what's the difference between ring and polygon in boost geometry?

Question

I'm confused about ring and polygon in Boost.Geometry.

In the documentation, there's no figure showing what is a ring, and what is a polygon.

Can anyone draw a figure explaining the difference between two concepts?

Answer 1

In Boost.Geometry a Polygon is defined as an areal Geometry containing an exterior ring and some number of interior rings (holes).

Ring represents one of the Polygon's rings. It's different than the OGC LinearRing because it's an areal Geometry. When used alone it can be seen as a Polygon without holes.