As someone mentioned before, "You only NEED to include the full header if you have a variable of that type which is not a pointer or reference, because in that case the class needs to know the size of the other class, for which it needs to full declearation".

So if you're using a vector of the actual classes (and not a pointer to the classes), you need the entire class definition - a forward declaration won't do.

As someone else suggested, I think the best thing to do it to try to make the smallest possible test case that reproduces the error - then we might be able to figure out what's going on.