Test, implement and document 'at'.

[SVN r41125]

include/boost/unordered/detail/hash_table.hpp

@@ -17,6 +17,7 @@
 #include <cmath>
 #include <algorithm>
 #include <utility>
+#include <stdexcept>
 
 #include <boost/iterator.hpp>
 #include <boost/iterator/iterator_categories.hpp>

include/boost/unordered/detail/hash_table_impl.hpp

@@ -1839,6 +1839,17 @@ namespace boost {
                     return this->end();
             }
 
+            value_type& at(key_type const& k) const
+            {
+                bucket_ptr bucket = get_bucket(k);
+                local_iterator_base it = find_iterator(bucket, k);
+
+                if (it.not_finished())
+                    return *it;
+                else
+                    throw std::out_of_range("Unable to find key in unordered_map.");
+            }
+
             // equal_range
             //
             // strong exception safety, no side effects

include/boost/unordered_map.hpp

@@ -211,6 +211,16 @@ namespace boost
             return base[k].second;
         }
 
+        mapped_type& at(const key_type& k)
+        {
+            return base.at(k).second;
+        }
+
+        mapped_type const& at(const key_type& k) const
+        {
+            return base.at(k).second;
+        }
+
         // lookup
 
         iterator find(const key_type& k)