Changeset 268:986d30f5c1c0 in lemon-1.2
- Timestamp:
- 09/21/08 08:49:57 (16 years ago)
- Branch:
- default
- Phase:
- public
- File:
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- 1 edited
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doc/named-param.dox
r267 r268 23 23 \section named-func-param Named Function Parameters 24 24 25 C++ makes it possible to use default parameter values when calling a 26 function. In such a case we do not have to give value for parameters, 27 the program will use the default ones. Unfortunately sometimes this 28 is not enough. If we do not want to give values for all the 29 parameters, only for some of them we come across problems, because an 30 arbitrary set of parameters cannot be omitted. On the other hand 31 parameters have a fixed order in the head of the function. C++ can 32 apply the default values only in the back of the order, if we do not 33 give other value for them. So we can not give the function for 34 example the value of the first, and the third parameter, expecting 35 that the program will aplly the default value for the second 36 parameter. However sometimes we would like to use some functinos 37 exactly in this way. With a crafty trick and with some little 38 inconvenience this is possible. We have implemented this little trick 39 as an example below. 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. 40 33 41 34 \code … … 53 46 54 47 run() { 55 printf("Here is the function itself."); 48 std::cout << "Here comes the function itself\n" << 49 << "With parameters " 50 << _id << ", " << _val << ", " << _dim << std::endl; 56 51 } 57 52 }; 58 53 \endcode 59 54 55 Then you can use it like this. 60 56 61 The usage is the following.62 63 We have to define a class, let's call it \c namedFn. Let us assume that64 we would like to use a parameter, called \c X. In the \c namedFn class we65 have to define an \c _X attribute, and a function \c X. The function66 expects a parameter with the type of \c _X, and sets the value of67 \c _X. After setting the value the function returns the class itself. The68 class also have to have a function, called for example <tt>run()</tt>, we have69 to implement here the original function itself. The constructor of the70 class have to give all the attributes like \c _X the default values of71 them.72 73 If we instantiate this class, the default values will be set for the74 attributes (originally the parameters), initially. If we call function75 \c X, we get a class with the modified parameter value of76 \c X. Therefore we can modify any parameter-value, independently from the77 order. To run the algorithm we have to call the <tt>run()</tt> function at the78 end of the row.79 80 Example:81 57 \code 82 58 namedFn().id(3).val(2).run(); 83 59 \endcode 84 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 85 65 \note Although it is a class, namedFn is used pretty much like as it were 86 a function. That it why it is called namedFn and not\c NamedFn.66 a function. That it why we called it namedFn instead of \c NamedFn. 87 67 88 \note In fact, the final <tt>.run()</tt> could be made unnecessary if the 89 actual function code were put in the destructor instead. This however would make 90 hard to implement functions with return values, and would also make the 91 implementation of \ref named-templ-func-param "named template parameters" 92 very problematic. <b>Therefore, by convention, <tt>.run()</tt> must be used 93 to explicitly execute function having named parameters in Lemon.</b> 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> 94 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. 95 85 96 86 \section traits-classes Traits Classes 97 87 98 The procedure above can also be applied when defining classes. In this 99 case the type of the attributes can be changed. Initially we have to 100 define a class with the default attribute types. This is the so called 101 Traits Class. Later on the types of these attributes can be changed, 102 as described below. In our software \ref lemon::DijkstraDefaultTraits is an 103 example of how a traits class looks like. 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. 104 96 105 97 \section named-templ-param Named Class Template Parameters … … 107 99 If we would like to change the type of an attribute in a class that 108 100 was instantiated by using a traits class as a template parameter, and 109 the class contains named parameters, we do not have to reinstantiate110 the class with new traits class . Instead of that,adaptor classes can111 be used like in the following cases.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. 112 104 113 105 \code … … 125 117 DistMap and the PredMap is modified. 126 118 127 \section named-templ-func-param Named Function Template Parameters128 129 If the class has so called wizard functions, the new class with the130 modified tpye of attributes can be returned by the appropriate wizard131 function. The usage of these wizard functions is the following:132 133 119 */
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