CODEWITHSUNDEEP
c++boostgraph
mkostya
mkostya

Reputation: 972

Printing boost::graph edge_descriptor

I am using a boost::graph and I noticed that when I am just using std::cout << *ei where ei is an edges iterator obtained, for example, from boost::edges command, I see that something like "(x, y)" is printed, where x and y are integers. I assume that these numbers represent vertices descriptors. According to this, I have some questions:

Thanks

Upvotes: 1

Views: 954

Answers (1)

sehe
sehe

Reputation: 392911

Q. does the default type for vertex_descriptor representation in boost::graph is int?

boost::graph is not a type.

Many graph types exist (BGL is a generic template library).

The actual type of vertex_descriptor is opaque, and can be integral. E.g. it is integral¹ with vecS vertex container selection in the adjacecy_list template argument list.

Q. Is there any operator<< for edge_descriptor is defined inside boost to make the output look like I described above? Otherwise, I cannot understand how it works in this way.

Yes. For such a graph type, the following overload of operator<< is injected into namespace std (see boost/graph/detail/edge.hpp):

namespace std {

  template <class Char, class Traits, class D, class V>
  std::basic_ostream<Char, Traits>& 
  operator<<(std::basic_ostream<Char, Traits>& os,
             const boost::detail::edge_desc_impl<D,V>& e)
  {
    return os << "(" << e.m_source << "," << e.m_target << ")";
  }

}

¹ Note that it's likely size_t, not int

Upvotes: 1

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