In a constrained triangulation, the information about constrained edges is stored in the faces of the triangulation. The base face of a constrained triangulation has to be a model of the concept ConstrainedTriangulationFaceBase_2 which refines the concept TriangulationFaceBase_2 providing functionalities to deal with constraints.
bool  f.is_constrained ( int i) 
returns true if the edge between f and its neighbor
f.neighbor(i) is constrained.

void  f.set_constraint ( int i, bool b)  
sets the edge between f and its neighbor f.neighbor(i) as a constrained or unconstrained edge according to b.  
void  f.set_constraints ( bool c0, bool c1, bool c2)  
sets the status (constrained or unconstrained) of the three edges of f.  
void  f.reorient ()  Changes the orientation of f by exchanging vertex(0) with vertex(1) and neighbor(0) with neighbor(1) and the corresponding constrained status. 
void  f.ccw_permute ()  performs a counterclockwise permutation of the vertices, neighbors and constrained status of f. 
void  f.cw_permute ()  performs a clockwise permutation of the vertices and neighbors and constrained status of f. 
CGAL::Constrained_triangulation_face_base_2<Traits>