Boost C++ Libraries

Checks if the first geometry is completely inside the second geometry.

Description

The free function within checks if the first geometry is completely inside the second geometry.

Synopsis

template<typename Geometry1, typename Geometry2>
bool within(Geometry1 const & geometry1, Geometry2 const & geometry2)

Parameters

Returns

true if geometry1 is completely contained within geometry2, else false

Header

Either

#include <boost/geometry/geometry.hpp>

Or

#include <boost/geometry/algorithms/within.hpp>

Supported geometries

Notes

If a point is located exactly on the border of a geometry, the result depends on the strategy. The default strategy (Winding (coordinate system agnostic)) returns false in that case.

If a polygon has a reverse oriented (e.g. counterclockwise for a clockwise typed polygon), the result also depends on the strategy. The default strategy returns still true if a point is completely within the reversed polygon. There is a specific strategy which returns false in this case.

Complexity

Linear

Example

Shows how to detect if a point is inside a polygon, or not

#include <iostream>
#include <list>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/geometry/geometries/polygon.hpp>

#include <boost/geometry/domains/gis/io/wkt/wkt.hpp>


int main()
{
    typedef boost::geometry::model::d2::point_xy<double> point_type;
    typedef boost::geometry::model::polygon<point_type> polygon_type;

    polygon_type poly;
    boost::geometry::read_wkt(
        "POLYGON((2 1.3,2.4 1.7,2.8 1.8,3.4 1.2,3.7 1.6,3.4 2,4.1 3,5.3 2.6,5.4 1.2,4.9 0.8,2.9 0.7,2 1.3)"
            "(4.0 2.0, 4.2 1.4, 4.8 1.9, 4.4 2.2, 4.0 2.0))", poly);

    point_type p(4, 1);

    std::cout << "within: " << (boost::geometry::within(p, poly) ? "yes" : "no") << std::endl;
    
    return 0;
}

Output:

within: yes