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Calculates the number of interior rings of a geometry.
The free function num_interior_rings calculates the number of interior rings of a geometry.
template<typename Geometry> std::size_t num_interior_rings(Geometry const & geometry)
| Type | Concept | Name | Description | 
|---|---|---|---|
| Geometry const & | Any type fulfilling a Geometry Concept | geometry | A model of the specified concept | 
The calculated number of interior rings
Either
          #include <boost/geometry.hpp>
        
Or
          #include <boost/geometry/algorithms/num_interior_rings.hpp>
        
The function num_interior_ring implements function NumInteriorRing from the OGC Simple Feature Specification.
| ![[Note]](../../../../../../../doc/src/images/note.png) | Note | 
|---|---|
| Boost.Geometry adds an "s" | 
| Case | Behavior | 
|---|---|
| Polygon | Returns the number of its interior rings | 
| Multi Polygon | Returns the number of the interior rings of all polygons | 
| Other geometries | Returns 0 | 
Constant
Get the number of interior rings in a multi-polygon
#include <iostream> #include <boost/geometry.hpp> #include <boost/geometry/geometries/point_xy.hpp> #include <boost/geometry/geometries/polygon.hpp> #include <boost/geometry/geometries/multi_polygon.hpp> int main() { boost::geometry::model::multi_polygon < boost::geometry::model::polygon < boost::geometry::model::d2::point_xy<double> > > mp; boost::geometry::read_wkt("MULTIPOLYGON(((0 0,0 10,10 0,0 0),(1 1,1 9,9 1,1 1)),((10 10,10 7,7 10,10 10)))", mp); std::cout << "Number of interior rings: " << boost::geometry::num_interior_rings(mp) << std::endl; return 0; }
Output:
Number of interior rings: 1