Add hashable base for std::hash of strong typedefs (foonathan#74)

example/strong_typedef.cpp

@@ -4,6 +4,7 @@
 
 #include <iostream>
 #include <string>
+#include <unordered_set>
 
 #include <type_safe/strong_typedef.hpp>
 
@@ -55,12 +56,24 @@ std::ostream& operator<<(std::ostream& out, const distance& d)
     return out << static_cast<unsigned>(d) << " distance";
 }
 
+namespace std
+{
+    // we want to use it with the std::unordered_* containers
+    template <>
+    struct hash<::distance> : type_safe::hashable<::distance>
+    {
+    };
+
+} // namespace std
+
 int main()
 {
     distance d(4);
     //    int      val = d; // error
     //    d += 3;           // error
     d += distance(3); // works
 
-    std::cout << d << '\n';
+    std::unordered_set<distance> set{d};
+
+    std::cout << *set.find(d) << '\n';
 }

include/type_safe/strong_typedef.hpp

@@ -141,8 +141,8 @@ namespace type_safe
 
     /// Some operations for [ts::strong_typedef]().
     ///
-    /// They all generate operators forwarding to the underlying type,
-    /// inherit from then in the typedef definition.
+    /// They all generate operators forwarding to the underlying type.
+    /// Inherit from them in the typedef definition.
     namespace strong_typedef_op
     {
         /// \exclude
@@ -729,6 +729,21 @@ namespace type_safe
 #undef TYPE_SAFE_DETAIL_MAKE_OP_COMPOUND
 #undef TYPE_SAFE_DETAIL_MAKE_STRONG_TYPEDEF_OP
     } // namespace strong_typedef_op
+
+    /// Inherit from it in the `std::hash<StrongTypedef>` specialization to make
+    /// it hashable like the underlying type. See example/strong_typedef.cpp.
+    template <class StrongTypedef>
+    struct hashable
+    {
+        using underlying_type = type_safe::underlying_type<StrongTypedef>;
+        using underlying_hash = std::hash<underlying_type>;
+
+        std::size_t operator()(const StrongTypedef& lhs) const
+            noexcept(noexcept(underlying_hash{}(std::declval<underlying_type>())))
+        {
+            return underlying_hash{}(static_cast<const underlying_type&>(lhs));
+        }
+    };
 } // namespace type_safe
 
 #endif // TYPE_SAFE_STRONG_TYPEDEF_HPP_INCLUDED