, and the third parameter, expecting
17 that the program will aplly the default value for the second
18 parameter. However sometimes we would like to use some functinos
19 exactly in this way. With a crafty trick and with some little
20 inconvenience this is possible. We have implemented this little trick
31 namedFn() : _id(0), _val(1), _dim(2) {}
32 namedFn& id(int p) { _id = p ; return *this; }
33 namedFn& val(double p) { _val = p ; return *this; }
34 namedFn& dim(int p) { _dim = p ; return *this; }
37 printf("Here is the function itself.");
43 The usage is the following.
45 We have to define a class, let's call it \c namedFn. Let us assume that
46 we would like to use a parameter, called \c X. In the \c namedFn class we
47 have to define an \c _X attribute, and a function \c X. The function
48 expects 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
50 class also have to have a function, called for example <tt>run()</tt>, we have
51 to implement here the original function itself. The constructor of the
52 class have to give all the attributes like \c _X the default values of
55 If we instantiate this class, the default values will be set for the
56 attributes (originally the parameters), initially. If we call function
57 \c X, we get a class with the modified parameter value of
58 \c X. Therefore we can modify any parameter-value, independently from the
59 order. To run the algorithm we have to call the <tt>run()</tt> function at the
64 namedFn().id(3).val(2).run();
67 \note Although it is a class, namedFn is used pretty much like as it were
68 a function. That it why it is called namedFn and not \c NamedFn.
70 \note In fact, the final <tt>.run()</tt> could be made unnecessary if the
71 actual function code were put in the destructor instead. This however would make
72 hard to implement functions with return values, and would also make the
73 implementation of \ref named-templ-func-param "named template parameters"
74 very problematic. Therefore, by convention, <tt>.run()</tt> is used
75 to explicitly execute function having named parameters in Lemon.
78 \section traits-classes Traits Classes
80 The procedure above can also be applied when defining classes. In this
81 case the type of the attributes can be changed. Initially we have to
82 define a class with the default attribute types. This is the so called
83 Traits Class. Later on the types of these attributes can be changed,
84 as described below. In our software \ref lemon::DijkstraDefaultTraits is an
85 example of how a traits class looks like.
87 \section named-templ-param Named Class Template Parameters
89 If we would like to change the type of an attribute in a class that
90 was instantiated by using a traits class as a template parameter, and
91 the class contains named parameters, we do not have to reinstantiate
92 the class with new traits class. Instead of that, adaptor classes can
93 be used like in the following cases.
96 Dijkstra<>::SetPredNodeMap<NullMap<Node,Node> >::Create
99 It can also be used in conjunction with other named template
100 parameters in arbitrary order.
103 Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Edge> >::Create
106 The result will be an instantiated Dijkstra class, in which the
107 DistMap and the PredMap is modified.
109 \section named-templ-func-param Named Function Template Parameters
111 If the class has so called wizard functions, the new class with the
112 modified tpye of attributes can be returned by the appropriate wizard
113 function. The usage of these wizard functions is the following: