Introduce CGAL_GENERATE_MEMBER_DETECTOR macro

STL_Extension/include/CGAL/Has_member.h

@@ -0,0 +1,43 @@
+// Copyright (c) 2017 Inria (France)
+// All rights reserved.
+//
+// This file is part of CGAL (www.cgal.org); 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.
+//
+// Licensees holding a valid commercial license may use this file in
+// accordance with the commercial license agreement provided with the software.
+//
+// This file is provided AS IS with NO WARRANTY OF ANY KIND, INCLUDING THE
+// WARRANTY OF DESIGN, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
+//
+//
+//
+// Author(s)     : Clement Jamin
+
+#ifndef CGAL_HAS_MEMBER_H
+#define CGAL_HAS_MEMBER_H
+
+// Macro used to check if a type T has a member named `X`
+// It generates a class has_X<T> where has_X<T>::value is a boolean
+// See example in Concurrent_compact_container.h
+#define CGAL_GENERATE_MEMBER_DETECTOR(X)                                      \
+template<typename T> class has_##X {                                          \
+    struct Fallback { int X; };                                               \
+    struct Derived : T, Fallback { };                                         \
+                                                                              \
+    template<typename U, U> struct Check;                                     \
+                                                                              \
+    typedef char ArrayOfOne[1];                                               \
+    typedef char ArrayOfTwo[2];                                               \
+                                                                              \
+    template<typename U> static ArrayOfOne & func(                            \
+                                            Check<int Fallback::*, &U::X> *); \
+    template<typename U> static ArrayOfTwo & func(...);                       \
+  public:                                                                     \
+    typedef has_##X type;                                                     \
+    enum { value = sizeof(func<Derived>(0)) == 2 };                           \
+} // semicolon is after the macro call
+
+#endif // CGAL_HAS_MEMBER_H