graphml attribute type mapping for types not in specification

[Sergey Mitsyn <svm at jinr.ru>]

At the moment, write_graphml tries to convert non-standard attribute value types (that are not in specification) which it got from dynamic_properties, to standard ones. A type is converted to a simple form, say unsigned int to int, etc, by a predefined mapping in write_graphml: value_types and type_names. The conversion only affects graph metainformation (<graphml>'s <key> elements): conversion of an attribute value to a string is done by dynamic properties in dynamic_property->get_string() (that is, the value is actually never converted by the writing function).

On the other hand, read_graphml tries to use type info from <key> elements and converts a string to the specified type and then passes it to dynamic property map as a boost::any instance. If a property map is passed to dynamic_properties that has non-standard value_type (not in boost::mutate_graph_impl::value_types list), but still "supported", like unsigned int, the read_graphml first checks if value types match and then, because they are not, unconditionally any_cast's to std::string, which fails with bad_any_cast being thrown.

This is unfortunate - a user has a promise that some attribute types are supported during writing, but then she can't read it back to the same property map.

On the contrary, read_graphvis doesn't try to convert an attribute value before passing it to dynamic_property_map, due to the fact that graphvis files contain no type information :). In its turn, a dynamic_property_map converts passed std::string to its value type successfully.

Maybe a better strategy is:

1) in write_graphml, write attribute types as currently done.

2) in read_graphml, disregard attribute type information and convert string representations to values in a dynamic_property_map. A more sophisticated and maybe type-safe alternative is to use a more complicated logic and check dynamic_property_map->value() for every possible matching value type in write_graphml::value_types for a given type name from write_graphml::type_names.