Question

I have a C# front end and a C++ backend for performance reasons.
Now I would like to call a C++ function like for example:

void findNeighbors(Point p, std::vector<Point> &neighbors, double maxDist);

What I’d like to have is a C# wrapper function like:

List<Point> FindNeigbors(Point p, double maxDist);

I could pass a flat array like Point[] to the unmanaged C++ dll, but the problem is, that I don’t know how much memory to allocate, because I don’t know the number of elements the function will return…

Is there an elegant way to handle this without having troubles with memory leaks?

Thanks for your help!

Benjamin

Answer 1

The best solution here is to write a wrapper function in C which is limited to non-C++ classes. Non-trivial C++ classes are essentially unmarshable via the PInvoke layer [1]. Instead have the wrapper function use a more traditional C signature which is easy to PInvoke against

void findNeigborsWrapper(
  Point p,
  double maxDist, 
  Point** ppNeighbors,
  size_t* pNeighborsLength)

[1] Yes there are certain cases where you can get away with it but that’s the exception and not the rule.