[269] | 1 | /* -*- mode: C++; indent-tabs-mode: nil; -*- |
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[267] | 2 | * |
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[269] | 3 | * This file is a part of LEMON, a generic C++ optimization library. |
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[267] | 4 | * |
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[463] | 5 | * Copyright (C) 2003-2009 |
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[267] | 6 | * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport |
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| 7 | * (Egervary Research Group on Combinatorial Optimization, EGRES). |
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| 8 | * |
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| 9 | * Permission to use, modify and distribute this software is granted |
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| 10 | * provided that this copyright notice appears in all copies. For |
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| 11 | * precise terms see the accompanying LICENSE file. |
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| 12 | * |
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| 13 | * This software is provided "AS IS" with no warranty of any kind, |
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| 14 | * express or implied, and with no claim as to its suitability for any |
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| 15 | * purpose. |
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| 16 | * |
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| 17 | */ |
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| 18 | |
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| 19 | /*! |
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| 20 | |
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| 21 | \page named-param Named Parameters |
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| 22 | |
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| 23 | \section named-func-param Named Function Parameters |
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| 24 | |
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[268] | 25 | Several modern languages provide a convenient way to refer the |
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| 26 | function parameters by name also when you call the function. It is |
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| 27 | especially comfortable in case of a function having tons of parameters |
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[269] | 28 | with natural default values. Sadly, C++ lack this amenity. |
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[268] | 29 | |
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| 30 | However, with a crafty trick and with some little |
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| 31 | inconvenience, it is possible to emulate is. |
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| 32 | The example below shows how to do it. |
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[267] | 33 | |
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| 34 | \code |
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[269] | 35 | class namedFn |
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[267] | 36 | { |
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| 37 | int _id; |
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| 38 | double _val; |
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| 39 | int _dim; |
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[269] | 40 | |
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[267] | 41 | public: |
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| 42 | namedFn() : _id(0), _val(1), _dim(2) {} |
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| 43 | namedFn& id(int p) { _id = p ; return *this; } |
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| 44 | namedFn& val(double p) { _val = p ; return *this; } |
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| 45 | namedFn& dim(int p) { _dim = p ; return *this; } |
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| 46 | |
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| 47 | run() { |
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[269] | 48 | std::cout << "Here comes the function itself\n" << |
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| 49 | << "With parameters " |
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| 50 | << _id << ", " << _val << ", " << _dim << std::endl; |
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[267] | 51 | } |
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| 52 | }; |
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| 53 | \endcode |
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| 54 | |
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[268] | 55 | Then you can use it like this. |
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[267] | 56 | |
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| 57 | \code |
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| 58 | namedFn().id(3).val(2).run(); |
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| 59 | \endcode |
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| 60 | |
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[268] | 61 | The trick is obvious, each "named parameter" changes one component of |
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| 62 | the underlying class, then gives back a reference to it. Finally, |
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| 63 | <tt>run()</tt> executes the algorithm itself. |
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| 64 | |
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[267] | 65 | \note Although it is a class, namedFn is used pretty much like as it were |
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[268] | 66 | a function. That it why we called it namedFn instead of \c NamedFn. |
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[267] | 67 | |
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[268] | 68 | \note In fact, the final <tt>.run()</tt> could be made unnecessary, |
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| 69 | because the algorithm could also be implemented in the destructor of |
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| 70 | \c namedFn instead. This however would make it impossible to implement |
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| 71 | functions with return values, and would also cause serious problems when |
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| 72 | implementing \ref named-templ-func-param "named template parameters". |
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| 73 | <b>Therefore, by convention, <tt>.run()</tt> must be used |
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| 74 | explicitly to execute a function having named parameters |
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| 75 | everywhere in LEMON.</b> |
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[267] | 76 | |
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[268] | 77 | \section named-templ-func-param Named Function Template Parameters |
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| 78 | |
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[269] | 79 | A named parameter can also be a template function. The usage is |
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[268] | 80 | exactly the same, but the implementation behind is a kind of black |
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| 81 | magic and they are the dirtiest part of the LEMON code. |
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| 82 | |
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| 83 | You will probably never need to know how it works, but if you really |
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| 84 | committed, have a look at \ref lemon/graph_to_eps.h for an example. |
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[267] | 85 | |
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| 86 | \section traits-classes Traits Classes |
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| 87 | |
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[268] | 88 | A similar game can also be played when defining classes. In this case |
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| 89 | the type of the class attributes can be changed. Initially we have to |
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| 90 | define a special class called <em>Traits Class</em> defining the |
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| 91 | default type of the attributes. Then the types of these attributes can |
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| 92 | be changed in the same way as described in the next section. |
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| 93 | |
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| 94 | See \ref lemon::DijkstraDefaultTraits for an |
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| 95 | example how a traits class implementation looks like. |
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[267] | 96 | |
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| 97 | \section named-templ-param Named Class Template Parameters |
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| 98 | |
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| 99 | If we would like to change the type of an attribute in a class that |
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| 100 | was instantiated by using a traits class as a template parameter, and |
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[268] | 101 | the class contains named parameters, we do not have to instantiate again |
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| 102 | the class with new traits class, but instead adaptor classes can |
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| 103 | be used as shown in the following example. |
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[267] | 104 | |
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| 105 | \code |
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[269] | 106 | Dijkstra<>::SetPredMap<NullMap<Node,Arc> >::Create |
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[267] | 107 | \endcode |
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| 108 | |
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| 109 | It can also be used in conjunction with other named template |
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| 110 | parameters in arbitrary order. |
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| 111 | |
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| 112 | \code |
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[269] | 113 | Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Arc> >::Create |
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[267] | 114 | \endcode |
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| 115 | |
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| 116 | The result will be an instantiated Dijkstra class, in which the |
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| 117 | DistMap and the PredMap is modified. |
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| 118 | |
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| 119 | */ |
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