LEDA Guide: Handle Types, Identity and Equality

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Handle Types, Identity and Equality

All geometric objects in LEDA are so-called handle types, i.e., an object of any geometric type is a (smart) pointer to a representation object. A rat_point, for example, is a pointer to a rat_point_rep and a segment is a pointer to a segment_rep. The objects of the representation class contain the defining information about the geometric object and possibly additional information for internal use.

Distinction between Identical and Equal Objects

In the following we use points as an example. Similar statements are true for all geometric objects.

Two points p and q are identical (function identical(p,q)) if they point to the same point_rep.
Two points p and q are equal (binary operator ==) if they agree as geometric objects, i.e., if they have the same Cartesian coordinates.

The assignment statement and the copy constructor preserve identity. They are implemented by pointer assignments.

Example

The following program illustrates the difference between identity and equality. See Writing Kernel Independent Code for an explanation for writing POINT.

#include <LEDA/point.h>
#include <LEDA/geo/float_kernel_names.h>

using namespace leda;

int main()
{ 
  POINT p(0,0);
  POINT q(0,0);
  POINT r=p;
	
  if (identical(p,q)) std::cout << "p and q are identical" << std::endl;
  else std::cout << "p and q are not identical" << std::endl;
  //evaluates to false

  if (p==q) std::cout << "p and q are equal" << std::endl;
  else std::cout << "p and q are not equal" << std::endl;
  //evaluates to true

  if (identical(p,r)) std::cout << "p and r are identical" << std::endl;
  else std::cout << "p and q are not identical" << std::endl; 
  //evaluates to true

  if (p==r) std::cout << "p and q are equal" << std::endl;
  else std::cout << "p and q are not equal" << std::endl;            
  //evaluates to true

  return 0;
}

Handle Types, Identity and Equality

Distinction between Identical and Equal Objects

Example

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