You can use assert() to help with debugging. Just put:
assert(something_that_I_believe_is_true);
in the program (which will need #include <cassert>) and it will warn you if it turns out to be false.
Additionally, FLTK then allows you to choose to Abort the program or Ignore the warning (Retry does nothing with Quincy)
e.g.:
assert(i>=0); assert(j<MAX);
There is no need to remove the asserts from the final program (they will always be true in a correct program!) but they can be turned off (for speed reasons) by defining NDEBUG under Tools|Options|Build
MSC 15 Oct 2003