CGAL::Object Class Reference

#include <CGAL/Object.h>

Definition

Some functions can return different types of objects.

A typical C++ solution to this problem is to derive all possible return types from a common base class, to return a pointer to this class and to perform a dynamic cast on this pointer. The class Object provides an abstraction. An object obj of the class Object can represent an arbitrary class. The only operations it provides is to make copies and assignments, so that you can put them in lists or arrays. Note that Object is NOT a common base class for the elementary classes. Therefore, there is no automatic conversion from these classes to Object. Rather this is done with the global function make_object(). This encapsulation mechanism requires the use of assign or object_cast to use the functionality of the encapsulated class.

This class is similar in spirit to std::any.

Example

In the following example, the object class is used as return value for the intersection computation, as there are possibly different return values.

{
typedef Cartesian<double> K;
typedef K::Point_2 Point_2;
typedef K::Segment_2 Segment_2;
 
Point_2 point;
Segment_2 segment, segment_1, segment_2;
 
std::cin >> segment_1 >> segment_2;
 
Object obj = intersection(segment_1, segment_2);
 
if (assign(point, obj)) {
  //do something with point
} else if (assign(segment, obj)) {
  // do something with segment
}
  //there was no intersection
}

A more efficient way to access the object is to use object_cast, which allows to skip a default construction and assignment:

{
typedef Cartesian<double> K;
typedef K::Point_2 Point_2;
typedef K::Segment_2 Segment_2;
 
Segment_2 segment_1, segment_2;
 
std::cin >> segment_1 >> segment_2;
 
Object obj = intersection(segment_1, segment_2);
 
if (const Point_2 * point = object_cast<Point_2>(&obj)) {
  // do something with *point
} else if (const Segment_2 * segment = object_cast<Segment_2>(&obj)) {
  // do something with *segment
}
  // there was no intersection
}

The intersection routine itself looks roughly as follows:

template < class Kernel >
Object intersection(Segment_2<Kernel> s1, Segment_2<Kernel> s2)
{
  if ( intersection_is_a_point ) {
    Point_2<Kernel> p = ... ;
    return make_object(p);
  } else if ( intersection_is_a_segment ) {
    Segment_2<Kernel> s = ... ;
    return make_object(s);
  }
 
  // empty intersection
  return Object();
}

Related Functions
(Note that these are not member functions.)
template<class T >
Object	make_object (const T &t)
	Creates an object that contains t.
template<class T >
bool	assign (T &c, const Object &o)
	assigns o to c if o was constructed from an object of type T.
template<class T >
const T *	object_cast (const Object *o)
	Returns a pointer to the object of type T stored by o, if any, otherwise returns NULL.
template<class T >
T	object_cast (const Object &o)
	Returns a copy of the object of type T stored by o, if any, otherwise throws an exception of type Bad_object_cast.

Creation
Objects of type Object are normally created using the global function make_object().
	Object ()
	introduces an empty object.
	Object (const Object &o)
	Copy constructor.
	Object (std::variant< T... >)
	Implicit converting constructor for compatibility with std::variant.
	Object (std::optional< std::variant< T... > >)
	Implicit converting constructor for compatibility with std::optional and std::variant.

Operations
Object &	operator= (const Object &o)
	Assignment.
bool	empty ()
	returns true, if obj does not contain an object of type T.
template<class T >
bool	is ()
	returns true, iff obj contains an object of type T.
const std::type_info &	type () const
	returns the type information of the contained type, or typeid(void) if empty.

Friends And Related Function Documentation

assign()

template<class T >

bool assign ( T &  c, const Object &  o  )

related

assigns o to c if o was constructed from an object of type T.

Returns true, if the assignment was possible. For efficiency reasons, we recommend using object_cast instead.