The cleanest was to do this is to inherit from the C struct:
struct point
{
long x, y;
};
class Point : public struct point
{
public:
Point(long x, long y)
{ this->x=x; this->y=y; }
float Distance(Point &point)
{ return ....; }
}
The C++ compiler guarantees the C style struct point has the same layout as with the C compiler. The C++ class Point inherits this layout for its base class portion (and since it adds no data or virtual members, it will have the same layout). A pointer to class Point will be converted to a pointer to struct point without a cast, since conversion to a base class pointer is always supported. So, you can use class Point objects and freely pass pointers to them to C functions expecting a pointer to struct point.
Of course, if there is already a C header file defining struct point, then you can just include this instead of repeating the definition.