Version 5.0 The LEDA User Manual

dictionary D0; // default ordering
dictionary D1; // polar ordering
In general the macro call
DEFINE_LINEAR_ORDER(T, cmp, T1)
introduces a new type T 1 equivalent to type T with the linear order defined by the
compare function cmp.
2. As a new feature all order based data types like dictionaries, priority queues, and
sorted sequences offer a constructor which allows a user to set the internally used
ordering at construction time.
dictionary D0;
// default ordering
dictionary D1(pol_cmp); // polar ordering
This alternative handles the cases where two or more different orderings are needed
more elegantly.
3. Instead of passing a compare function cmp(const T &, const T &) to the sorted type
one can also pass an object (a so-called compare object) of a class that is derived
from the class leda cmp base and that overloads the function-call operator
int operator()(const T &, const T &) to define a linear order for T .
This variant is helpful when the compare function depends on a global parameter.
We give an example. More examples can be found in several sections of the LEDA
book [64]. Assume that we want to compare edges of a graph GRAPH < point, int >
(in this type every node has an associated point in the plane; the point associated
with a node v is accessed as G[v]) according to the distance of their endpoints. We
write
using namespace leda;
class cmp_edges_by_length: public leda_cmp_base {
const GRAPH& G;
public:
cmp_edges_by_length(const GRAPH& g): G(g){}
int operator()(const edge& e, const edge& f) const
{ point pe = G[G.source(e)]; point qe = G[G.target(e)];
point pf = G[G.source(f)]; point qf = G[G.target(f)];
return compare(pe.sqr_dist(qe),pf.sqr_dist(qf));
}
};
int main(){
GRAPH G;