[glibmm] Glib::Variant: Add template specialization for std::tuple

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 6e205b721ebb26264bb00bdf294e04255de34f34
Author: Alexander Rössler <alexander roessler zuehlke com>
Date:   Mon Mar 6 15:13:27 2017 +0100

    Glib::Variant: Add template specialization for std::tuple
    
    Bug 777791

 glib/src/variant.hg |  186 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 186 insertions(+), 0 deletions(-)
---
diff --git a/glib/src/variant.hg b/glib/src/variant.hg
index 92e0b6b..4843398 100644
--- a/glib/src/variant.hg
+++ b/glib/src/variant.hg
@@ -26,6 +26,7 @@ _DEFS(glibmm,glib)
 #include <utility>
 #include <vector>
 #include <map>
+#include <tuple>
 #include <stdexcept>
 #include <typeinfo>
 
@@ -944,6 +945,71 @@ public:
   VariantIter get_iter() const;
 };
 
+/** Specialization of Variant containing a tuple.
+ * @newin{2,52}
+ * @ingroup Variant
+ */
+template <class... Types>
+class Variant<std::tuple<Types...>> : public VariantContainerBase
+{
+public:
+  using CppContainerType = std::tuple<Types...>;
+
+  /// Default constructor
+  Variant<std::tuple<Types...>>()
+  : VariantContainerBase()
+  {}
+
+  /** GVariant constructor.
+   * @param castitem The GVariant to wrap.
+   * @param take_a_reference Whether to take an extra reference of the GVariant
+   *        or not (not taking one could destroy the GVariant with the wrapper).
+   */
+  explicit Variant<std::tuple<Types...>>(GVariant* castitem, bool take_a_reference = false)
+  : VariantContainerBase(castitem, take_a_reference)
+  {}
+
+  /** Creates a new Variant containing a tuple.
+   * @param data The tuple to use for creation.
+   * @return The new Variant holding a tuple.
+   * @newin{2,52}
+   */
+  static Variant<std::tuple<Types...>> create(const std::tuple<Types...>& data);
+
+  /** Gets the VariantType.
+   * @return The VariantType.
+   * @newin{2,52}
+   */
+  static const VariantType& variant_type() G_GNUC_CONST;
+
+  /** Gets a specific element from the tuple.
+   * It is an error if @a index is greater than or equal to the number of
+   * elements in the tuple. See VariantContainerBase::get_n_children().
+   *
+   * @param index The index of the element.
+   * @return The tuple element at index @a index.
+   * @throw std::out_of_range
+   * @newin{2,52}
+   */
+  template<class T>
+  T get_child(gsize index) const;
+
+  template<class T>
+  Variant<T> get_child_variant(gsize index) const;
+
+  /** Gets the tuple of the Variant.
+   * @return The tuple.
+   * @newin{2,52}
+   */
+  std::tuple<Types...> get() const;
+
+  /** Gets a VariantIter of the Variant.
+   * @return The VariantIter.
+   * @newin{2,52}
+   */
+  VariantIter get_iter() const;
+};
+
 } // namespace Glib
 
 
@@ -1229,4 +1295,124 @@ VariantIter Variant< std::map<K, V> >::get_iter() const
   return VariantContainerBase::get_iter(variant_type());
 }
 
+/*---------------------Variant<std::tuple<class... Types>> --------------------*/
+
+// static
+template <class... Types>
+const VariantType& Variant<std::tuple<Types...>>::variant_type()
+{
+  std::vector<VariantType> types;
+  auto expander = [&types](const VariantType &type) mutable -> int
+  {
+    types.push_back(type);
+    return 0;
+  };
+
+  // expands the variadic template parameters
+  using swallow = int[]; // ensures left to right order
+  swallow{(expander(Variant<Types>::variant_type()))...};
+  static auto type = VariantType::create_tuple(types);
+
+  return type;
+}
+
+namespace detail
+{
+template <class Tuple, std::size_t... Is>
+void expand_tuple(std::vector<VariantBase> &variants, const Tuple & t,
+                  std::index_sequence<Is...>)
+{
+  using swallow = int[]; // ensures left to right order
+  auto expander = [&variants](const VariantBase &variant) -> int
+  {
+    variants.push_back(variant);
+    return 0;
+  };
+  (void)swallow {(expander(Variant<typename std::tuple_element<Is, 
Tuple>::type>::create(std::get<Is>(t))))...};
+}
+} // namespace detail
+
+template <class... Types>
+Variant<std::tuple<Types...>>
+Variant<std::tuple<Types...>>::create(const std::tuple<Types...>& data)
+{
+  // create a vector containing all tuple values as variants
+  std::vector<Glib::VariantBase> variants;
+  detail::expand_tuple(variants, data, std::index_sequence_for<Types...>{});
+
+  using var_ptr = GVariant*;
+  var_ptr* const var_array = new var_ptr[sizeof... (Types)];
+
+  for (std::vector<VariantBase>::size_type i = 0; i < variants.size(); i++)
+    var_array[i] = const_cast<GVariant*>(variants[i].gobj());
+
+  Variant<std::tuple<Types...>> result = Variant<std::tuple<Types...>>(
+          g_variant_new_tuple(var_array, variants.size()));
+
+  return result;
+}
+
+template <class... Types>
+template <class T>
+T Variant<std::tuple<Types...>>::get_child(gsize index) const
+{
+  Variant<T> entry;
+  VariantContainerBase::get_child(entry, index);
+  return entry.get();
+}
+
+template <class... Types>
+template <class T>
+Variant<T> Variant<std::tuple<Types...>>::get_child_variant(gsize index) const
+{
+  Variant<T> entry;
+  VariantContainerBase::get_child(entry, index);
+  return entry;
+}
+
+namespace detail
+{
+// swallows any argument
+template <class T>
+constexpr int any_arg(T&& arg)
+{
+  (void)arg;
+  return 0;
+}
+
+template <class Tuple, std::size_t... Is>
+void assign_tuple(std::vector<VariantBase> &variants, Tuple & t, std::index_sequence<Is...>)
+{
+  int i = 0;
+  using swallow = int[]; // ensures left to right order
+  (void)swallow {(any_arg(std::get<Is>(t) = VariantBase::cast_dynamic<Variant<typename 
std::tuple_element<Is, Tuple>::type > >(variants[i++]).get()))...};
+}
+} // namespace detail
+
+template <class... Types>
+std::tuple<Types...> Variant<std::tuple<Types...>>::get() const
+{
+  std::tuple<Types...> data;
+  int i = 0;
+
+  std::vector<VariantBase> variants;
+  using swallow = int[]; // ensures left to right order
+  auto expander = [&variants, &i](const VariantBase &variant) -> int
+  {
+    variants.push_back(variant);
+    return i++;
+  };
+  swallow{(expander(get_child_variant<Types>(i)))...};
+  detail::assign_tuple(variants, data, std::index_sequence_for<Types...>{});
+
+  return data;
+}
+
+template< class... Types>
+VariantIter Variant<std::tuple<Types...>>::get_iter() const
+{
+  const auto type = variant_type();
+  return VariantContainerBase::get_iter(type);
+}
+
 } // namespace Glib