doc/named-param.dox
author ladanyi
Tue, 12 Jul 2005 13:36:01 +0000
changeset 1545 e8378402f97e
parent 1438 826bdac3525a
child 1619 f0700b9e6418
permissions -rw-r--r--
Added the individual png files to EXTRA_DIST instead of the icons directory. This way the .svn directories do not get in the tarball. An alternative solution would be to delete these in a dist-hook target.
     1 /*!
     2 
     3 \page named-param Named Parameters
     4 
     5 \section named-func-param Named "Function" Parameters
     6 
     7 C++ makes it possible to use default parameter values when calling a
     8 function. In such a case we do not have to give value for parameters,
     9 the program will use the default ones.  Unfortunately sometimes this
    10 is not enough. If we do not want to give values for all the
    11 parameters, only for some of them we come across problems, because an
    12 arbitrary set of parameters cannot be omitted. On the other hand
    13 parameters have a fixed order in the head of the function.  C++ can
    14 apply the default values only in the back of the order, if we do not
    15 give other value for them.  So we can not give the function for
    16 example the value of the first, and the third parameter, expecting
    17 that the program will aplly the default value for the second
    18 parameter.  However sometimes we would like to use some functinos
    19 exactly in this way. With a crafty trick and with some little
    20 inconvenience this is possible. We have implemented this little trick
    21 as an example below.
    22 
    23 \code
    24 class named_fn 
    25 {
    26   int _id;
    27   double _val;
    28   int _dim;
    29   
    30   public:
    31   named_fn() : _id(0), _val(1), _dim(2) {}
    32   named_fn& id(int p)     { _id  = p ; return *this; }
    33   named_fn& val(double p) { _val = p ; return *this; }
    34   named_fn& dim(int p)    { _dim = p ; return *this; }
    35 
    36   run() {
    37     printf("Here is the function itself.");
    38   }
    39 };
    40 \endcode
    41 
    42 
    43 The usage is the following.
    44 
    45 We have to define a class, let's call it named_fn.  Let us assume that
    46 we would like to use a parameter, called X. In the named_fn class we
    47 have to define an _X attribute, and an X function. The function
    48 expects a parameter with the type of _X, and sets the value of
    49 _X. After setting the value the function returns the class itself. The
    50 class also have to have a function, called for example run(), we have
    51 to implement here the original function itself. The constructor of the
    52 class have to give all the attributes like _X the default values of
    53 them.
    54 
    55 If we instantiate this class, the default values will be set for the
    56 attributes (originally the parameters), initially. If we call the X
    57 function, we get a class with the modified parameter value of
    58 X. Therefore we can modify any parameter-value, independent from the
    59 order. To run the algorithm we have to call the run() function at the
    60 end of the row.
    61 
    62 Example: named_fn().id(3).val(2).run();
    63 
    64 \section traits-classes Traits Classes
    65 
    66 The procedure above can also be applied when defining classes. In this
    67 case the type of the attributes can be changed.  Initially we have to
    68 define a class with the default attribute types. This is the so called
    69 Traits Class. Later on the types of these attributes can be changed,
    70 as described below. In our software \ref lemon::DijkstraDefaultTraits is an
    71 example of how a traits class looks like.
    72 
    73 \section named-templ-param Named Class Template Parameters
    74 
    75 If we would like to change the type of an attribute in a class that
    76 was instantiated by using a traits class as a template parameter, and
    77 the class contains named parameters, we do not have to reinstantiate
    78 the class with new traits class. Instead of that, adaptor classes can
    79 be used like in the following cases.
    80 
    81 \code
    82 Dijkstra<>::SetPredNodeMap<NullMap<Node,Node> >
    83 \endcode
    84 
    85 It can also be used in conjunction with other named template
    86 parameters in arbitrary order.
    87 
    88 \code
    89 Dijkstra<>::SetDistMap<MyMap>::SetPredMap<NullMap<Node,Edge> >
    90 \endcode
    91 
    92 The result will be an instantiated Dijkstra class, in which the
    93 DistMap and the PredMap is modified.
    94 
    95 \section named-templ-func-param Named "Function" Template Parameters
    96 
    97 If the class has so called wizard functions, the new class with the
    98 modified tpye of attributes can be returned by the appropriate wizard
    99 function. The usage of these wizard functions is the following:
   100 
   101 */