CGAL 6.0.1  2D Triangulations on the Sphere

TriangulationDSFaceBase_2
The concept TriangulationOnSphereFaceBase_2
describes the requirements for a face class of a triangulation data structure that is itself plugged into a triangulation on the sphere.
The data structure concept TriangulationDataStructure_2
was primarily designed to serve as a data structure for the 2D triangulation classes of CGAL, which are triangulations embedded in the 2D Euclidean plane. However, its genericity makes it usable for any orientable triangulated surface without boundary regardless of the dimensionality of the space the triangulation is embedded in, and thus it is a valid data structure for the triangulations on the sphere of this package.
A departing feature is that if the vertices of the triangulation are all lying on one hemisphere, then the triangulation is not an orientable triangulated surface without boundary. In this case, fictitious faces are added to the triangulation, called ghost faces, such that the triangulation is a topological sphere.
CGAL::Triangulation_on_sphere_face_base_2<Traits, Fb>
Public Member Functions  
bool  is_ghost () 
provides read access to a Boolean used to indicate if the face is a ghost face.  
void  set_ghost (const bool b) 
provides write access to a Boolean used to indicate if the face is a ghost face.  
std::istream &  operator>> (std::istream &is, TriangulationOnSphereFaceBase_2 &v) 
inputs the noncombinatorial information given by the face: its ghost status and other possible information.  
std::ostream &  operator<< (std::ostream &os, const TriangulationOnSphereFaceBase_2 &v) 
outputs the non combinatorial operation given by the face: its ghost status and other possible information.  