clear

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#### clear

Clears a linestring, ring or polygon (exterior+interiors) or multi*.

###### Description

Generic function to clear a geometry. All points will be removed from the collection or collections making up the geometry. In most cases this is equivalent to the .clear() method of a std::vector<...>. In the case of a polygon, this clear functionality is automatically called for the exterior ring, and for the interior ring collection. In the case of a point, boxes and segments, nothing will happen.

###### Synopsis

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

###### Parameters

Type | Concept | Name | Description
--- | --- | --- | ---
Geometry & | Any type fulfilling a Geometry Concept | geometry | A model of the specified concept which will be cleared

###### Header

Either

`#include <boost/geometry.hpp>`

Or

`#include <boost/geometry/algorithms/clear.hpp>`

###### Conformance

The function clear is not defined by OGC.

The function clear conforms to the clear() method of the C++ std-library.

###### Supported geometries

Geometry | Status
--- | ---
Point |
Segment |
Box |
Linestring |
Ring |
Polygon |
MultiPoint |
MultiLinestring |
MultiPolygon |
Variant |

###### Behavior

Case | Behavior
--- | ---
Point | Nothing happens, geometry is unchanged
Segment | Nothing happens, geometry is unchanged
Box | Nothing happens, geometry is unchanged
Linestring | Linestring is cleared
Ring | Ring is cleared
Polygon | The exterior ring is cleared and all interior rings are removed
Multi Point | Multi Point is cleared
Multi Linestring | Multi Linestring is cleared
Multi Polygon | Multi Polygon is cleared

###### Complexity

Constant

###### Example

Shows how to clear 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<float, float> point;
        typedef boost::geometry::model::polygon<point> polygon;
        typedef boost::geometry::model::ring<point> ring;

        polygon poly;

        // Fill the polygon (using its own methods + Boost.Assign)
        poly.outer() = tuple_list_of(0, 0)(0, 9)(10, 10)(0, 0);
        poly.inners().push_back(tuple_list_of(1, 2)(4, 6)(2, 8)(1, 2));

        std::cout << boost::geometry::dsv(poly) << std::endl;
        boost::geometry::clear(poly);
        std::cout << boost::geometry::dsv(poly) << std::endl;

        // Create a ring using Boost.Assign
        ring r = tuple_list_of(0, 0)(0, 9)(8, 8)(0, 0);

        std::cout << boost::geometry::dsv(r) << std::endl;
        boost::geometry::clear(r);
        std::cout << boost::geometry::dsv(r) << std::endl;

        return 0;
    }

Output:

    (((0, 0), (0, 10), (11, 11), (0, 0)), ((0, 0), (0, 10), (11, 11), (0, 0)))
    (())
    ((0, 0), (0, 9), (8, 8), (0, 0))
    ()