What is a property map in BOOST?
boost, boost-graph, boost-property-map, c++
Solution
PropertyMaps at their core are an abstraction of data access. A problem that comes up very quickly in generic programming is: How do I get data associated with some object? It could be stored in the object itself, the object could be a pointer, it could be outside of the object in some mapping structure.
You can of course encapsulate data-access in a functor, but that becomes tedious very quickly and you look for a more narrow solution, the one chosen in Boost are PropertyMaps.
Remember this is just the concept. Concrete instances are for example an `std::map` (with some syntactic adaption), a function returning a member of the key (again, with some syntactic adaption).
Towards your edit: `make_iterator_property_map` builds an iterator_property_map. The first argument provides an iterator for a basis of offset calculations. The second argument is again a property_map to do the offset calculation. Together this provides a way to use an `vertex_descriptor` to write data to the `vector` based on the index of the `vertex_descriptor`.
Problem
Can someone explain to a Boost beginner like me what is a property map is in Boost? I came across this when trying to use the BGL for calculating strong connected components. I went through the documentation for the property map and graph module and still don't know what to make of it. Take this code, for example: - what is the make_iterator_property_map function doing? - and what is the meaning of this code: get(vertex_index, G)? ``` #include <boost/config.hpp> #include <vector> #include <iostream> #include <boost/graph/strong_components.hpp> #include <boost/graph/adjacency_list.hpp> int main() { using namespace boost; typedef adjacency_list < vecS, vecS, directedS > Graph; const int N = 6; Graph G(N); add_edge(0, 1, G); add_edge(1, 1, G); add_edge(1, 3, G); add_edge(1, 4, G); add_edge(3, 4, G); add_edge(3, 0, G); add_edge(4, 3, G); add_edge(5, 2, G); std::vector<int> c(N); int num = strong_components (G, make_iterator_property_map(c.begin(), get(vertex_index, G), c[0])); std::cout << "Total number of components: " << num << std::endl; std::vector < int >::iterator i; for (i = c.begin(); i != c.end(); ++i) std::cout << "Vertex " << i - c.begin() << " is in component " << *i << std::endl; return EXIT_SUCCESS; } ```