在使用 BGL 实现一些算法后,我尝试使用 GraphML 提供 io 函数。但是,我没有设法编译一个合适的 operator<<,它需要一个 const Graph 引用。
这是一个简单的例子:
// use bundled properties for vertices and edges
struct VertexProperty
{
double error;
};
typedef boost::adjacency_list< boost::setS, boost::setS, boost::undirectedS, VertexProperty> Graph;
typedef typename boost::graph_traits<Graph>::edge_descriptor edge_descriptor;
typedef typename boost::graph_traits<Graph>::vertex_descriptor vertex_descriptor;
std::ostream& operator<<(std::ostream& os, const Graph& graph)
{
typedef std::map<vertex_descriptor, std::size_t> IndexMap;
IndexMap index_map;
boost::associative_property_map<IndexMap> index_properties(index_map);
std::size_t i = 0;
for (const vertex_descriptor& v : boost::make_iterator_range(boost::vertices(graph)))
index_properties[v] = i++;
boost::dynamic_properties dp;
typename boost::property_map<Graph, double VertexProperty::*>::const_type error_map = get(&VertexProperty::error, graph);
dp.property("error", error_map);
boost::write_graphml(os, graph,index_properties,dp);
return os;
}
int main()
{
Graph g;
std::cout << g <<std::endl;
}
编译失败:
boost/property_map/property_map.hpp:309:44: error: assignment of read-only location '(&((const boost::adj_list_vertex_property_map, double, const double&, double VertexProperty::*>&)pa))->boost::adj_list_vertex_property_map::operator[], double, const double&, double VertexProperty::*>(k)' static_cast(pa)[k] = v;
据我了解 dynamic_properties 文档,那些只读检查应该在运行时发生(这不是整个类型删除的目标之一)。当然,如果有人试图修改不可变属性,它们应该会失败。但是对 write write_graphml() 的调用采用对动态属性的 const ref,并且不应更改任何内容。
陈述问题:
- 为什么会编译失败?
- 我该如何正确地做到这一点?
- 通过使用其他一些 property_map(是/否/哪个)?
对于(非)运行示例@ coliru.stacked-crooked.com:See here!
问候, 马蒂
最佳答案
手头的真正问题是顶点属性映射的类别被推断为 LvaluePropertyMap
(它是)。
但是,LvaluePropertyMap
概念有望成为 ReadablePropertyMap
和 WritablePropertyMap
的超集。当使用图形属性的 const_type
时,这会带来问题:它们是左值,但不可写。
我想出的唯一可行的解决方案是包装属性映射类型以否决类别:
namespace detail {
template <typename Map>
struct readable_only_pmap : Map {
readable_only_pmap(Map map) : Map(map) { }
// overrule the category tag
typedef boost::readable_property_map_tag category;
};
}
现在,您可以像这样使用它:
using pmap_t = typename boost::property_map<Graph, double VertexProperty::*>::const_type;
detail::readable_only_pmap<pmap_t> error_map = get(&VertexProperty::error, graph);
虽然几乎相同,但现在 dynamic_properties::property
检测到 map 是只读的,并且不会尝试生成放置助手(相反,如果 将引发异常code>put
被尝试)。
带有演示图的完整代码:
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <iostream>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/graphml.hpp>
// #include <Eigen/Core>
// use bundled properties for vertices and edges
struct VertexProperty
{
double error;
// Eigen::Matrix<real,dim,1> location;
};
typedef boost::adjacency_list< boost::setS, boost::setS, boost::undirectedS, VertexProperty> Graph;
typedef typename boost::graph_traits<Graph>::edge_descriptor edge_descriptor;
typedef typename boost::graph_traits<Graph>::vertex_descriptor vertex_descriptor;
namespace detail {
template <typename Map>
struct readable_only_pmap : Map {
readable_only_pmap(Map map) : Map(map) { }
// overrule the category tag
typedef boost::readable_property_map_tag category;
};
}
std::ostream& operator<<(std::ostream& os, const Graph& graph)
{
typedef std::map<vertex_descriptor, std::size_t> IndexMap;
IndexMap index_map;
boost::associative_property_map<IndexMap> index_properties(index_map);
std::size_t i = 0;
for (const vertex_descriptor& v : boost::make_iterator_range(boost::vertices(graph)))
index_properties[v] = i++;
using pmap_t = typename boost::property_map<Graph, double VertexProperty::*>::const_type;
detail::readable_only_pmap<pmap_t> error_map = get(&VertexProperty::error, graph);
boost::dynamic_properties dp;
dp.property("error", error_map);
boost::write_graphml(os, graph, index_properties, dp);
return os;
}
int main()
{
Graph g;
auto v1 = boost::add_vertex(VertexProperty{0.1}, g);
auto v2 = boost::add_vertex(VertexProperty{0.2}, g);
auto v3 = boost::add_vertex(VertexProperty{0.3}, g);
auto v4 = boost::add_vertex(VertexProperty{0.4}, g);
auto v5 = boost::add_vertex(VertexProperty{0.5}, g);
add_edge(v1,v2,g);
add_edge(v5,v2,g);
add_edge(v4,v2,g);
add_edge(v2,v3,g);
add_edge(v3,v4,g);
add_edge(v4,v1,g);
std::cout << g <<std::endl;
}
输出:(稍微重新格式化)
<?xml version="1.0" encoding="UTF-8"?>
<graphml xmlns="http://graphml.graphdrawing.org/xmlns" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://graphml.graphdrawing.org/xmlns http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd">
<key id="key0" for="node" attr.name="error" attr.type="double" />
<graph id="G" edgedefault="undirected" parse.nodeids="free" parse.edgeids="canonical" parse.order="nodesfirst">
<node id="n0"> <data key="key0">0.1</data> </node>
<node id="n1"> <data key="key0">0.2</data> </node>
<node id="n2"> <data key="key0">0.3</data> </node>
<node id="n3"> <data key="key0">0.4</data> </node>
<node id="n4"> <data key="key0">0.5</data> </node>
<edge id="e0" source="n0" target="n1"> </edge>
<edge id="e1" source="n4" target="n1"> </edge>
<edge id="e2" source="n3" target="n1"> </edge>
<edge id="e3" source="n1" target="n2"> </edge>
<edge id="e4" source="n2" target="n3"> </edge>
<edge id="e5" source="n3" target="n0"> </edge>
</graph>
</graphml>
关于c++ - boost::dynamic_properties 和不可变图形对象,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/27232196/