From e1ba5baf65c2aca4c04fa2fd7263eed32ca6c63b Mon Sep 17 00:00:00 2001
From: Luca Heltai <luca.heltai@sissa.it>
Date: Thu, 8 Aug 2019 10:48:13 -0600
Subject: [PATCH] Added std_cxx17::apply() method.

---
 doc/news/changes/minor/20190808LucaHeltai |  3 ++
 include/deal.II/base/std_cxx17/tuple.h    | 58 +++++++++++++++++++++++
 tests/base/stdcxx17_apply.cc              | 39 +++++++++++++++
 tests/base/stdcxx17_apply.output          |  2 +
 4 files changed, 102 insertions(+)
 create mode 100644 doc/news/changes/minor/20190808LucaHeltai
 create mode 100644 include/deal.II/base/std_cxx17/tuple.h
 create mode 100644 tests/base/stdcxx17_apply.cc
 create mode 100644 tests/base/stdcxx17_apply.output

diff --git a/doc/news/changes/minor/20190808LucaHeltai b/doc/news/changes/minor/20190808LucaHeltai
new file mode 100644
index 0000000000..c974d5151e
--- /dev/null
+++ b/doc/news/changes/minor/20190808LucaHeltai
@@ -0,0 +1,3 @@
+New: Added std_cxx17::apply() method.
+<br>
+(Luca Heltai, 2019/08/08)
diff --git a/include/deal.II/base/std_cxx17/tuple.h b/include/deal.II/base/std_cxx17/tuple.h
new file mode 100644
index 0000000000..038ffcb43a
--- /dev/null
+++ b/include/deal.II/base/std_cxx17/tuple.h
@@ -0,0 +1,58 @@
+// ---------------------------------------------------------------------
+//
+// Copyright (C) 2019 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// The deal.II library is free software; you can use it, 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 2.1 of the License, or (at your option) any later version.
+// The full text of the license can be found in the file LICENSE.md at
+// the top level directory of deal.II.
+//
+// ---------------------------------------------------------------------
+#ifndef dealii_cxx17_tuple_h
+#define dealii_cxx17_tuple_h
+
+#include <deal.II/base/config.h>
+
+#ifndef DEAL_II_WITH_CXX17
+#  include <deal.II/base/std_cxx14/utility.h>
+#endif
+
+#include <cmath>
+
+DEAL_II_NAMESPACE_OPEN
+namespace std_cxx17
+{
+#ifndef DEAL_II_WITH_CXX17
+  template <typename F, typename Tuple, size_t... S>
+  auto
+  apply_impl(F &&fn, Tuple &&t, std_cxx14::index_sequence<S...>)
+    -> decltype(std::forward<F>(fn)(std::get<S>(std::forward<Tuple>(t))...))
+  {
+    return std::forward<F>(fn)(std::get<S>(std::forward<Tuple>(t))...);
+  }
+
+  template <typename F, typename Tuple>
+  auto
+  apply(F &&fn, Tuple &&t) -> decltype(apply_impl(
+    std::forward<F>(fn),
+    std::forward<Tuple>(t),
+    std_cxx14::make_index_sequence<
+      std::tuple_size<typename std::remove_reference<Tuple>::type>::value>()))
+  {
+    std::size_t constexpr tSize =
+      std::tuple_size<typename std::remove_reference<Tuple>::type>::value;
+    return apply_impl(std::forward<F>(fn),
+                      std::forward<Tuple>(t),
+                      std_cxx14::make_index_sequence<tSize>());
+  }
+#else
+  using std::apply;
+#endif // DEAL_II_WITH_CXX17
+} // namespace std_cxx17
+DEAL_II_NAMESPACE_CLOSE
+
+#endif
diff --git a/tests/base/stdcxx17_apply.cc b/tests/base/stdcxx17_apply.cc
new file mode 100644
index 0000000000..7426c87f19
--- /dev/null
+++ b/tests/base/stdcxx17_apply.cc
@@ -0,0 +1,39 @@
+// ---------------------------------------------------------------------
+//
+// Copyright (C) 2017 - 2018 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// The deal.II library is free software; you can use it, 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 2.1 of the License, or (at your option) any later version.
+// The full text of the license can be found in the file LICENSE.md at
+// the top level directory of deal.II.
+//
+// ---------------------------------------------------------------------
+
+
+// test the implementation of the std_cxx17::apply function
+
+
+#include <deal.II/base/point.h>
+#include <deal.II/base/std_cxx17/tuple.h>
+
+#include "../tests.h"
+
+
+void
+example_function(const Point<2> &p, const double &d, const unsigned int i = 3)
+{
+  deallog << "P: " << p << ", d: " << d << ", i: " << i << std::endl;
+}
+
+
+int
+main()
+{
+  initlog();
+  auto tup = std::make_tuple(Point<2>(.5, .5), 2.0, 3u);
+  std_cxx17::apply(example_function, tup);
+}
diff --git a/tests/base/stdcxx17_apply.output b/tests/base/stdcxx17_apply.output
new file mode 100644
index 0000000000..8f2ac511fb
--- /dev/null
+++ b/tests/base/stdcxx17_apply.output
@@ -0,0 +1,2 @@
+
+DEAL::P: 0.500000 0.500000, d: 2.00000, i: 3
-- 
2.39.5