All this however does not bring you any closer to solving your original problem.
Ok.
...the compiler might put the debug statement between any two instructions in the whole section
Didn't know that.

So I had some more debugging fun.
The problem is, that calculation of "a" is a little off compared to sa.
I don't know why this happens cause both angles are calculated using the SAME function and the SAME
point.

Qt Code: 
  1. double sa = center.angleTo(sp);
  2. ...
  3. double a;
  4. ...
  5. a = center.angleTo(*v);
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Qt Code: 
  1. double angleTo(const RS_Vector& v) const;
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here is sp and v from the watch list (they are the same point/coordinates)

sp RS_Vector
valid true bool
x 170.15564366033121 double
y 20.000000048775643 double
z 0 double


v std::shared_ptr<RS_Vector>
__shared_ptr<RS_Vector, (__gnu_cxx::_Lock_policy)2> std::__shared_ptr<RS_Vector, (__gnu_cxx::_Lock_policy)2>
_M_ptr @0x8ac7ff0 RS_Vector
valid true bool
x 170.15564366033121 double
y 20.000000048775643 double
z 0 double
_M_refcount std::__shared_count<(__gnu_cxx::_Lock_policy)2>



here is the result of a - sa (as you can see it's e-16) (should be 0 as it is the same point in this case)

result of a - sa: -4.4408920985006261617e-16

As I already told, adding qDebug("angle centerToNextIntersection: %f",a) will somehow lead to
the result 0 of a - sa. Does anybody have an idea what qDebug does to "a"?
I don't get how qDebug does influence the calculation of "a" and why the reseults of calculating
the same angle of the same point using the same function, differ from each other (without qDebug())?