I have repeatedly found myself debugging frustrating problems and thinking "I swear I've seen this problem before, but I can't for the life of me remember how I fixed it!". So, this page is a record of random programming problems I've encountered that had non-intuitive solutions. The emphasis is on describing what happened as specifically as possible so that future me has all the facts. I shall not presume to draw too many inferences about things I still don't fully understand.

Error:

/home/micaiah/Desktop/hexed/libhexed/Block.cpp:187:9: runtime error: member access within address 0x72c03c52ac80 which does not point to an object of type 'Block'

0x72c03c52ac80: note: object has invalid vptr

Solution: Initialization of base class referenced one of its own members:

class Block
{
  public:
  int n_dim
  Block(int, ...);
};
 
class Mesh_element : public Block
{
  ...
}
 
Mesh_element::Mesh_element(...) : Block(n_dim, ...)

Error:

undefined reference to `vtable for hexed::Mortal'

Solution: hexed::Mortal had declared a virtual destructor in the header file which was never defined in the .cpp file.

Error: GCC mistakenly thinks some Eigen object may be uninitialized:

/home/micaiah/Desktop/hexed/build/include/Eigen/src/Core/functors/BinaryFunctors.h:80:122: warning: ‘unit_avg’ may be used uninitialized [-Wmaybe-uninitialized]
   80 |   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE result_type operator() (const LhsScalar& a, const RhsScalar& b) const { return a * b; }
      |                                                                                                                        ~~^~~
/home/micaiah/Desktop/hexed/libhexed/brep.cpp: In member function ‘void hexed::brep::Trimmed_surface::_recursive_nearest(hexed::Nearest_point<3>&, hexed::Mat<2, 1>&, hexed::Int, hexed::Int, hexed::Int, bool) const’:
/home/micaiah/Desktop/hexed/libhexed/brep.cpp:634:10: note: ‘unit_avg’ declared here
  634 |   Mat<3> unit_avg = average(1).vector()/norm;

Clearly, unit_avg is in fact initialized.

Solution: Replace

average(1).vector() += (average(1) - average(0)).vector().norm()*sin/cos*math::sign(scale > 0)*unit_avg;

with

double scale1 = (average(1) - average(0)).vector().norm()*sin/cos*math::sign(scale > 0);

for (int i = 0; i < 3; ++i) average(1)[i] += scale1*unit_avg(i);

Not sure why this works, or if it will be in any way repeatable, but at least for this time, it shuts GCC up.