From 36f0494e8f680e63dec5d44b5f3c256ea426fb87 Mon Sep 17 00:00:00 2001
From: Christos Houtouridis <houtouridis.ch@gmail.com>
Date: Tue, 23 Oct 2018 13:55:25 +0300
Subject: [PATCH] meta: Small changes and detect idiom

---
 include/utl/meta/bool.h     |  2 +-
 include/utl/meta/detect.h   | 70 +++++++++++++++++++++++++++++++++++++
 include/utl/meta/if.h       |  2 +-
 include/utl/meta/integral.h |  2 +-
 include/utl/meta/logical.h  |  2 +-
 include/utl/meta/sfinae.h   |  2 +-
 include/utl/meta/void.h     |  2 +-
 7 files changed, 76 insertions(+), 6 deletions(-)
 create mode 100644 include/utl/meta/detect.h

diff --git a/include/utl/meta/bool.h b/include/utl/meta/bool.h
index 1767115..5e91e00 100644
--- a/include/utl/meta/bool.h
+++ b/include/utl/meta/bool.h
@@ -20,7 +20,7 @@
 #ifndef __utl_meta_bool_h__
 #define __utl_meta_bool_h__
 
-#include <utl/impl/version.h>
+#include <utl/impl/impl.h>
 #include <utl/meta/integral.h>
 
 /*!
diff --git a/include/utl/meta/detect.h b/include/utl/meta/detect.h
new file mode 100644
index 0000000..49964a4
--- /dev/null
+++ b/include/utl/meta/detect.h
@@ -0,0 +1,70 @@
+/*!
+ * \file    utl/meta/detect.h
+ * \brief   Template meta-programming void helpers
+ *
+ * Copyright (C) 2018 Christos Choutouridis
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as
+ * published by the Free Software Foundation, either version 3
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef __utl_meta_detect_h__
+#define __utl_meta_detect_h__
+
+#include <utl/impl/impl.h>
+#include <utl/meta/sfinae.h>
+
+/*!
+ * \ingroup meta
+ * \defgroup detect
+ * Implementation of the detection idiom
+ */
+//! @{
+
+namespace utl {
+
+   //! detection idiom (negative case).
+   template <typename _Def,
+             typename _AlwaysVoid,
+             template<typename...> class _Op,
+             typename... _Args>
+   struct detector_ {
+      using value_ = false_;
+      using type = _Def;
+   };
+
+   //! detection idiom (positive case).
+   template <typename _Def,
+             template<typename...> class _Op,
+             typename... _Args>
+   struct detector_ <_Def, void_t<_Op<_Args...>>, _Op, _Args...> {
+      using value_ = true_;
+      using type = _Op<_Args...>;
+   };
+
+   // Detect whether _Op<_Args...> is a valid type, use _Def if not.
+   template <typename _Def,
+             template<typename...> class _Op,
+             typename... _Args>
+   using detected_or_ = detector_ <_Def, void, _Op, _Args...>;
+
+   // template alias
+   template <typename _Def,
+             template<typename...> class _Op,
+             typename... _Args>
+   using detected_or_t = typename detected_or_<_Def, _Op, _Args...>::type;
+
+
+
+}
+//! @}
+#endif /* __utl_meta_detect_h__ */
diff --git a/include/utl/meta/if.h b/include/utl/meta/if.h
index 11c0b89..66e5034 100644
--- a/include/utl/meta/if.h
+++ b/include/utl/meta/if.h
@@ -20,7 +20,7 @@
 #ifndef __utl_meta_if_h__
 #define __utl_meta_if_h__
 
-#include <utl/impl/version.h>
+#include <utl/impl/impl.h>
 
 
 /*!
diff --git a/include/utl/meta/integral.h b/include/utl/meta/integral.h
index 061b55f..11a2993 100644
--- a/include/utl/meta/integral.h
+++ b/include/utl/meta/integral.h
@@ -20,7 +20,7 @@
 #ifndef __utl_meta_integral_h__
 #define __utl_meta_integral_h__
 
-#include <utl/impl/version.h>
+#include <utl/impl/impl.h>
 
 /*!
  * \ingroup meta
diff --git a/include/utl/meta/logical.h b/include/utl/meta/logical.h
index 9960c39..6b8ce35 100644
--- a/include/utl/meta/logical.h
+++ b/include/utl/meta/logical.h
@@ -20,7 +20,7 @@
 #ifndef __utl_meta_logical_h__
 #define __utl_meta_logical_h__
 
-#include <utl/impl/version.h>
+#include <utl/impl/impl.h>
 #include <utl/meta/bool.h>
 #include <utl/meta/if.h>
 
diff --git a/include/utl/meta/sfinae.h b/include/utl/meta/sfinae.h
index 9dc127e..3f9f0ac 100644
--- a/include/utl/meta/sfinae.h
+++ b/include/utl/meta/sfinae.h
@@ -20,7 +20,7 @@
 #ifndef __utl_meta_use_h__
 #define __utl_meta_use_h__
 
-#include <utl/impl/version.h>
+#include <utl/impl/impl.h>
 #include <utl/meta/logical.h>
 
 
diff --git a/include/utl/meta/void.h b/include/utl/meta/void.h
index b5df113..2211b47 100644
--- a/include/utl/meta/void.h
+++ b/include/utl/meta/void.h
@@ -20,7 +20,7 @@
 #ifndef __utl_meta_void_h__
 #define __utl_meta_void_h__
 
-#include <utl/impl/version.h>
+#include <utl/impl/impl.h>
 #include <utl/meta/bool.h>
 
 /*! \ingroup meta