doc/named-param.dox
 author Alpar Juttner Sun, 21 Sep 2008 07:49:57 +0100 changeset 268 986d30f5c1c0 parent 267 1eb606fe5591 child 269 f5965bbf1353 permissions -rw-r--r--
Improvements in named-param.dox
     1 /* -*- C++ -*-

     2  *

     3  * This file is a part of LEMON, a generic C++ optimization library

     4  *

     5  * Copyright (C) 2003-2008

     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  */

    18

    19 /*!

    20

    21 \page named-param Named Parameters

    22

    23 \section named-func-param Named Function Parameters

    24

    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++ lack this amenity.

    29

    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.

    33

    34 \code

    35 class namedFn

    36 {

    37   int _id;

    38   double _val;

    39   int _dim;

    40

    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; }

    46

    47   run() {

    48   std::cout << "Here comes the function itself\n" <<

    49             << "With parameters "

    50             << _id << ", " << _val << ", " << _dim << std::endl;

    51   }

    52 };

    53 \endcode

    54

    55 Then you can use it like this.

    56

    57 \code

    58 namedFn().id(3).val(2).run();

    59 \endcode

    60

    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.

    64

    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.

    67

    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>

    76

    77 \section named-templ-func-param Named Function Template Parameters

    78

    79 A named parameter can also be a template functions. 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.

    82

    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.

    85

    86 \section traits-classes Traits Classes

    87

    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.

    93

    94 See \ref lemon::DijkstraDefaultTraits for an

    95 example how a traits class implementation looks like.

    96

    97 \section named-templ-param Named Class Template Parameters

    98

    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.

   104

   105 \code

   106 Dijkstra<>::SetPredNodeMap<NullMap<Node,Node> >::Create

   107 \endcode

   108

   109 It can also be used in conjunction with other named template

   110 parameters in arbitrary order.

   111

   112 \code

   113 Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Edge> >::Create

   114 \endcode

   115

   116 The result will be an instantiated Dijkstra class, in which the

   117 DistMap and the PredMap is modified.

   118

   119 */