void_t, real_t, field_t in dune/common/typetraits.hh

Contains:

• void_t (+voider)

• template <class T> using field_t = typename FieldTraits::field_type;

• template <class T> using real_t = typename FieldTraits::real_type;

Requires C++11.

dune/common/typetraits.hh

@@ -531,6 +531,38 @@ namespace Dune
#endif // defined(DOXYGEN) or HAVE_IS_INDEXABLE_SUPPORT
+  namespace detail
+  {
+    ///
+    /**
+     * @internal
+     * @brief Helper to make void_t work with gcc versions prior to gcc 5.0.
+     *
+     * This was not a compiler bug, but an accidental omission in the C++11 standard (see N3911, CWG issue 1558).
+     * It is not clearly specified what happens
+     * with unused template arguments in template aliases. The developers of GCC decided to ignore them, thus making void_t equivalent to void.
+     * With gcc 5.0 this was changed and the voider-hack is no longer needed.
+     */
+    template <class...>
+    struct voider
+    {
+      using type = void;
+    };
+  }
+
+  template <class> struct FieldTraits;
+
+  //! Is void for all valid input types (see N3911). The workhorse for C++11 SFINAE-techniques.
+  template <class... Types>
+  using void_t = typename detail::voider<Types...>::type;
+
+  //! Convenient access to FieldTraits<Type>::field_type.
+  template <class Type>
+  using field_t = typename FieldTraits<Type>::field_type;
+
+  //! Convenient access to FieldTraits<Type>::real_type.
+  template <class Type>
+  using real_t = typename FieldTraits<Type>::real_type;
/** @} */
}