hash/std: add a trivial adapter for std::hash

CMakeLists.txt

@@ -425,6 +425,8 @@ list (
     hash/murmur/murmur3.hpp
     hash/siphash.cpp
     hash/siphash.hpp
+    hash/std.cpp
+    hash/std.hpp
     hash/table.cpp
     hash/table.hpp
     hash/tuple.cpp

hash/std.cpp

@@ -0,0 +1 @@
+#include "./std.hpp"

hash/std.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <utility>
+#include <type_traits>
+#include <functional>
+
+
+namespace cruft::hash {
+    // A trivial adapter around the `std::hash` that avoids the need to
+    // specify the type up front.
+    //
+    // Useful as a component in more complex hash objects (eg, hash::tuple)
+    struct std {
+        template <typename ValueT>
+        auto operator () (ValueT &&val)
+        {
+            using inner_t = ::std::remove_cvref_t<ValueT>;
+            return ::std::hash<inner_t> {} (::std::forward<ValueT> (val));
+        }
+    };
+}