# `read_graphml`

    void read_graphml(std::istream& in, MutableGraph& graph,
                      dynamic_properties& dp);

The `read_graphml` function interprets a graph described using the graphml format and builds a BGL graph that captures that description. Using this function, you can initialize a graph using data stored as text.

The graphml format can specify both directed and undirected graphs, and `read_graphml` differentiates between the two. One must pass `read_graphml` an undirected graph when reading an undirected graph; the same is true for directed graphs. Furthermore, `read_graphml` will throw an exception if it encounters parallel edges and cannot add them to the graph.

To handle attributes expressed in the graphml format, `read_graphml` takes a dynamic_properties object and operates on its collection of property maps. The reader passes all the properties encountered to this object, using the graphml attribute names as the property keys, and with the appropriate C++ type based on the graphml attribute type definition.

Requirements:
  - The type of the graph must model the Mutable Graph concept.
  - The type of the iterator must model the Multi-Pass Iterator concept.
  - The property map value types must be default-constructible.

Contents

- Where Defined
- Exceptions
- Building the graphml reader
- Notes
- See Also
- Future Work

# Where Defined

`<boost/graph/graphml.hpp>`

# Exceptions

    struct graph_exception : public std::exception {
      virtual ~graph_exception() throw();
      virtual const char* what() const throw() = 0;
    };

    struct bad_parallel_edge : public graph_exception {
      std::string from;
      std::string to;

      bad_parallel_edge(const std::string&, const std::string&);
      virtual ~bad_parallel_edge() throw();
      const char* what() const throw();
    };

    struct directed_graph_error : public graph_exception {
      virtual ~directed_graph_error() throw();
      virtual const char* what() const throw();
    };

    struct undirected_graph_error : public graph_exception {
      virtual ~undirected_graph_error() throw();
      virtual const char* what() const throw();
    };

    struct parse_error : public graph_exception {
      parse_error(const std::string&);
      virtual ~parse_error() throw() {}
      virtual const char* what() const throw();
      std::string statement;
    };

Under certain circumstances, `read_graphml` will throw one of the above exceptions. The three concrete exceptions can all be caught using the general `graph_exception` moniker when greater precision is not needed. In addition, all of the above exceptions derive from the standard `std::exception` for even more generalized error handling.

The `bad_parallel_edge` exception is thrown when an attempt to add a parallel edge to the supplied MutableGraph fails. The graphml format supports parallel edges, but some BGL-compatible graph types do not. One example of such a graph is `boost::adjacency_list<setS,vecS>`, which allows at most one edge can between any two vertices.

The `directed_graph_error` exception occurs when an undirected graph type is passed to `read_graph` but the textual representation of the graph is directed, as indicated by the `edgedefault="directed"` graph attribute in the graphml format.

The `undirected_graph_error` exception occurs when a directed graph type is passed to `read_graph` but the textual representation of the graph is undirected, as indicated by the `edgedefault="undirected"` graph attribute in the graphml format.

# Building the graphml reader

To use the graphml reader, you will need to build and link against the "bgl-graphml" library. The library can be built by following the Boost Jam Build Instructions for the subdirectory `libs/graph/build`.

# Notes

> - On successful reading of a graph, every vertex and edge will have an associated value for every respective edge and vertex property encountered while interpreting the graph. These values will be set using the `dynamic_properties` object. Some properties may be `put` multiple times during the course of reading in order to ensure the graphml semantics. Those edges and vertices that are not explicitly given a value for a property (and that property has no default) will be given the default constructed value of the value type. Be sure that property map value types are default constructible.
> - Nested graphs are supported as long as they are exactly of the same type as the root graph, i.e., are also directed or undirected. Note that since nested graphs are not directly supported by BGL, they are in fact completely ignored when building the graph, and the internal vertices or edges are interpreted as belonging to the root graph.
> - Hyperedges and Ports are not supported.

# See Also

write_graphml

# Future Work

> - Better expat error detection.