[Gmsh] Implementing loop direction in C++

[Tolga Kurt]

In some other thread, it has been showed that the equivalent of a GEO 
script in C++ is something as follows.

   // Add lines. Equivalent .geo directives are
   // Line(1) = {1, 2};
   // Line(2) = {2, 3};
   // Line(3) = {3, 4};
   // Line(4) = {4, 1};
   GEdge *ge1 = m->addLine(gv1, gv2);
   GEdge *ge2 = m->addLine(gv2, gv3);
   GEdge *ge3 = m->addLine(gv3, gv4);
   GEdge *ge4 = m->addLine(gv4, gv1);

   // Add line loop. Equivalent .geo directive is
   // Line Loop(1) = {1, 2, 3, 4};

   std::vector<std::vector<GEdge *>  >  lineLoop1(1);
   lineLoop1[0].push_back(ge1);

My question is, how can I set a sign for a line's orientation in the 
loop. In other words I need to convert
Line Loop(1) = {1, -2, 3, -4}
to c++ code. Explicitly saying, I have 4 points that are corners of a 
tetrahedron and I have added the lines
     GEdge* gedge[6];

     gedge[0] = m->addLine(gv[0], gv[1]);
     gedge[1] = m->addLine(gv[1], gv[2]);
     gedge[2] = m->addLine(gv[2], gv[0]);
     gedge[3] = m->addLine(gv[0], gv[3]);
     gedge[4] = m->addLine(gv[1], gv[3]);
     gedge[5] = m->addLine(gv[2], gv[3]);

So that, when I add the first surface as (Line Loop(1) = {1,2,3})
     vector<GEdge*> ge[4];
     ge[0].push_back(gedge[0]);
     ge[0].push_back(gedge[1]);
     ge[0].push_back(gedge[2]);
there is no problem.

But when it comes to add the second surface with these existing lines, I 
need to add the loop (Line Loop(2) = {1, 5, -4} )
Otherwise, it says that the loop is wrong (when I just add the line 4 
without a sign)

Any help would be great.

Thanks in advance,
Tolga Kurt.