Boost C++ Libraries

Reverses the points within a geometry.

Description

Generic function to reverse a geometry. It resembles the std::reverse functionality, but it takes the geometry type into account. Only for a ring or for a linestring it is the same as the std::reverse.

Synopsis

template<typename Geometry>
void reverse(Geometry & geometry)

Parameters

Header

Either

#include <boost/geometry.hpp>

Or

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

Conformance

The function reverse is not defined by OGC.

The function reverse conforms to the std::reverse function of the C++ std-library.

Behavior

	Note
	The reverse of a (multi)polygon or ring might make a valid geometry invalid because the (counter)clockwise orientation reverses.

Complexity

Linear

Example

Shows how to reverse a ring or polygon

#include <iostream>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/polygon.hpp>
#include <boost/geometry/geometries/ring.hpp>
#include <boost/geometry/geometries/adapted/boost_tuple.hpp>

BOOST_GEOMETRY_REGISTER_BOOST_TUPLE_CS(cs::cartesian)

#include <boost/assign.hpp>

int main()
{
    using boost::assign::tuple_list_of;

    typedef boost::tuple<int, int> point;
    typedef boost::geometry::model::polygon<point> polygon;
    typedef boost::geometry::model::ring<point> ring;


    polygon poly;
    boost::geometry::exterior_ring(poly) = tuple_list_of(0, 0)(0, 9)(10, 10)(0, 0);
    boost::geometry::interior_rings(poly).push_back(tuple_list_of(1, 2)(4, 6)(2, 8)(1, 2));

    double area_before = boost::geometry::area(poly);
    boost::geometry::reverse(poly);
    double area_after = boost::geometry::area(poly);
    std::cout << boost::geometry::dsv(poly) << std::endl;
    std::cout << area_before << " -> " << area_after << std::endl;

    ring r = tuple_list_of(0, 0)(0, 9)(8, 8)(0, 0);

    area_before = boost::geometry::area(r);
    boost::geometry::reverse(r);
    area_after = boost::geometry::area(r);
    std::cout << boost::geometry::dsv(r) << std::endl;
    std::cout << area_before << " -> " << area_after << std::endl;

    return 0;
}

Output:

(((0, 0), (10, 10), (0, 9), (0, 0)), ((1, 2), (2, 8), (4, 6), (1, 2)))
38 -> -38
((0, 0), (8, 8), (0, 9), (0, 0))
36 -> -36