Add overload() and orderedOverload()

These helper functions allow to create overload sets from lambdas.
E.g. one can merge to lambdas for different argument types into
one overload set in order to easily implement type-specific behaviour
when looping over a tuple.

While the result of overload(f1, f2,...) will pick the best match
for a given set of arguments, orderedOverload(f1,f2,...) will pick
the first match which allows to prevent ambiguity.

This functionality is proposed for the standard in P0051R2
(open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0051r2.pdf).
There overload(...) is proposed and an upcomming proposal
with the functionality of orderedOverload(...) is announced.
Since the former is not accepted and the latter is not even written
I did explicitly implement this in Dune:: instead of Dune::Std::.

dune/common/CMakeLists.txt

@@ -74,6 +74,7 @@ install(FILES
         math.hh
         matvectraits.hh
         nullptr.hh
+        overloadset.hh
         parametertree.hh
         parametertreeparser.hh
         path.hh

dune/common/overloadset.hh

+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
+#ifndef DUNE_COMMON_OVERLOADSET_HH
+#define DUNE_COMMON_OVERLOADSET_HH
+
+#include <utility>
+#include <type_traits>
+
+#include <dune/common/std/type_traits.hh>
+
+
+
+namespace Dune {
+
+namespace Impl {
+
+  // This overload set derives from
+  // all passed functions. Since we
+  // cannot do argument pack expansion
+  // on using statements this is done recursively.
+  template<class F0, class... F>
+  class OverloadSet: public OverloadSet<F...>, F0
+  {
+    using Base = OverloadSet<F...>;
+  public:
+
+    template<class FF0, class... FF>
+    OverloadSet(FF0&& f0, FF&&... ff) :
+      Base(std::forward<FF>(ff)...),
+      F0(std::forward<FF0>(f0))
+    {}
+
+    using F0::operator();
+    using Base::operator();
+  };
+
+  template<class F0>
+  class OverloadSet<F0>: public F0
+  {
+  public:
+
+    template<class FF0>
+    OverloadSet(FF0&& f0) :
+      F0(std::forward<FF0>(f0))
+    {}
+
+    using F0::operator();
+  };
+
+} // end namespace Impl
+
+
+
+/**
+ * \brief Create an overload set
+ *
+ * \tparam F List of function object types
+ * \param f List of function objects
+ *
+ * This returns an object that contains all
+ * operator() implementations of the passed
+ * functions. All those are available when
+ * calling operator() of the returned object.
+ *
+ * The returned object derives from
+ * those implementations such that it contains
+ * all operator() implementations in its
+ * overload set. When calling operator()
+ * this will select the best overload.
+ * If multiple overload are equally good this
+ * will lead to ambiguity.
+ *
+ * Notice that the passed function objects are
+ * stored by value and must be copy-constructible.
+ */
+template<class... F>
+auto overload(F&&... f)
+{
+  return Impl::OverloadSet<std::decay_t<F>...>(std::forward<F>(f)...);
+}
+
+
+
+namespace Impl {
+
+  template<class F0, class... F>
+  class OrderedOverloadSet: public OrderedOverloadSet<F...>, F0
+  {
+    using Base = OrderedOverloadSet<F...>;
+  public:
+
+    template<class FF0, class... FF>
+    OrderedOverloadSet(FF0&& f0, FF&&... ff) :
+      Base(std::forward<FF>(ff)...),
+      F0(std::forward<FF0>(f0))
+    {}
+
+    template<class...  Args,
+        std::enable_if_t<Std::is_callable<F0(Args&&...)>::value, int> = 0>
+    decltype(auto) operator()(Args&&... args)
+    {
+      return F0::operator()(std::forward<Args>(args)...);
+    }
+
+    template<class...  Args,
+        std::enable_if_t< not Std::is_callable<F0(Args&&...)>::value, int> = 0>
+    decltype(auto) operator()(Args&&... args)
+    {
+      return Base::operator()(std::forward<Args>(args)...);
+    }
+
+  };
+
+  template<class F0>
+  class OrderedOverloadSet<F0>: public F0
+  {
+  public:
+
+    template<class FF0>
+    OrderedOverloadSet(FF0&& f0) :
+      F0(std::forward<FF0>(f0))
+    {}
+
+    template<class...  Args>
+    decltype(auto) operator()(Args&&... args)
+    {
+      static_assert(Std::is_callable<F0(Args&&...)>::value, "No matching overload found in OrderedOverloadSet");
+      return F0::operator()(std::forward<Args>(args)...);
+    }
+  };
+
+} // end namespace Impl
+
+
+
+/**
+ * \brief Create an ordered overload set
+ *
+ * \tparam F List of function object types
+ * \param f List of function objects
+ *
+ * This returns an object that contains all
+ * operator() implementations of the passed
+ * functions. All those are available when
+ * calling operator() of the returned object.
+ *
+ * In contrast to overload() these overloads
+ * are ordered in the sense that the first
+ * matching overload for the given arguments
+ * is selected and later ones are ignored.
+ * Hence such a call is never ambiguous.
+ *
+ * Notice that the passed function objects are
+ * stored by value and must be copy-constructible.
+ */
+template<class... F>
+auto orderedOverload(F&&... f)
+{
+  return Impl::OrderedOverloadSet<std::decay_t<F>...>(std::forward<F>(f)...);
+}
+
+
+
+} // end namespace Dune
+
+#endif // DUNE_COMMON_OVERLOADSET_HH

dune/common/test/CMakeLists.txt

@@ -131,6 +131,9 @@ dune_add_test(NAME mpihelpertest2
               LINK_LIBRARIES dunecommon
               )
 
+dune_add_test(SOURCES overloadsettest.cc
+              LINK_LIBRARIES dunecommon)
+
 dune_add_test(SOURCES parametertreelocaletest.cc
               LINK_LIBRARIES dunecommon)
 

dune/common/test/overloadsettest.cc

+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+
+#include <tuple>
+
+#include <dune/common/overloadset.hh>
+#include <dune/common/hybridutilities.hh>
+
+#include <dune/common/test/testsuite.hh>
+
+
+int main()
+{
+  Dune::TestSuite test;
+
+  {
+    auto foo = Dune::overload(
+            [](double i) { return 0; },
+            [](int i) { return 1; },
+            [](long i) { return 2; });
+
+    test.check(foo(3.14) == 0)
+      << "incorrect overload selected from OverloadSet";
+    test.check(foo(int(42)) == 1)
+      << "incorrect overload selected from OverloadSet";
+    test.check(foo(long(42)) == 2)
+      << "incorrect overload selected from OverloadSet";
+  }
+
+  {
+    auto foo = Dune::orderedOverload(
+            [](double i) { return 0; },
+            [](int i) { return 1; },
+            [](long i) { return 2; });
+
+    test.check(foo(3.14) == 0)
+      << "incorrect overload selected from OverloadSet";
+    test.check(foo(int(42)) == 0)
+      << "incorrect overload selected from OverloadSet";
+    test.check(foo(long(42)) == 0)
+      << "incorrect overload selected from OverloadSet";
+  }
+
+  {
+    auto foo = Dune::overload(
+            [](const int& i) { return 0; },
+            [](int&& i) { return 1; });
+
+    int i = 0;
+    test.check(foo(long(42)) == 1)
+      << "incorrect overload selected from OverloadSet";
+    test.check(foo(int(42)) == 1)
+      << "incorrect overload selected from OverloadSet";
+    test.check(foo(i) == 0)
+      << "incorrect overload selected from OverloadSet";
+  }
+
+  {
+    auto foo = Dune::orderedOverload(
+            [](const int& i) { return 0; },
+            [](int&& i) { return 1; });
+
+    int i = 0;
+    test.check(foo(long(42)) == 0)
+      << "incorrect overload selected from OverloadSet";
+    test.check(foo(int(42)) == 0)
+      << "incorrect overload selected from OverloadSet";
+    test.check(foo(i) == 0)
+      << "incorrect overload selected from OverloadSet";
+  }
+
+  {
+    auto t = std::make_tuple(42, "foo", 3.14);
+
+    auto typeToName = Dune::overload(
+            [](int) { return "int"; },
+            [](long) { return "long"; },
+            [](std::string) { return "string"; },
+            [](float) { return "float"; },
+            [](double) { return "double"; });
+
+    std::string tupleTypes;
+    Dune::Hybrid::forEach(t, [&](auto&& ti) {
+      tupleTypes += typeToName(ti);
+    });
+
+    test.check(tupleTypes == "intstringdouble")
+      << "traversal of tuple called incorrect overloads";
+  }
+
+
+  return test.exit();
+}