促进：：几何学：：交集返回

Question

boost::geometry::intersection( 3.html )的文档表示该函数返回一个bool。但是，文档没有说明返回值指示的是什么。我猜如果找到了一个交集，它就会返回true。

错了！

这段代码

#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;
}

输出

no intersection point!

真实指示的返回是什么？

Answer 1

返回值为true，表示没有错误。例如，在呼叫链的深处：

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/stackoverflow/test.cpp:40

我查看了Boost地质测量的OGC简单特性规范

图书馆遵循现有的公约：

• 来自boost的会议
• 科技促进发展图书馆公约和
• 名称来自OGC几何学标准之一，更确切地说是来自OGC简单特征规范

它在概念上建模算法，没有返回值：

​

​

我检查了算法/细节/交叉(areal_areal.hpp、box_box.hpp、implementation.hpp、interface.hpp、multi.hpp)中的所有实现，没有任何返回为false。

TL;DR综述

返回值是明确无文档化的，换句话说:它是一个您可能不依赖的实现细节。

在库接口方面，文档化的接口不能在新版本中更改(没有警告)。许多通过标头“可发现”的东西都是没有文档的--最常见的是detail::名称空间和/或detail/头文件夹。