What does boost::geometry::intersection return

问题

The documentation for boost::geometry::intersection( https://www.boost.org/doc/libs/1_73_0/libs/geometry/doc/html/geometry/reference/algorithms/intersection/intersection_3.html ) says the function returns a bool. However the docs do NOT say what the return value indicates. I guessed it would return true if an intersection was found.

Wrong!!!

This code

#include <iostream>
#include <deque>
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/register/point.hpp>

namespace bg = boost::geometry;
using namespace std;

class cxy
{
public:
    double x;
    double y;
    cxy( double X, double Y )
        : x( X )
        , y( Y )
    {

    }
    /// boost geometry insists on a default constructor
    cxy()
        : cxy(0,0)
    {

    }
};

BOOST_GEOMETRY_REGISTER_POINT_2D( cxy, double, bg::cs::cartesian, x, y )
typedef bg::model::segment<cxy> segment_t;

int main()
{
    cxy a(1,0);
    cxy b(1,1);
    cxy c(0,0.5);
    cxy d(0.5,0.5) ;

    segment_t ab( a, b );
    segment_t cd( c, d );
    std::vector<cxy> out;
    if( ! bg::intersection( ab, cd, out ) ) {
       std::cout << "intersection returned false\n";
       return 1;
    }
    if( ! out.size() ) {
        std::cout << "no intersection point!\n";
        return 2;
    }
    std::cout << "intersection at " << out[0].x <<" " << out[0].y << "\n";

    return 0;
}

outputs

no intersection point!

What is the return of true indicating?

回答1:

The return value is true indicating no errors. E.g. deep down the call chain:

template <typename RobustPolicy, typename GeometryOut, typename Strategy>
static inline bool apply(Geometry1 const& geometry1,
        Geometry2 const& geometry2,
        RobustPolicy const& robust_policy,
        GeometryOut& geometry_out,
        Strategy const& strategy)
{
    typedef typename geometry::detail::output_geometry_value
        <
            GeometryOut
        >::type SingleOut;

    intersection_insert
        <
            Geometry1, Geometry2, SingleOut,
            overlay_intersection
        >::apply(geometry1, geometry2, robust_policy,
                 geometry::detail::output_geometry_back_inserter(geometry_out),
                 strategy);

    return true;
}

That's at

#0  0x000055555555598c in boost::geometry::dispatch::intersection<boost::geometry::model::segment<cxy>, boost::geometry::model::segment<cxy>, boost::geometry::segment_tag, boost::geometry::segment_tag, false>::apply<boost::geometry::detail::no_rescale_policy, std::vector<cxy, std::allocator<cxy> >, boost::geometry::strategy::intersection::cartesian_segments<void> > (geometry1=..., geometry2=..., robust_policy=..., geometry_out=std::vector of length 0, capacity 0, strategy=...) at /home/sehe/custom/boost_1_73_0/boost/geometry/algorithms/detail/intersection/interface.hpp:63
#1  0x0000555555555842 in boost::geometry::resolve_strategy::intersection::apply<boost::geometry::model::segment<cxy>, boost::geometry::model::segment<cxy>, std::vector<cxy, std::allocator<cxy> > > (geometry1=..., geometry2=..., geometry_out=std::vector of length 0, capacity 0) at /home/sehe/custom/boost_1_73_0/boost/geometry/algorithms/detail/intersection/interface.hpp:175
#2  0x00005555555556ed in boost::geometry::resolve_variant::intersection<boost::geometry::model::segment<cxy>, boost::geometry::model::segment<cxy> >::apply<std::vector<cxy, std::allocator<cxy> >, boost::geometry::default_strategy> (geometry1=..., geometry2=..., geometry_out=std::vector of length 0, capacity 0, strategy=...) at /home/sehe/custom/boost_1_73_0/boost/geometry/algorithms/detail/intersection/interface.hpp:198
#3  0x00005555555554f3 in boost::geometry::intersection<boost::geometry::model::segment<cxy>, boost::geometry::model::segment<cxy>, std::vector<cxy, std::allocator<cxy> > > (geometry1=..., geometry2=..., geometry_out=std::vector of length 0, capacity 0) at /home/sehe/custom/boost_1_73_0/boost/geometry/algorithms/detail/intersection/interface.hpp:403
#4  0x0000555555554eab in main () at /home/sehe/Projects/stackoverflow/test.cpp:40

I looked at the OGC Simple Feature Specification that Boost Geoetry follows

The library follows existing conventions:

conventions from boost

conventions from the std library conventions and

names from one of the OGC standards on geometry and, more specificly, from the OGC Simple Feature Specification

It conceptually models the algorithm without the return value:

I checked all the implementations in algorithms/detail/intersection (areal_areal.hpp, box_box.hpp, implementation.hpp, interface.hpp, multi.hpp) and nothing returns false.

TL;DR Summary

The return-value is specifically undocumented, in other words: it's an implementation detail you may not depend on.

In terms of the library interface, documented interface may not change (without warning) in new versions. A lot of things that are 'discoverable' through the headers are undocumented - most often indicated by a detail:: namespace and/or detail/ header folder(s).

来源：https://stackoverflow.com/questions/63118550/what-does-boostgeometryintersection-return

标签

c++

boost-geometry