什么是属性映射在BOOST？ [英] What is a property map in BOOST?

问题描述

有人能解释一个升压初学者喜欢我什么是属性映射在加速？
尝试使用BGL计算强连接的部件，当我遇到这来了。
我去扔了属性映射和图形模块的文档，仍然不知道这是怎么回事。
借此code，例如：
- 什么是make_iterator_property_map功能做什么？
- 而这是什么code的含义：获得（的vertex_index，G）？

 的#include＆LT;升压/ config.hpp＆GT;
＃包括LT＆;矢量＆GT;
＃包括LT＆;＆iostream的GT;
＃包括LT＆;升压/图/ strong_components.hpp＆GT;
＃包括LT＆;升压/图/ adjacency_list.hpp＆GT;INT
主要（）
{
  使用名字空间boost;
  的typedef的adjacency_list＆LT;血管内皮细胞，血管内皮细胞，directedS＆GT;图形;
  const int的N = 6;
  图G（N）;
  的add_edge（0，1，G）;
  的add_edge（1，1，G）;
  的add_edge（1，3，G）;
  的add_edge（1，4，G）;
  的add_edge（3，4，G）;
  的add_edge（3,0，G）;
  的add_edge（4，3，G）;
  的add_edge（5，2，G）;  的std ::矢量＆lt;＆INT GT; C（N）;
  INT NUM = strong_components
    （G，make_iterator_property_map（c.begin（），拿到（的vertex_index，G），C [0]））;  性病::法院LT＆;＆LT; 总数部分组成：＆所述;＆下; NUM＆LT;＆LT;的std :: ENDL;
  的std ::矢量＆lt; INT＆GT; ::迭代器I;
  对于（I = c.begin（！）; I = c.end（）; ++ I）
    性病::法院LT＆;＆LT; 顶点＆LT;＆LT;我 -  c.begin（）
      ＆LT;＆LT; 是组件＆LT;＆LT; * I＆LT;＆LT;的std :: ENDL;
  返回EXIT_SUCCESS;
}
 

解决方案

其核心PropertyMaps是数据访问的抽象。在泛型编程非常迅速出现的一个问题是：我如何获得一些对象相关的数据？它可以被存储在对象本身，该对象可以是一个指针，它可能是在一些映射结构的对象之外。

您当然可以封装在一个仿函数数据访问，但很快变得单调乏味，你找一个更窄的解决方案，一个在升压选择是PropertyMaps。

记住，这仅仅是概念。具体的实例是例如一个的std ::地图（有一些语法适应），（再次，一些语法自适应）返回键的成员函数。

对你的编辑： make_iterator_property_map 构建一个<一个href=\"http://www.boost.org/doc/libs/1_55_0/libs/property_map/doc/iterator_property_map.html\">iterator_property_map.第一个参数为偏移计算的基础上的迭代器。第二个参数又是一个property_map做偏移计算。总之这提供了一种使用 vertex_descriptor的写根据<$ C指数数据到矢量$ C> vertex_descriptor的。

Can someone explain to a Boost beginner like me what is a property map is in Boost? I came across this when trying to use the BGL for calculating strong connected components. I went throw the documentation for the property map and graph module and still don't know what to make of it. Take this code, for example: - what is the make_iterator_property_map function doing? - and what is the meaning of this code: get(vertex_index, G) ?

#include <boost/config.hpp>
#include <vector>
#include <iostream>
#include <boost/graph/strong_components.hpp>
#include <boost/graph/adjacency_list.hpp>

int
main()
{
  using namespace boost;
  typedef adjacency_list < vecS, vecS, directedS > Graph;
  const int N = 6;
  Graph G(N);
  add_edge(0, 1, G);
  add_edge(1, 1, G);
  add_edge(1, 3, G);
  add_edge(1, 4, G);
  add_edge(3, 4, G);
  add_edge(3, 0, G);
  add_edge(4, 3, G);
  add_edge(5, 2, G);

  std::vector<int> c(N);
  int num = strong_components
    (G, make_iterator_property_map(c.begin(), get(vertex_index, G), c[0]));

  std::cout << "Total number of components: " << num << std::endl;
  std::vector < int >::iterator i;
  for (i = c.begin(); i != c.end(); ++i)
    std::cout << "Vertex " << i - c.begin()
      << " is in component " << *i << std::endl;
  return EXIT_SUCCESS;
}

解决方案

PropertyMaps at their core are an abstraction of data access. A problem that comes up very quickly in generic programming is: How do I get data associated with some object? It could be stored in the object itself, the object could be a pointer, it could be outside of the object in some mapping structure.

You can of course encapsulate data-access in a functor, but that becomes tedious very quickly and you look for a more narrow solution, the one chosen in Boost are PropertyMaps.

Remember this is just the concept. Concrete instances are for example an std::map (with some syntactic adaption), a function returning a member of the key (again, with some syntactic adaption).

Towards your edit: make_iterator_property_map builds an iterator_property_map. The first argument provides an iterator for a basis of offset calculations. The second argument is again a property_map to do the offset calculation. Together this provides a way to use an vertex_descriptor to write data to the vector based on the index of the vertex_descriptor.

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