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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)

    int main()
    {
        using point = boost::tuple<float, float>;
        using polygon = boost::geometry::model::polygon<point>;
        using ring = boost::geometry::model::ring<point>;

        // Create a square as exterior ring, with a triangle as interior ring
        polygon poly{{{0, 0}, {0, 10}, {10, 10}, {10, 0}, {0, 0}}, {{{1, 2}, {8, 2}, {4, 6}, {1, 2}}}};

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

        // Create a triangle
        ring r{{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), (10, 10), (10, 0), (0, 0)), ((1, 2), (8, 2), (4, 6), (1, 2)))
    (())
    ((0, 0), (0, 9), (8, 8), (0, 0))
    ()