From f381c24f29f27e3fd6893addefa3e835956b7e7a Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Wed, 27 Feb 2019 11:14:45 +1100
Subject: [PATCH] types: add a 'has_native' trait

Returns true if 'ValueT::native ()' is available.
---
 types.hpp | 35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/types.hpp b/types.hpp
index 37581cf7..31ce63bf 100644
--- a/types.hpp
+++ b/types.hpp
@@ -3,11 +3,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  *
- * Copyright 2011-2014 Danny Robson <danny@nerdcruft.net>
+ * Copyright 2011-2019 Danny Robson <danny@nerdcruft.net>
  */
 
-#ifndef CRUFT_UTIL_TYPES_HPP
-#define CRUFT_UTIL_TYPES_HPP
+#pragma once
 
 #include <cstdint>
 #include <cstdlib>
@@ -96,6 +95,32 @@ namespace cruft {
 
         return sizeof... (t);
     }
-}
 
-#endif
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// A trait that tests for the presence of a 'native' member function.
+    ///
+    /// Not terrifically useful by itself, but it is use reasonably frequently
+    /// in various library wrappers that we maintain (to return the underlying
+    /// native objects).
+    template <typename ValueT, typename = std::void_t<>>
+    struct has_native : public std::false_type
+    {};
+
+
+    //-------------------------------------------------------------------------
+    template <
+        typename ValueT
+    > struct has_native<
+        ValueT,
+        std::void_t<
+            decltype(std::declval<ValueT> ().native ())
+        >
+    > : public std::true_type {};
+
+
+    //-------------------------------------------------------------------------
+    template <typename ValueT>
+    constexpr auto has_native_v = has_native<ValueT>::value;
+}