Add a property map with bool as value_type based on a set-like container

Property_map/include/CGAL/property_map.h

@@ -456,8 +456,52 @@ struct Default_property_map{
   get (const Default_property_map&, const key_type&){ return ValueType(); }
 };
 
+/// Read-write Property map turning a set (such a `std::set`,
+/// `boost::unordered_set`, `std::unordered_set`) into a property map
+/// associating a Boolean to the value type of the set. The function `get` will
+/// return `true` if the key is inside the set and `false` otherwise. The `put`
+/// function will insert an element in the set if `true` is passed and erase it
+/// otherwise.
+///
+/// `operator()(key k)` calling the get function with `k`
+template<class Set>
+struct Boolean_property_map
+{
+  typedef typename Set::value_type key_type;
+  typedef bool value_type;
+  typedef bool reference;
+  typedef boost::read_write_property_map_tag category;
 
-/// \endcond
+  Set* set_ptr;
+  /// Constructor taking a copy of the set. Note that `set_` must be valid
+  /// while the property map is in use.
+  Boolean_property_map() : set_ptr(NULL) {}
+  Boolean_property_map(Set& set_) : set_ptr(&set_) {}
+
+  friend bool get(const Boolean_property_map<Set>& pm, const key_type& k)
+  {
+    CGAL_assertion(pm.set_ptr!=NULL);
+    return pm.set_ptr->count(k) != 0;
+  }
+
+  friend void put(Boolean_property_map<Set>& pm, const key_type& k, bool v)
+  {
+    CGAL_assertion(pm.set_ptr!=NULL);
+    if (v)
+      pm.set_ptr->insert(k);
+    else
+      pm.set_ptr->erase(k);
+  }
+};
+
+/// \ingroup PkgProperty_map
+/// returns `Boolean_property_map<Set>(set_)`
+template <class Set>
+Boolean_property_map<Set>
+make_boolean_property_map(Set& set_)
+{
+  return Boolean_property_map<Set>(set_);
+}
 
 } // namespace CGAL