COIN-OR::LEMON - Graph Library

source: lemon/doc/named-param.dox @ 1351:2f479109a71d

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Documentation for VF2 (#597)

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[269]1/* -*- mode: C++; indent-tabs-mode: nil; -*-
[267]2 *
[269]3 * This file is a part of LEMON, a generic C++ optimization library.
[267]4 *
[463]5 * Copyright (C) 2003-2009
[267]6 * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
7 * (Egervary Research Group on Combinatorial Optimization, EGRES).
8 *
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.
12 *
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
15 * purpose.
16 *
17 */
21\page named-param Named Parameters
23\section named-func-param Named Function Parameters
[268]25Several modern languages provide a convenient way to refer the
26function parameters by name also when you call the function. It is
27especially comfortable in case of a function having tons of parameters
[1351]28with natural default values. Sadly, C++ lacks this amenity.
30However, with a crafty trick and with some little
31inconvenience, it is possible to emulate is.
32The example below shows how to do it.
[269]35class namedFn
37  int _id;
38  double _val;
39  int _dim;
[267]41  public:
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; }
47  run() {
[269]48    std::cout << "Here comes the function itself\n" <<
49              << "With parameters "
50              << _id << ", " << _val << ", " << _dim << std::endl;
[267]51  }
[268]55Then you can use it like this.
[268]61The trick is obvious, each "named parameter" changes one component of
62the underlying class, then gives back a reference to it. Finally,
63<tt>run()</tt> executes the algorithm itself.
[267]65\note Although it is a class, namedFn is used pretty much like as it were
[268]66a function. That it why we called it namedFn instead of \c NamedFn.
[268]68\note In fact, the final <tt>.run()</tt> could be made unnecessary,
69because the algorithm could also be implemented in the destructor of
70\c namedFn instead. This however would make it impossible to implement
71functions with return values, and would also cause serious problems when
72implementing \ref named-templ-func-param "named template parameters".
73<b>Therefore, by convention, <tt>.run()</tt> must be used
74explicitly to execute a function having named parameters
75everywhere in LEMON.</b>
[268]77\section named-templ-func-param Named Function Template Parameters
[269]79A named parameter can also be a template function. The usage is
[268]80exactly the same, but the implementation behind is a kind of black
81magic and they are the dirtiest part of the LEMON code.
83You will probably never need to know how it works, but if you really
84committed, have a look at \ref lemon/graph_to_eps.h for an example.
86\section traits-classes Traits Classes
[268]88A similar game can also be played when defining classes. In this case
89the type of the class attributes can be changed. Initially we have to
90define a special class called <em>Traits Class</em> defining the
91default type of the attributes. Then the types of these attributes can
92be changed in the same way as described in the next section.
94See \ref lemon::DijkstraDefaultTraits for an
95example how a traits class implementation looks like.
97\section named-templ-param Named Class Template Parameters
99If we would like to change the type of an attribute in a class that
100was instantiated by using a traits class as a template parameter, and
[268]101the class contains named parameters, we do not have to instantiate again
102the class with new traits class, but instead adaptor classes can
103be used as shown in the following example.
[269]106Dijkstra<>::SetPredMap<NullMap<Node,Arc> >::Create
109It can also be used in conjunction with other named template
110parameters in arbitrary order.
[269]113Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Arc> >::Create
116The result will be an instantiated Dijkstra class, in which the
117DistMap and the PredMap is modified.
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