1 /* -*- mode: C++; indent-tabs-mode: nil; -*-
3 * This file is a part of LEMON, a generic C++ optimization library.
5 * Copyright (C) 2003-2009
6 * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
7 * (Egervary Research Group on Combinatorial Optimization, EGRES).
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.
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
21 \page named-param Named Parameters
23 \section named-func-param Named Function Parameters
25 Several modern languages provide a convenient way to refer the
26 function parameters by name also when you call the function. It is
27 especially comfortable in case of a function having tons of parameters
28 with natural default values. Sadly, C++ lacks this amenity.
30 However, with a crafty trick and with some little
31 inconvenience, it is possible to emulate is.
32 The example below shows how to do it.
42 namedFn() : _id(0), _val(1), _dim(2) {}
43 namedFn& id(int p) { _id = p ; return *this; }
44 namedFn& val(double p) { _val = p ; return *this; }
45 namedFn& dim(int p) { _dim = p ; return *this; }
48 std::cout << "Here comes the function itself\n" <<
50 << _id << ", " << _val << ", " << _dim << std::endl;
55 Then you can use it like this.
58 namedFn().id(3).val(2).run();
61 The trick is obvious, each "named parameter" changes one component of
62 the underlying class, then gives back a reference to it. Finally,
63 <tt>run()</tt> executes the algorithm itself.
65 \note Although it is a class, namedFn is used pretty much like as it were
66 a function. That it why we called it namedFn instead of \c NamedFn.
68 \note In fact, the final <tt>.run()</tt> could be made unnecessary,
69 because the algorithm could also be implemented in the destructor of
70 \c namedFn instead. This however would make it impossible to implement
71 functions with return values, and would also cause serious problems when
72 implementing \ref named-templ-func-param "named template parameters".
73 <b>Therefore, by convention, <tt>.run()</tt> must be used
74 explicitly to execute a function having named parameters
75 everywhere in LEMON.</b>
77 \section named-templ-func-param Named Function Template Parameters
79 A named parameter can also be a template function. The usage is
80 exactly the same, but the implementation behind is a kind of black
81 magic and they are the dirtiest part of the LEMON code.
83 You will probably never need to know how it works, but if you really
84 committed, have a look at \ref lemon/graph_to_eps.h for an example.
86 \section traits-classes Traits Classes
88 A similar game can also be played when defining classes. In this case
89 the type of the class attributes can be changed. Initially we have to
90 define a special class called <em>Traits Class</em> defining the
91 default type of the attributes. Then the types of these attributes can
92 be changed in the same way as described in the next section.
94 See \ref lemon::DijkstraDefaultTraits for an
95 example how a traits class implementation looks like.
97 \section named-templ-param Named Class Template Parameters
99 If we would like to change the type of an attribute in a class that
100 was instantiated by using a traits class as a template parameter, and
101 the class contains named parameters, we do not have to instantiate again
102 the class with new traits class, but instead adaptor classes can
103 be used as shown in the following example.
106 Dijkstra<>::SetPredMap<NullMap<Node,Arc> >::Create
109 It can also be used in conjunction with other named template
110 parameters in arbitrary order.
113 Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Arc> >::Create
116 The result will be an instantiated Dijkstra class, in which the
117 DistMap and the PredMap is modified.