mirror of https://github.com/CGAL/cgal
114 lines
3.0 KiB
TeX
114 lines
3.0 KiB
TeX
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\begin{ccRefClass}{Object}
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\ccInclude{CGAL/Object.h}
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%\ccInclude{CGAL/basic.h}
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\ccDefinition
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Some functions can return different types of objects. A typical
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\CC\ solution to this problem is to derive all possible return
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types from a common base class, to return a pointer to this
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class and to perform a dynamic cast on this pointer. The class
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\ccRefName\ provides an abstraction.
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An object \ccStyle{obj} of the class \ccRefName\ can
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represent an arbitrary class. The only operations it provides is
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to make copies and assignments, so that you can put them in lists
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or arrays. Note that \ccRefName\ is NOT a common base class for the
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elementary classes. Therefore, there is no
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automatic conversion from these classes to \ccRefName. Rather
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this is done with the global function \ccc{make_object}. This
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encapsulation mechanism requires the use of \ccc{assign} to use
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the functionality of the encapsulated class.
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\ccCreation
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\ccCreationVariable{obj}
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\ccConstructor{Object();}
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{introduces an uninitialized variable.}
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\ccConstructor{Object(const Object &o);}
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{Copy constructor.}
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Objects of type \ccRefName\ are normally created via the global function
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\ccc{make_object}.
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\ccOperations
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\ccMethod{Object &operator=(const Object &o);}
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{Assignment.}
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Assignment of an object of type \ccRefName\ to an object of type \ccc{T}
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is done using \ccc{assign}.
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There is also a member function to check whether an object of type \ccRefName\
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contains an object.
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\ccMethod{bool is_empty();}{returns true, if \ccc{object} does not
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contain an object.}
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\ccExample
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In the following example, the object class is used as return value for the
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\ccHtmlNoLinksFrom{intersection} computation, as there are possibly different return values.
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\begin{cprog}
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{
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Point_2< Cartesian<double> > point;
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Segment_2< Cartesian<double> > segment, segment_1, segment_2;
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std::cin >> segment_1 >> segment_2;
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Object obj = intersection(segment_1, segment_2);
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if (assign(point, obj)) {
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/* do something with point */
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} else if ((assign(segment, obj)) {
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/* do something with segment*/
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}
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\end{cprog}
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\ccHtmlLinksOff%
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\begin{cprog}
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/* there was no intersection */
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}
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\end{cprog}
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\ccHtmlLinksOn%
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\medskip
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The \ccHtmlNoLinksFrom{intersection} routine itself looks roughly as follows:
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\begin{cprog}
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template < class Kernel >
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Object intersection(Segment_2<Kernel> s1, Segment_2<Kernel> s2)
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{
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\end{cprog}
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\ccHtmlLinksOff%
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\begin{cprog}
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if (/* intersection in a point */ ) {
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\end{cprog}
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\ccHtmlLinksOn%
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\begin{cprog}
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Point_2<Kernel> p = ... ;
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return make_object(p);
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\end{cprog}
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\ccHtmlLinksOff%
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\begin{cprog}
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} else if (/* intersection in a segment */ ) {
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\end{cprog}
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\ccHtmlLinksOn%
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\begin{cprog}
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Segment_2<Kernel> s = ... ;
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return make_object(s);
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}
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return Object();
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}
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\end{cprog}
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\ccSeeAlso
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\ccRefIdfierPage{CGAL::assign} \\
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\ccRefIdfierPage{CGAL::make_object}\\
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\ccRefConceptPage{Kernel::Object_2}\\
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\ccRefConceptPage{Kernel::Object_3}\\
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\end{ccRefClass}
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