Re: [Boost-users] [graph] How to find out at compile time if a graph has some concrete internal property?

On Thu, 19 Mar 2009, Andrew Sutton wrote: (snip)

Unfortunately, I don't think such a function actually exists. However, if you can detect these at compile time, you can easily reflect that as a runtime query:

template bool has_property(Graph const& g) { return has_property_metafunc::value; }

It appears that the value_type of an undefined non-bundled property map is always boost::detail::error_property_not_found, so you can use the following metafunction to detect them: #include #include #include #include template struct graph_has_property: public boost::mpl::not_< boost::is_same< typename boost::property_traits< typename boost::property_map::type >::value_type, boost::detail::error_property_not_found > > {}; This always returns true when given a bundled property, however (it does work for testing for non-bundled properties in graphs that have bundled properties). I have not figured out how to test for bundled properties; there is a compile error when trying to get a particular value from a nonexistent member of a bundled property map. Is that a case that is important to you? There are several Boost graph algorithms that make a decision based on whether a property exists; look for the handling of the color map at the bottom of boost/graph/breadth_first_search.hpp for an example (look at the definition of boost::detail::bfs_dispatch). The code there is a more complicated version of the metafunction above, but can also handle an optionally user-specified color map. -- Jeremiah Willcock