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CGAL::Object

#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 boost::any.

Creation

Object obj;
introduces an empty object.


Object obj ( o);
Copy constructor.

Objects of type Object are normally created using the global function make_object.

Operations

Object & obj = o Assignment.

bool obj.empty () returns true, if obj does not contain an object.

template<class T>
bool obj.is () returns true, iff obj contains an object of type T.

std::type_info obj.type () returns the type information of the contained type, or typeid(void) if empty.

Construction of an Object storing an object of type T can be performed using the make_object global function :

template <class T>
Object make_object ( T t) Creates an object that contains t.

Assignment of an object of type Object to an object of type T can be done using assign :

template <class T>
bool assign ( T& c, o) 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.

Another possibility to access the encapsulated object is to use object_cast, which avoids the default constructor and assignment required by assign :

template <class T>
const T * object_cast ( const * o) Returns a pointer to the object of type T stored by o, if any, otherwise returns NULL.

template <class T>
T object_cast ( o) Returns a copy of the object of type T stored by o, if any, otherwise throws an exception of type Bad_object_cast.

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();
}

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CGAL Open Source Project. Release 3.7. 14 October 2010.