doc/named-param.dox
author Alpar Juttner <alpar@cs.elte.hu>
Wed, 17 Oct 2018 22:55:02 +0200
changeset 1414 73e29215aaa4
parent 463 88ed40ad0d4f
permissions -rw-r--r--
Add citation for Vf2pp (#597)
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/* -*- mode: C++; indent-tabs-mode: nil; -*-
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 *
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 * This file is a part of LEMON, a generic C++ optimization library.
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 *
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 * Copyright (C) 2003-2009
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 * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
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 * (Egervary Research Group on Combinatorial Optimization, EGRES).
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 *
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 * Permission to use, modify and distribute this software is granted
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 * provided that this copyright notice appears in all copies. For
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 * precise terms see the accompanying LICENSE file.
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 *
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 * This software is provided "AS IS" with no warranty of any kind,
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 * express or implied, and with no claim as to its suitability for any
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 * purpose.
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 *
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 */
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/*!
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\page named-param Named Parameters
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\section named-func-param Named Function Parameters
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Several modern languages provide a convenient way to refer the
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function parameters by name also when you call the function. It is
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especially comfortable in case of a function having tons of parameters
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with natural default values. Sadly, C++ lacks this amenity.
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However, with a crafty trick and with some little
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inconvenience, it is possible to emulate is.
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The example below shows how to do it.
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\code
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class namedFn
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{
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  int _id;
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  double _val;
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  int _dim;
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  public:
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  namedFn() : _id(0), _val(1), _dim(2) {}
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  namedFn& id(int p)     { _id  = p ; return *this; }
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  namedFn& val(double p) { _val = p ; return *this; }
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  namedFn& dim(int p)    { _dim = p ; return *this; }
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  run() {
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    std::cout << "Here comes the function itself\n" <<
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              << "With parameters "
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              << _id << ", " << _val << ", " << _dim << std::endl;
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  }
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};
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\endcode
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Then you can use it like this.
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\code
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namedFn().id(3).val(2).run();
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\endcode
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The trick is obvious, each "named parameter" changes one component of
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the underlying class, then gives back a reference to it. Finally,
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<tt>run()</tt> executes the algorithm itself.
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\note Although it is a class, namedFn is used pretty much like as it were
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a function. That it why we called it namedFn instead of \c NamedFn.
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\note In fact, the final <tt>.run()</tt> could be made unnecessary,
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because the algorithm could also be implemented in the destructor of
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\c namedFn instead. This however would make it impossible to implement
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functions with return values, and would also cause serious problems when
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implementing \ref named-templ-func-param "named template parameters".
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<b>Therefore, by convention, <tt>.run()</tt> must be used
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explicitly to execute a function having named parameters
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everywhere in LEMON.</b>
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\section named-templ-func-param Named Function Template Parameters
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A named parameter can also be a template function. The usage is
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exactly the same, but the implementation behind is a kind of black
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magic and they are the dirtiest part of the LEMON code.
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You will probably never need to know how it works, but if you really
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committed, have a look at \ref lemon/graph_to_eps.h for an example.
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\section traits-classes Traits Classes
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A similar game can also be played when defining classes. In this case
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the type of the class attributes can be changed. Initially we have to
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define a special class called <em>Traits Class</em> defining the
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default type of the attributes. Then the types of these attributes can
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be changed in the same way as described in the next section.
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See \ref lemon::DijkstraDefaultTraits for an
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example how a traits class implementation looks like.
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\section named-templ-param Named Class Template Parameters
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If we would like to change the type of an attribute in a class that
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was instantiated by using a traits class as a template parameter, and
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the class contains named parameters, we do not have to instantiate again
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the class with new traits class, but instead adaptor classes can
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be used as shown in the following example.
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\code
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Dijkstra<>::SetPredMap<NullMap<Node,Arc> >::Create
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\endcode
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It can also be used in conjunction with other named template
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parameters in arbitrary order.
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\code
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Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Arc> >::Create
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\endcode
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The result will be an instantiated Dijkstra class, in which the
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DistMap and the PredMap is modified.
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*/