Named Parameters

Named Function Parameters

C++ makes it possible to use default parameter values when calling a function. In such a case we do not have to give value for parameters, the program will use the default ones. Unfortunately sometimes this is not enough. If we do not want to give values for all the parameters, only for some of them we come across problems, because an arbitrary set of parameters cannot be omitted. On the other hand parameters have a fixed order in the head of the function. C++ can apply the default values only in the back of the order, if we do not give other value for them. So we can not give the function for example the value of the first, and the third parameter, expecting that the program will aplly the default value for the second parameter. However sometimes we would like to use some functinos exactly in this way. With a crafty trick and with some little inconvenience this is possible. We have implemented this little trick as an example below.

class namedFn 
{
  int _id;
  double _val;
  int _dim;
  
  public:
  namedFn() : _id(0), _val(1), _dim(2) {}
  namedFn& id(int p)     { _id  = p ; return *this; }
  namedFn& val(double p) { _val = p ; return *this; }
  namedFn& dim(int p)    { _dim = p ; return *this; }

  run() {
    printf("Here is the function itself.");
  }
};

The usage is the following.

We have to define a class, let's call it namedFn. Let us assume that we would like to use a parameter, called X. In the namedFn class we have to define an _X attribute, and a function X. The function expects a parameter with the type of _X, and sets the value of _X. After setting the value the function returns the class itself. The class also have to have a function, called for example run(), we have to implement here the original function itself. The constructor of the class have to give all the attributes like _X the default values of them.

If we instantiate this class, the default values will be set for the attributes (originally the parameters), initially. If we call function X, we get a class with the modified parameter value of X. Therefore we can modify any parameter-value, independently from the order. To run the algorithm we have to call the run() function at the end of the row.

Example:

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

Note:

Although it is a class, namedFn is used pretty much like as it were a function. That it why it is called namedFn and not NamedFn.

In fact, the final .run() could be made unnecessary if the actual function code were put in the destructor instead. This however would make hard to implement functions with return values, and would also make the implementation of named template parameters very problematic. Therefore, by convention, .run() must be used to explicitly execute function having named parameters in Lemon.

Traits Classes

The procedure above can also be applied when defining classes. In this case the type of the attributes can be changed. Initially we have to define a class with the default attribute types. This is the so called Traits Class. Later on the types of these attributes can be changed, as described below. In our software lemon::DijkstraDefaultTraits is an example of how a traits class looks like.

Named Class Template Parameters

If we would like to change the type of an attribute in a class that was instantiated by using a traits class as a template parameter, and the class contains named parameters, we do not have to reinstantiate the class with new traits class. Instead of that, adaptor classes can be used like in the following cases.

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

It can also be used in conjunction with other named template parameters in arbitrary order.

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

The result will be an instantiated Dijkstra class, in which the DistMap and the PredMap is modified.

Named Function Template Parameters

If the class has so called wizard functions, the new class with the modified tpye of attributes can be returned by the appropriate wizard function. The usage of these wizard functions is the following: