boost::geometry::intersection return 是什么意思

Question

boost::geometry::intersection(https://www.boost.org/doc/libs/1_73_0/libs/geometry/doc/html/geometry/reference/algorithms/intersection/intersection_3.html) 的文档说函数 return 是一个布尔值。但是文档没有说明 return 值表示什么。我猜如果找到交集，它会 return 正确。

错了！！！

这个代码

#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;
}

产出

no intersection point!

true表示的return是什么？

Answer 1

return 值为真，表示没有错误。例如。深入调用链：

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;
}

那是在

#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/Whosebug/test.cpp:40

我查看了 Boost Geoetry follows

的 OGC 简单特征规范

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

它在概念上模拟了没有 return 值的算法：

我检查了 algorithms/detail/intersection 中的所有实现（areal_areal.hpp、box_box.hpp、implementation.hpp、interface.hpp、multi.hpp），但没有 returns false.

TL;DR 总结

return-value 具体 未记录，换句话说：它是您可能不依赖的实现细节。

就库界面而言，文档化的界面在新版本中可能不会更改（没有警告）。很多 'discoverable' 到 header 的东西都是 未记录的 - 通常由 detail:: 命名空间 and/or detail/ header 个文件夹。

boost::geometry::intersection return 是什么意思

What does boost::geometry::intersection return

c++

boost-geometry

TL;DR 总结