Question

I try to figure out how boost::geometry‘s for_each_segment ist working. The documentation tells me, that for_each_segment expects a geometry and a Functor. This functor is called polylength_helper in my example, as long as this snippet is not compiling I just increment a number there to keep things simple until it compiles.

// foo.h

typedef boost::geometry::model::point<double, 2, bg::cs::cartesian> GeographicPoint;
typedef boost::geometry::model::linestring<GeographicPoint> GeographicPolyLine;
typedef boost::geometry::model::segment<GeographicPoint> GeographicSegment;

double poly_length(const GeographicPolyLine&);

template<typename Segment>
struct polylength_helper{
    polylength_helper() : length(0){};

    inline void operator()(Segment s){
        length += 1;
    };

    double length;
};

// foo.cpp

double poly_length(GeographicPolyLine &poly){
    polylength_helper<GeographicSegment> helper;
    bg::for_each_segment(poly, helper);
    return helper.length;
}

Well, this does not compile. I used clang for a more understandable output, it says:

note: candidate function not viable: no known
conversion from 'model::referring_segment<point_type>' to
'boost::geometry::model::segment<boost::geometry::model::point<double, 2,
  boost::geometry::cs::cartesian> >' for 1st argument
inline void operator()(Segment s){
            ^

Can anyone help me out? Especially I have no idea where the referring_segment in the message comes from.

Here is an example from the docs:

http://www.boost.org/doc/libs/1_48_0/libs/geometry/doc/html/geometry/reference/algorithms/for_each/for_each_segment_2_const_version.html

But I cannot figure out how this differs from my version, except for the typedefs.

Answer 1

Change the line

typedef boost::geometry::model::segment<GeographicPoint> GeographicSegment;

to

typedef boost::geometry::model::referring_segment<GeographicPoint> GeographicSegment;

That will get you compiling.

---

From the documentation on segment and referring_segment, the only difference between the two is that referring_segment holds a reference to the points. This is what is needed in a for each that modifies the segment since the points modified should be reflected in the linestring. In a for each that does not modify the points, it should still take a reference (most likely a const reference) since it reduces the amount of copying.