# `read_graphviz`

    template <typename MutableGraph>
    bool read_graphviz(std::istream& in, MutableGraph& graph,
                       dynamic_properties& dp,
                       const std::string& node_id = "node_id");

    // Only available if BOOST_GRAPH_READ_GRAPHVIZ_ITERATORS is defined
    template <typename MultiPassIterator, typename MutableGraph>
    bool read_graphviz(MultiPassIterator begin, MultiPassIterator end,
                       MutableGraph& graph, dynamic_properties& dp,
                       const std::string& node_id = "node_id");

    // Deprecated GraphViz readers
    void read_graphviz(const std::string& file, GraphvizDigraph& g);
    void read_graphviz(FILE* file, GraphvizDigraph& g);
    void read_graphviz(const std::string& file, GraphvizGraph& g);
    void read_graphviz(FILE* file, GraphvizGraph& g);

The `read_graphviz` function interprets a graph described using the GraphViz DOT language and builds a BGL graph that captures that description. Using this function, you can initialize a graph using data stored as text. To use the iterator version of `read_graphviz`, you will need to define the macro BOOST_GRAPH_READ_GRAPHVIZ_ITERATORS before including the header `<boost/graph/graphviz.hpp>`. Doing so may greatly increase the amount of time required to compile the GraphViz reader.

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

To handle properties expressed in the DOT language, `read_graphviz` 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 GraphViz string keys as the property keys. Furthermore, `read_graphviz` stores node identifier names under the vertex property map named node_id.

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 GraphViz Readers
- Deprecated Readers
- Notes
- See Also
- Future Work

# Where Defined

`<boost/graph/graphviz.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();
    };

Under certain circumstances, `read_graphviz` 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 DOT language 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 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 `digraph` keyword in the DOT language.

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 `graph` keyword in the DOT language.

# Building the GraphViz Readers

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

# Deprecated Readers

The deprecated readers do not provide exceptions on error (they abort), they require the use of one of the predefined graph types (`GraphvizDigraph` or `GraphvizGraph`), and they do not support arbitrary properties. They will be removed in a future Boost version.

# Notes

> - The `read_graphviz` function does not use any code from the GraphViz distribution to interpret the DOT Language. Rather, the implementation was based on documentation found on the GraphViz web site, as well as experiments run using the dot application. The resulting interpretation may be subtly different from dot for some corner cases that are not well specified.
> - `read_graphviz` treats subgraphs as syntactic sugar. It does not reflect subgraphs as actual entities in the BGL. Rather, they are used to shorten some edge definitions as well as to give a subset of all nodes or edges certain properties. For example, the DOT graphs `digraph { a -> subgraph {b -> c} -> e }` and `digraph { a -> b -> e ; a -> c -> e ; b -> c}` are equivalent.
> - Subgraph IDs refer to subgraphs defined earlier in the graph description. Undefined subgraphs behave as empty subgraphs (`{}`).
> - 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 same semantics as the GraphViz tools. 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.

# See Also

write_graphviz

# Future Work

> - Currently the parser relies upon lowercase language keywords (i.e. "graph", "edge", etc.). The DOT Language specifies that they are case-insensitive. This change should be easy given the proper understanding of Spirit and some free time (hint hint!)
> - The parser currently does not handle continuation lines as defined in the DOT Language. Some more sophisticated parsing of identifiers (so-called "ID" in the source) is required to support this.