File: tuple.h

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/******************************************************************************
The MIT License(MIT)

Embedded Template Library.
https://github.com/ETLCPP/etl
https://www.etlcpp.com

Copyright(c) 2014 John Wellbelove

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files(the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and / or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions :

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
******************************************************************************/

#ifndef ETL_TUPLE_INCLUDED
#define ETL_TUPLE_INCLUDED

#include "platform.h"

#if ETL_NOT_USING_CPP11 && !defined(ETL_IN_UNIT_TEST)
  #error NOT SUPPORTED FOR C++03 OR BELOW
#endif

#if ETL_USING_CPP11

#if ETL_USING_STL
  #include <tuple>
#endif

#include "nth_type.h"
#include "type_traits.h"
#include "utility.h"
#include "functional.h"

#include "private/tuple_element.h"
#include "private/tuple_size.h"

namespace etl
{
  //***************************************************************************
  /// A tuple type
  /// etl::tuple<TTypes...>
  //***************************************************************************
  template <typename... TTypes>
  class tuple;

  //***************************************************************************
  /// Type trait to check if a type is an etl::tuple
  /// Default implementation.
  //***************************************************************************
  template <typename T>
  struct is_tuple : etl::false_type
  {
  };

  //***************************************************************************
  /// Type trait to check if a type is an etl::tuple
  /// Specialisation for etl::tuple
  //***************************************************************************
  template <typename... TTypes>
  struct is_tuple<etl::tuple<TTypes...>> : etl::true_type
  {
  };

  namespace private_tuple
  {
    //***************************************************************************
    /// Get the base of a tuple type whose head type is T.
    //***************************************************************************
    template <typename T, typename TTuple>
    struct tuple_type_base;

    // Specialisation for an empty tuple
    template <typename T>
    struct tuple_type_base<T, tuple<>>
    {
      using type = void;
    };

    // Recursive definition of the type.
    template <typename T, typename THead, typename... TTail>
    struct tuple_type_base<T, tuple<THead, TTail...>>
    {
      using type = etl::conditional_t<etl::is_same<T, THead>::value,
                                      tuple<THead, TTail...>,
                                      typename tuple_type_base<T, tuple<TTail...>>::type>;
    };

    // Get the base of a tuple type whose head type is T.
    template <typename T, typename TTuple>
    using tuple_type_base_t = typename tuple_type_base<T, TTuple>::type;

    //***************************************************************************
    /// ignore
    /// An object of unspecified type such that any value can be assigned to it with no effect. 
    /// Intended for use with etl::tie when unpacking a etl::tuple, as a placeholder for the arguments that are not used.
    /// https://en.cppreference.com/w/cpp/utility/tuple/ignore
    //***************************************************************************
    struct ignore_t
    {
      template <typename T>
      ETL_CONSTEXPR ignore_t operator =(T&&) const ETL_NOEXCEPT
      {
        return *this;
      }
    };
  }

  //***************************************************************************
  /// Empty tuple
  //***************************************************************************
  template<>
  class tuple<>
  {
  public:

    using value_type          = void;                        ///< The type contained by this tuple.
    using this_type           = tuple<>;                     ///< The type of this tuple.
    using base_type           = void;                        ///< The type of the base tuple.
    using index_sequence_type = etl::make_index_sequence<0>; ///< The index_sequence type for this tuple.

    //*********************************
    // No-op copy_assignment for the base case
    //*********************************
    ETL_CONSTEXPR14
    void copy_assignment(const this_type& /*other*/)
    {
    }

    //*********************************
    // No-op forward_assignment for the base case
    //*********************************
    ETL_CONSTEXPR14
    void forward_assignment(this_type&& /*other*/)
    {
    }

    //*********************************
    // No-op swap for the base case
    //*********************************
    ETL_CONSTEXPR14
    void swap(this_type& /*other*/)
    {
    }

    //*********************************
    // Returns the size of the base case.
    // Always zero.
    //*********************************
    ETL_NODISCARD 
    ETL_CONSTEXPR
    static size_t size()
    {
      return 0U;
    }
  };

  //***************************************************************************
  /// Tuple
  //***************************************************************************
  template<typename THead, typename... TTail>
  class tuple<THead, TTail...> : public tuple<TTail...>
  {
  private:

    //*********************************
    /// Helper function to calculate the number
    /// of types from a type list.
    //*********************************
    template <typename... UTypes>
    static constexpr size_t number_of_types()
    {
      return sizeof...(UTypes);
    }

  public:

    //*********************************
    /// Friends
    //*********************************
    template<typename... UTypes>
    friend class tuple;

    template <size_t Index, typename... TTypes>
    ETL_CONSTEXPR14
    friend etl::tuple_element_t<Index, etl::tuple<TTypes...>>& get(tuple<TTypes...>&);

    template <size_t Index, typename... TTypes>
    ETL_CONSTEXPR14 
    friend etl::tuple_element_t<Index, etl::tuple<TTypes...>>&& get(tuple<TTypes...>&&);

    template <size_t Index, typename... TTypes>
    ETL_CONSTEXPR14 
    friend const etl::tuple_element_t<Index, etl::tuple<TTypes...>>& get(const tuple<TTypes...>&);
        
    template <size_t Index, typename... TTypes>
    ETL_CONSTEXPR14 
    friend const etl::tuple_element_t<Index, etl::tuple<TTypes...>>&& get(const tuple<TTypes...>&&);

    template <typename T, typename... TTypes>
    ETL_CONSTEXPR14
    friend T& get(tuple<TTypes...>&);

    template <typename T, typename... TTypes>
    ETL_CONSTEXPR14
    friend T&& get(tuple<TTypes...>&&);

    template <typename T, typename... TTypes>
    ETL_CONSTEXPR14
    friend const T& get(const tuple<TTypes...>&);

    template <typename T, typename... TTypes>
    ETL_CONSTEXPR14
    friend const T&& get(const tuple<TTypes...>&&);

    //*********************************
    /// Types
    //*********************************
    using value_type          = THead;                  ///< The type contained by this tuple.
    using this_type           = tuple<THead, TTail...>; ///< The type of this tuple.
    using base_type           = tuple<TTail...>;        ///< The type of the base tuple.
    using index_sequence_type = etl::make_index_sequence<number_of_types<THead, TTail...>()>; ///< The index_sequence type for this tuple.

    //*********************************
    /// Default constructor.
    //*********************************
    ETL_CONSTEXPR14
    tuple()
      : value()
    {
    }

    //*********************************
    /// Copy constructor.
    //*********************************
    ETL_CONSTEXPR14
    tuple(const tuple<THead, TTail...>& other) = default;

    //*********************************
    /// Move constructor.
    //*********************************
    ETL_CONSTEXPR14
    tuple(tuple<THead, TTail...>&& other) = default;

    //*********************************
    /// Copy assignment.
    //*********************************
    ETL_CONSTEXPR14
    tuple& operator =(const tuple<THead, TTail...>& other) = default;

    //*********************************
    /// Move assignment.
    //*********************************
    ETL_CONSTEXPR14
    tuple& operator =(tuple<THead, TTail...>&& other) = default;

    //*********************************
    /// Copy construct from lvalue reference tuple type.
    /// Implicit conversion
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) && 
                                                                  etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(tuple<UHead, UTail...>& other)
      : base_type(other.get_base())
      , value(other.get_value())
    {
    }

    //*********************************
    /// Copy construct from lvalue reference tuple type.
    /// Explicit conversion
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) && 
                                                                  !etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14
    explicit tuple(tuple<UHead, UTail...>& other)
      : base_type(other.get_base())
      , value(other.get_value())
    {
    }

    //*********************************
    /// Copy construct from const lvalue reference tuple type.
    /// Implicit conversion
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) && 
                                                                  etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(const tuple<UHead, UTail...>& other)
      : base_type(other.get_base())
      , value(other.get_value())
    {
    }

    //*********************************
    /// Copy construct from const lvalue reference tuple type.
    /// Explicit conversion
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) && 
                                                                  !etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14 
    explicit tuple(const tuple<UHead, UTail...>& other)
      : base_type(other.get_base())
      , value(other.get_value())
    {
    }

    //*********************************
    /// Move construct from rvalue reference tuple type.
    /// Implicit conversion
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) && 
                                                                  etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(tuple<UHead, UTail...>&& other)
      : base_type(etl::forward<tuple<UTail...>>(other.get_base()))
      , value(etl::forward<UHead>(other.get_value()))
    {
    }

    //*********************************
    /// Move construct from rvalue reference tuple type.
    /// Explicit conversion
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) &&
                                                                  !etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14 
    explicit tuple(tuple<UHead, UTail...>&& other)
      : base_type(etl::forward<tuple<UTail...>>(other.get_base()))
      , value(etl::forward<UHead>(other.get_value()))
    {
    }

    //*********************************
    /// Construct from const rvalue reference tuple type.
    /// Implicit conversion
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) &&
                                                                  etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(const tuple<UHead, UTail...>&& other)
      : base_type(other.get_base())
      , value(other.get_value())
    {
    }

    //*********************************
    /// Construct from const rvalue reference tuple type.
    /// Explicit conversion
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) &&
                                                                  !etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14 
    explicit tuple(const tuple<UHead, UTail...>&& other)
      : base_type(other.get_base())
      , value(other.get_value())
    {
    }

    //*********************************
    /// Construct from arguments.
    //*********************************
    ETL_CONSTEXPR14
    tuple(const THead& head, const TTail&... tail)
      : base_type(tail...)
      , value(head)
    {
    }

    //*********************************
    /// Construct from arguments.
    /// Implicit conversion.
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<!is_tuple<etl::remove_reference_t<UHead>>::value &&
                                                                  (number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) && 
                                                                  (number_of_types<THead, TTail...>() >= 1U) && 
                                                                  etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(UHead&& head, UTail&&... tail) ETL_NOEXCEPT
      : base_type(etl::forward<UTail>(tail)...)
      , value(etl::forward<UHead>(head))
    {
    }

    //*********************************
    /// Construct from arguments.
    /// explicit conversion.
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<!is_tuple<etl::remove_reference_t<UHead>>::value &&
                                                                  (number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()) &&
                                                                  (number_of_types<THead, TTail...>() >= 1U) &&
                                                                  !etl::is_convertible<UHead, THead>::value, int> = 0>
    ETL_CONSTEXPR14
    explicit tuple(UHead&& head, UTail&&... tail) ETL_NOEXCEPT
      : base_type(etl::forward<UTail>(tail)...)
      , value(etl::forward<UHead>(head))
    {
    }

    //*********************************
    /// Construct from lvalue reference pair.
    /// Implicit conversion.
    //*********************************
    template <typename U1, typename U2, etl::enable_if_t<number_of_types<THead, TTail...>() == 2U &&
                                                         etl ::is_convertible<U1, THead>::value && 
                                                         etl ::is_convertible<U2, typename base_type::value_type>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(ETL_OR_STD::pair<U1, U2>& p) ETL_NOEXCEPT
      : base_type(p.second)
      , value(p.first)
    {
    }

    //*********************************
    /// Construct from lvalue reference pair.
    /// Explicit conversion.
    //*********************************
    template <typename U1, typename U2, etl::enable_if_t<number_of_types<THead, TTail...>() == 2U &&
                                                         (!etl ::is_convertible<U1, THead>::value || 
                                                          !etl ::is_convertible<U2, typename base_type::value_type>::value), int> = 0>
    ETL_CONSTEXPR14 
    explicit tuple(ETL_OR_STD::pair<U1, U2>& p) ETL_NOEXCEPT
      : base_type(p.second)
      , value(p.first)
    {
    }

    //*********************************
    /// Construct from const lvalue reference pair.
    /// Implicit conversion.
    //*********************************
    template <typename U1, typename U2, etl::enable_if_t<number_of_types<THead, TTail...>() == 2U && 
                                                         etl ::is_convertible<U1, THead>::value && 
                                                         etl ::is_convertible<U2, typename base_type::value_type>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(const ETL_OR_STD::pair<U1, U2>& p) ETL_NOEXCEPT
      : base_type(p.second)
      , value(p.first)
    {
    }

    //*********************************
    /// Construct from const lvalue reference pair.
    /// Explicit conversion.
    //*********************************
    template <typename U1, typename U2, etl::enable_if_t<number_of_types<THead, TTail...>() == 2U &&
                                                         (!etl ::is_convertible<U1, THead>::value || 
                                                          !etl ::is_convertible<U2, typename base_type::value_type>::value), int> = 0>
    ETL_CONSTEXPR14
    explicit tuple(const ETL_OR_STD::pair<U1, U2>& p) ETL_NOEXCEPT
      : base_type(p.second)
      , value(p.first)
    {
    }

    //*********************************
    /// Construct from rvalue reference pair.
    /// Implicit conversion.
    //*********************************
    template <typename U1, typename U2, etl::enable_if_t<number_of_types<THead, TTail...>() == 2U &&
                                                         etl ::is_convertible<U1, THead>::value && 
                                                         etl ::is_convertible<U2, typename base_type::value_type>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(ETL_OR_STD::pair<U1, U2>&& p) ETL_NOEXCEPT
      : base_type(etl::forward<U2>(p.second))
      , value(etl::forward<U1>(p.first))
    {
    }

    //*********************************
    /// Construct from rvalue reference pair.
    /// Explicit conversion.
    //*********************************
    template <typename U1, typename U2, etl::enable_if_t<number_of_types<THead, TTail...>() == 2U &&
                                                         (!etl ::is_convertible<U1, THead>::value ||
                                                          !etl ::is_convertible<U2, typename base_type::value_type>::value), int> = 0>
    ETL_CONSTEXPR14 
    explicit tuple(ETL_OR_STD::pair<U1, U2>&& p) ETL_NOEXCEPT
      : base_type(etl::forward<U2>(p.second))
      , value(etl::forward<U1>(p.first))
    {
    }

    //*********************************
    /// Construct from const rvalue reference pair.
    /// Implicit conversion.
    //*********************************
    template <typename U1, typename U2, etl::enable_if_t<number_of_types<THead, TTail...>() == 2U && 
                                                         etl ::is_convertible<U1, THead>::value &&
                                                         etl ::is_convertible<U2, typename base_type::value_type>::value, int> = 0>
    ETL_CONSTEXPR14
    tuple(const ETL_OR_STD::pair<U1, U2>&& p) ETL_NOEXCEPT
      : base_type(etl::forward<U2>(p.second))
      , value(etl::forward<U1>(p.first))
    {
    }

    //*********************************
    /// Construct from const rvalue reference pair.
    /// Explicit conversion.
    //*********************************
    template <typename U1, typename U2, etl::enable_if_t<number_of_types<THead, TTail...>() == 2U &&
                                                         (!etl ::is_convertible<U1, THead>::value || 
                                                          !etl ::is_convertible<U2, typename base_type::value_type>::value), int> = 0>
    ETL_CONSTEXPR14 
    explicit tuple(const ETL_OR_STD::pair<U1, U2>&& p) ETL_NOEXCEPT
      : base_type(p.second)
      , value(p.first)
    {
    }

    //*********************************
    /// Copy assign from other tuple type.
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()), int> = 0>
    ETL_CONSTEXPR14
    tuple& operator =(const tuple<UHead, UTail...>& other)
    {
      copy_assignment(other);

      return *this;
    }

    //*********************************
    /// Move assign from other tuple type.
    //*********************************
    template <typename UHead, typename... UTail, etl::enable_if_t<(number_of_types<THead, TTail...>() == number_of_types<UHead, UTail...>()), int> = 0>
    ETL_CONSTEXPR14
    tuple& operator =(tuple<UHead, UTail...>&& other)
    {
      forward_assignment(etl::forward<tuple<UHead, UTail...>>(other));

      return *this;
    }

    //*********************************
    /// Assign from lvalue pair tuple type.
    //*********************************
    template <typename U1, typename U2, size_t NTypes = number_of_types<THead, TTail...>, etl::enable_if_t<NTypes == 2U, int> = 0>
    ETL_CONSTEXPR14
    tuple& operator =(pair<U1, U2>& p)
    {
      get_value()            = p.first;
      get_base().get_value() = p.second;

      return *this;
    }

    //*********************************
    /// Assign from const lvalue pair tuple type.
    //*********************************
    template <typename U1, typename U2, size_t NTypes = number_of_types<THead, TTail...>, etl::enable_if_t<NTypes == 2U, int> = 0>
    ETL_CONSTEXPR14
    tuple& operator =(const pair<U1, U2>& p)
    {
      get_value()            = p.first;
      get_base().get_value() = p.second;

      return *this;
    }

    //*********************************
    /// Assign from rvalue pair tuple type.
    //*********************************
    template <typename U1, typename U2, size_t NTypes = number_of_types<THead, TTail...>, etl::enable_if_t<NTypes == 2U, int> = 0>
    ETL_CONSTEXPR14
    tuple& operator =(pair<U1, U2>&& p)
    {
      get_value()            = etl::forward<U1>(p.first);
      get_base().get_value() = etl::forward<U2>(p.second);

      return *this;
    }

    //*********************************
    /// Assign from const rvalue pair tuple type.
    //*********************************
    template <typename U1, typename U2, size_t NTypes = number_of_types<THead, TTail...>, etl::enable_if_t<NTypes == 2U, int> = 0>
    ETL_CONSTEXPR14
      tuple& operator =(const pair<U1, U2>&& p)
    {
      get_value()            = etl::forward<U1>(p.first);
      get_base().get_value() = etl::forward<U2>(p.second);

      return *this;
    }

    //*********************************
    /// Swaps this tuple with another.
    //*********************************
    ETL_CONSTEXPR14
    void swap(this_type& other)
    {
      using ETL_OR_STD::swap;

      // Swap the head
      swap(get_value(), other.get_value());

      auto& this_base = get_base();
      auto& other_base = other.get_base();

      // Recursively swap the tail by calling the base class's swap implementation.
      this_base.swap(other_base);
    }

    //*********************************
    /// Returns the number of elements in the tuple.
    //*********************************
    ETL_NODISCARD
    constexpr
    static size_t size()
    {
      return number_of_types<THead, TTail...>();
    }

  protected:

    //*********************************
    /// Returns a reference to the head value.
    //*********************************
    ETL_NODISCARD
    ETL_CONSTEXPR14
    THead& get_value()
    {
      return value;
    }

    //*********************************
    /// Returns a const reference to the head value.
    //*********************************
    ETL_CONSTEXPR
    const THead& get_value() const
    {
      return value;
    }

    //*********************************
    /// Get a reference to the base class.
    //*********************************
    ETL_NODISCARD
    ETL_CONSTEXPR14
    base_type& get_base()
    {
      return static_cast<base_type&>(*this);
    }

    //*********************************
    /// Get a const reference to the base class.
    //*********************************
    ETL_NODISCARD
    ETL_CONSTEXPR14
    const base_type& get_base() const
    {
      return static_cast<const base_type&>(*this);
    }

    //*********************************
    /// Handles copy assignment from another tuple.
    //*********************************
    template <typename UHead, typename... UTail>
    ETL_CONSTEXPR14
    void copy_assignment(const tuple<UHead, UTail...>& other)
    {
      // Assign the head
      this->value = other.get_value();

      // Get the base classes
      auto& this_base = get_base();
      const auto& other_base = other.get_base();

      // Recursively assign the tail by calling the base class's assignment implementation
      this_base.copy_assignment(other_base);
    }

    //*********************************
    /// Handles move assignment from another tuple.
    //*********************************
    template <typename UHead, typename... UTail>
    ETL_CONSTEXPR14
    void forward_assignment(tuple<UHead, UTail...>&& other)
    {
      // Assign the head
      this->value = etl::forward<UHead>(other.get_value());

      auto& this_base = get_base();
      auto&& other_base = other.get_base();

      // Recursively assign the tail by calling the base class's move assignment implementation
      this_base.forward_assignment(etl::forward<tuple<UTail...>>(other_base));
    }

  private:

    THead value;
  };

#if ETL_USING_CPP17
  //***************************************************************************
  /// Template deduction guideline from variadic arguments.
  //***************************************************************************
  template <typename... TArgs>
  tuple(TArgs... args) -> tuple<TArgs...>;

  //***************************************************************************
  /// Template deduction guideline from pair.
  //***************************************************************************
  template <typename T1, typename T2>
  tuple(ETL_OR_STD::pair<T1, T2>) -> tuple<T1, T2>;
#endif

  //***************************************************************************
  /// Gets the element type at the index in the tuple.
  //***************************************************************************
  template<size_t Index, typename... TTypes>
  struct tuple_element<Index, etl::tuple<TTypes...>>
  {
    using type = etl::nth_type_t<Index, TTypes...>;
  };

  //***************************************************************************
  /// Gets the size of the tuple.
  //***************************************************************************
  template <typename... TTypes>
  struct tuple_size<etl::tuple<TTypes...>>
    : etl::integral_constant<size_t, sizeof...(TTypes)>
  {
  };

  //***************************************************************************
  /// Gets the common type of a tuple.
  //***************************************************************************
  template<typename... Types>
  struct common_type<etl::tuple<Types...>> 
  {
    using type = etl::common_type_t<Types...>;
  };

  //***************************************************************************
  /// Extracts the element at Index from the tuple. 
  /// Index must be an integer value in sizeof...(TTypes)).
  /// Returns a reference.
  //***************************************************************************
  template <size_t Index, typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  etl::tuple_element_t<Index, etl::tuple<TTypes...>>& get(tuple<TTypes...>& t)
  {
    ETL_STATIC_ASSERT(Index < sizeof...(TTypes), "etl::get<Index> - Index out of range");

    // Get the type at this index.
    using tuple_type = etl::nth_base_t<Index, tuple<TTypes...>>&;

    // Cast the tuple to the selected type and get the value.
    return static_cast<tuple_type>(t).get_value();
  }

  //***************************************************************************
  /// Extracts the element at Index from the tuple. 
  /// Index must be an integer value in [?0?, sizeof...(TTypes)).
  /// Returns a const reference.
  //***************************************************************************
  template <size_t Index, typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  const etl::tuple_element_t<Index, etl::tuple<TTypes...>>& get(const tuple<TTypes...>& t)
  {
    ETL_STATIC_ASSERT(Index < sizeof...(TTypes), "etl::get<Index> - Index out of range");

    // Get the type at this index.
    using tuple_type = const etl::nth_base_t<Index, tuple<TTypes...>>&;

    // Cast the tuple to the selected type and get the value.
    return static_cast<tuple_type>(t).get_value();
  }

  //***************************************************************************
  /// Extracts the element at Index from the tuple. 
  /// Index must be an integer value in [?0?, sizeof...(TTypes)).
  /// Returns an rvalue reference.
  //***************************************************************************
  template <size_t Index, typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  etl::tuple_element_t<Index, etl::tuple<TTypes...>>&& get(tuple<TTypes...>&& t)
  {
    ETL_STATIC_ASSERT(Index < sizeof...(TTypes), "etl::get<Index> - Index out of range");

    // Get the type at this index.
    using tuple_type = etl::nth_base_t<Index, tuple<TTypes...>>&&;

    // Cast the tuple to the selected type and get the value.
    return etl::move(static_cast<tuple_type>(t).get_value());
  }

  //***************************************************************************
  /// Extracts the element at Index from the tuple. 
  /// Index must be an integer value in [?0?, sizeof...(TTypes)).
  /// Returns a const rvalue reference.
  //***************************************************************************
  template <size_t Index, typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  const etl::tuple_element_t<Index, etl::tuple<TTypes...>>&& get(const tuple<TTypes...>&& t)
  {
    ETL_STATIC_ASSERT(Index < sizeof...(TTypes), "etl::get<Index> - Index out of range");

    // Get the type at this index.
    using tuple_type = const etl::nth_base_t<Index, etl::tuple<TTypes...>>&&;

    // Cast the tuple to the selected type and get the value.
    return etl::move(static_cast<tuple_type>(t).get_value());
  }

  //***************************************************************************
  /// Extracts the element with type T from the tuple. 
  /// Static asserts if the tuple contain more than one T, or does not contain a T element.
  /// Returns a reference.
  //***************************************************************************
  template <typename T, typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  T& get(tuple<TTypes...>& t)
  {
    ETL_STATIC_ASSERT(!(etl::has_duplicates_of<T, TTypes...>::value), "etl::get<Type> - Tuple contains duplicate instances of T");
    ETL_STATIC_ASSERT((etl::is_one_of<T, TTypes...>::value),          "etl::get<Type> - Tuple does not contain the specified type");

    // Get the tuple base type that contains a T
    using tuple_type = etl::private_tuple::tuple_type_base_t<T, tuple<TTypes...>>&;

    // Cast the tuple to the selected type and get the value.
    return static_cast<tuple_type>(t).get_value();
  }

  //***************************************************************************
  /// Extracts the element with type T from the tuple. 
  /// Static asserts if the tuple contain more than one T, or does not contain a T element.
  /// Returns a const reference.
  //***************************************************************************
  template <typename T, typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  const T& get(const tuple<TTypes...>& t)
  {
    ETL_STATIC_ASSERT(!(etl::has_duplicates_of<T, TTypes...>::value), "etl::get<Type> - Tuple contains duplicate instances of T");
    ETL_STATIC_ASSERT((etl::is_one_of<T, TTypes...>::value),          "etl::get<Type> - Tuple does not contain the specified type");

    // Get the tuple base type that contains a T
    using tuple_type = const etl::private_tuple::tuple_type_base_t<T, tuple<TTypes...>>&;

    // Cast the tuple to the selected type and get the value.
    return static_cast<tuple_type>(t).get_value();
  }

  //***************************************************************************
  /// Extracts the element with type T from the tuple. 
  /// Static asserts if the tuple contain more than one T, or does not contain a T element.
  /// Returns an rvalue reference.
  //***************************************************************************
  template <typename T, typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  T&& get(tuple<TTypes...>&& t)
  {
    ETL_STATIC_ASSERT(!(etl::has_duplicates_of<T, TTypes...>::value), "etl::get<Type> - Tuple contains duplicate instances of T");
    ETL_STATIC_ASSERT((etl::is_one_of<T, TTypes...>::value),          "etl::get<Type> - Tuple does not contain the specified type");

    // Get the tuple base type that contains a T
    using tuple_type = etl::private_tuple::tuple_type_base_t<T, tuple<TTypes...>>&&;

    // Cast the tuple to the selected type and get the value.
    return etl::move(static_cast<tuple_type>(t).get_value());
  }

  //***************************************************************************
  /// Extracts the element with type T from the tuple. 
  /// Static asserts if the tuple contain more than one T, or does not contain a T element.
  /// Returns a const rvalue reference.
  //***************************************************************************
  template <typename T, typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  const T&& get(const tuple<TTypes...>&& t)
  {
    ETL_STATIC_ASSERT(!(etl::has_duplicates_of<T, TTypes...>::value), "etl::get<Type> - Tuple contains duplicate instances of T");
    ETL_STATIC_ASSERT((etl::is_one_of<T, TTypes...>::value),          "etl::get<Type> - Tuple does not contain the specified type");

    // Get the tuple base type that contains a T
    using tuple_type = const etl::private_tuple::tuple_type_base_t<T, tuple<TTypes...>>&&;

    // Cast the tuple to the selected type and get the value.
    return etl::move(static_cast<tuple_type>(t).get_value());
  }

#if ETL_USING_CPP17
  inline constexpr private_tuple::ignore_t ignore;
#else
  static constexpr private_tuple::ignore_t ignore;
#endif

  //***************************************************************************
  /// Creates a tuple of references to the provided arguments.
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR 
  etl::tuple<TTypes&...> tie(TTypes&... args)
  {
    return { args... };
  }

  //***************************************************************************
  // Creates a tuple from the provided arguments.
  //***************************************************************************
  template <typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  etl::tuple<etl::unwrap_ref_decay_t<TTypes>...> make_tuple(TTypes&&... args)
  {
    return etl::tuple<unwrap_ref_decay_t<TTypes>...>(etl::forward<TTypes>(args)...);
  }

  //***************************************************************************
  /// Creates a new tuple by selecting elements from another, given a run time index sequence.
  /// Static asserts if the number of indices does not match the tuple size.
  //***************************************************************************
  template <typename TTuple, size_t... Indices>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  auto select_from_tuple(TTuple&& tuple, etl::index_sequence<Indices...>)
    -> etl::tuple<etl::tuple_element_t<Indices, etl::decay_t<TTuple>>...> 
  {
    ETL_STATIC_ASSERT(sizeof...(Indices) <= etl::tuple_size<etl::decay_t<TTuple>>::value, "Number of indices is greater than the tuple size");

    return etl::make_tuple(etl::forward<etl::tuple_element_t<Indices, etl::decay_t<TTuple>>>(etl::get<Indices>(etl::forward<TTuple>(tuple)))...);
  }

  //***************************************************************************
  /// Creates a new tuple by selecting elements from another, given a template parameter index sequence.
  /// Static asserts if the number of indices does not match the tuple size.
  //***************************************************************************
  template <size_t... Indices, typename TTuple>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  auto select_from_tuple(TTuple&& tuple)
    -> etl::tuple<etl::tuple_element_t<Indices, etl::decay_t<TTuple>>...> 
  {
    return select_from_tuple(etl::forward<TTuple>(tuple), etl::index_sequence<Indices...>{});
  }

  //***************************************************************************
  /// Forwards the arguments as a tuple.
  //***************************************************************************
  template <typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  etl::tuple<TTypes&&...> forward_as_tuple(TTypes&&... args)
  {
    return tuple<TTypes&&...>(etl::forward<TTypes>(args)...);
  }

  namespace private_tuple
  {
    //**********************************
    // Helper to concatenate two tuples
    //**********************************
    template <typename Tuple1, typename Tuple2, size_t... Index1, size_t... Index2>
    ETL_CONSTEXPR14
    auto tuple_cat_impl(Tuple1&& t1, etl::index_sequence<Index1...>, Tuple2&& t2, etl::index_sequence<Index2...>)
      -> etl::tuple<etl::tuple_element_t<Index1, etl::decay_t<Tuple1>>..., etl::tuple_element_t<Index2, etl::decay_t<Tuple2>>...>
    {
      return etl::tuple<etl::tuple_element_t<Index1, etl::decay_t<Tuple1>>...,
                        etl::tuple_element_t<Index2, etl::decay_t<Tuple2>>...>
                        (etl::get<Index1>(etl::forward<Tuple1>(t1))..., etl::get<Index2>(etl::forward<Tuple2>(t2))...);
    }
  }

  //***************************************************************************
  /// Base case for concatenating one tuple
  //***************************************************************************
  template <typename Tuple>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  auto tuple_cat(Tuple&& t) -> Tuple
  {
    return etl::forward<Tuple>(t);
  }

  //***************************************************************************
  /// Recursive case for concatenating multiple tuples
  //***************************************************************************
  template <typename Tuple1, typename Tuple2, typename... Tuples>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  auto tuple_cat(Tuple1&& t1, Tuple2&& t2, Tuples&&... ts)
    -> decltype(private_tuple::tuple_cat_impl(etl::forward<Tuple1>(t1), 
                                              etl::make_index_sequence<etl::tuple_size<etl::decay_t<Tuple1>>::value>{},
                                              etl::forward<Tuple2>(t2),
                                              etl::make_index_sequence<etl::tuple_size<etl::decay_t<Tuple2>>::value>{}))
  {
    auto concatenated = private_tuple::tuple_cat_impl(etl::forward<Tuple1>(t1), 
                                                      etl::make_index_sequence<etl::tuple_size<etl::decay_t<Tuple1>>::value>{},
                                                      etl::forward<Tuple2>(t2),
                                                      etl::make_index_sequence<etl::tuple_size<etl::decay_t<Tuple2>>::value>{});

    return tuple_cat(etl::move(concatenated), etl::forward<Tuples>(ts)...);
  }

#if ETL_USING_STL
  //***************************************************************************
  // Tuple conversion functions.
  // From ETL to STL
  //***************************************************************************
  namespace private_tuple
  {
    ///*********************************
    template<typename TEtl_Tuple, size_t... Indices>
    ETL_NODISCARD
    ETL_CONSTEXPR14
    auto to_std_impl(const TEtl_Tuple& etl_tuple, etl::index_sequence<Indices...>)
      -> std::tuple<typename etl::tuple_element_t<Indices, TEtl_Tuple>...>
    {
      return std::tuple<etl::tuple_element_t<Indices, TEtl_Tuple>...>(etl::get<Indices>(etl_tuple)...);
    }

    ///*********************************
    template<typename TEtl_Tuple, size_t... Indices>
    ETL_NODISCARD
    ETL_CONSTEXPR14
    auto to_std_impl(TEtl_Tuple&& etl_tuple, etl::index_sequence<Indices...>)
      -> std::tuple<etl::tuple_element_t<Indices, TEtl_Tuple>...>
    {
      return std::tuple<etl::tuple_element_t<Indices, TEtl_Tuple>...>(etl::move(etl::get<Indices>(etl_tuple))...);
    }
  }

  //***************************************************************************
  /// Converts an etl::tuple to a std::tuple.
  //***************************************************************************
  template<typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  auto to_std(const etl::tuple<TTypes...>& etl_tuple)
    -> std::tuple<etl::decay_t<TTypes>...>
  {
    return private_tuple::to_std_impl(etl_tuple, etl::make_index_sequence_for<TTypes...>());
  }

  //***************************************************************************
  /// Converts an etl::tuple to a std::tuple.
  //***************************************************************************
  template<typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  auto to_std(etl::tuple<TTypes...>&& etl_tuple)
    -> std::tuple<etl::decay_t<TTypes>...>
  {
    return private_tuple::to_std_impl(etl::move(etl_tuple), etl::make_index_sequence_for<TTypes...>());
  }

  //***************************************************************************
  /// Tuple conversion functions.
  /// From STL to ETL
  //***************************************************************************
  namespace private_tuple
  {
    ///*********************************
    template<typename TStd_Tuple, size_t... Indices>
    ETL_NODISCARD
    ETL_CONSTEXPR14
    auto to_etl_impl(const TStd_Tuple& std_tuple, etl::index_sequence<Indices...>)
      -> etl::tuple<typename std::tuple_element<Indices, TStd_Tuple>::type...>
    {
      return etl::tuple<typename std::tuple_element<Indices, TStd_Tuple>::type...>(std::get<Indices>(std_tuple)...);
    }

    ///*********************************
    template<typename TStd_Tuple, size_t... Indices>
    ETL_NODISCARD
    ETL_CONSTEXPR14
    auto to_etl_impl(TStd_Tuple&& std_tuple, etl::index_sequence<Indices...>) 
      -> etl::tuple<typename std::tuple_element<Indices, TStd_Tuple>::type...>
    {
      return etl::tuple<typename std::tuple_element<Indices, TStd_Tuple>::type...>(std::move(std::get<Indices>(std_tuple))...);
    }
  }

  //***************************************************************************
  /// Converts a std::tuple to an etl::tuple.
  //***************************************************************************
  template<typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  auto to_etl(const std::tuple<TTypes...>& std_tuple)
    -> etl::tuple<etl::decay_t<TTypes>...>
  {
    return private_tuple::to_etl_impl(std_tuple, etl::make_index_sequence_for<TTypes...>());
  }

  //***************************************************************************
  /// Converts a std::tuple to an etl::tuple.
  //***************************************************************************
  template<typename... TTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  auto to_etl(std::tuple<TTypes...>&& std_tuple)
    -> etl::tuple<etl::decay_t<TTypes>...>
  {
    return private_tuple::to_etl_impl(etl::move(std_tuple), etl::make_index_sequence_for<TTypes...>());
  }
#endif

  namespace private_tuple
  {
    //***************************************************************************
    /// Equality
    //***************************************************************************
    // When there are no indices left to compare.
    template <typename TTuple1, typename TTuple2>
    ETL_NODISCARD
    ETL_CONSTEXPR14
    bool tuple_equality(const TTuple1& /*lhs*/, const TTuple2& /*rhs*/, etl::index_sequence<>)
    {
      return true;
    }

    // Recursive case: compare the current element and recurse.
    template <typename TTuple1, typename TTuple2, size_t Index, size_t... Indices>
    ETL_NODISCARD
    ETL_CONSTEXPR14
    bool tuple_equality(const TTuple1& lhs, const TTuple2& rhs, etl::index_sequence<Index, Indices...>)
    {
      return etl::get<Index>(lhs) == etl::get<Index>(rhs) && tuple_equality(lhs, rhs, etl::index_sequence<Indices...>{});
    }

    //***************************************************************************
    /// Less than
    //***************************************************************************
    // When there are no indices left to compare.
    template <typename TTuple1, typename TTuple2>
    ETL_NODISCARD
    ETL_CONSTEXPR14
    bool tuple_less_than(const TTuple1& /*lhs*/, const TTuple2& /*rhs*/, etl::index_sequence<>)
    {
      return false;
    }

    // Recursively compare the current element and the rest.
    template <typename TTuple1, typename TTuple2, size_t Index, size_t... Indices>
    ETL_NODISCARD
    ETL_CONSTEXPR14
    bool tuple_less_than(const TTuple1& lhs, const TTuple2& rhs, etl::index_sequence<Index, Indices...>)
    {
      if (get<Index>(lhs) < get<Index>(rhs))
      {
        return true;
      }

      if (get<Index>(rhs) < get<Index>(lhs))
      {
        return false;
      }

      return tuple_less_than(lhs, rhs, etl::index_sequence<Indices...>{});
    }
  }

  //***************************************************************************
  /// Equality operator.
  //*************************************************************************** 
  template <typename... TTypes, typename... UTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  bool operator ==(const etl::tuple<TTypes...>& lhs,
                   const etl::tuple<UTypes...>& rhs)
  {
    ETL_STATIC_ASSERT(sizeof...(TTypes) == sizeof...(UTypes), "Cannot compare tuples of different sizes");

    // Compare each element of the tuples.
    return private_tuple::tuple_equality(lhs, rhs, etl::make_index_sequence<etl::tuple<TTypes...>::size()>{});
  }

  //***************************************************************************
  /// Inequality operator.
  //*************************************************************************** 
  template <typename... TTypes, typename... UTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  bool operator !=(const etl::tuple<TTypes...>& lhs,
                   const etl::tuple<UTypes...>& rhs)
  {
    return !(lhs == rhs);
  }

  //***************************************************************************
  /// Less than operator.
  //*************************************************************************** 
  template <typename... TTypes, typename... UTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  bool operator <(const etl::tuple<TTypes...>& lhs,
                  const etl::tuple<UTypes...>& rhs)
  {
    ETL_STATIC_ASSERT(sizeof...(TTypes) == sizeof...(UTypes), "Cannot compare tuples of different sizes");

    // Compare the elements.
    return private_tuple::tuple_less_than(lhs, rhs, etl::make_index_sequence<etl::tuple<TTypes...>::size()>{});
  }

  //***************************************************************************
  /// Less than or equals operator.
  //*************************************************************************** 
  template <typename... TTypes, typename... UTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  bool operator <=(const etl::tuple<TTypes...>& lhs,
                   const etl::tuple<UTypes...>& rhs)
  {
    return !(rhs < lhs);
  }

  //***************************************************************************
  /// Greater than operator.
  //*************************************************************************** 
  template <typename... TTypes, typename... UTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  bool operator >(const etl::tuple<TTypes...>& lhs,
                  const etl::tuple<UTypes...>& rhs)
  {
    return (rhs < lhs);
  }

  //***************************************************************************
  /// Greater than or equals operator.
  //*************************************************************************** 
  template <typename... TTypes, typename... UTypes>
  ETL_NODISCARD
  ETL_CONSTEXPR14
  bool operator >=(const etl::tuple<TTypes...>& lhs,
                   const etl::tuple<UTypes...>& rhs)
  {
    return !(lhs < rhs);
  }

  //***************************************************************************
  /// Swap two tuples.
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR14
  void swap(etl::tuple<TTypes...>& lhs, etl::tuple<TTypes...>& rhs)
  {
    lhs.swap(rhs);
  }
}

namespace std
{
#if ETL_NOT_USING_STL && !((defined(ETL_DEVELOPMENT_OS_APPLE) || \
    (ETL_COMPILER_FULL_VERSION >= 190000)) && defined(ETL_COMPILER_CLANG))
  template <typename T>
  struct tuple_size;

  template <size_t Index, typename TType>
  struct tuple_element;
#endif

  //***************************************************************************
  /// Specialisation of tuple_size to allow the use of C++ structured bindings.
  //***************************************************************************
  template <typename... Types>
  struct tuple_size<etl::tuple<Types...>> : etl::integral_constant<size_t, sizeof...(Types)>
  {
  };

  //***************************************************************************
  /// Specialisation of tuple_element to allow the use of C++ structured bindings.
  //***************************************************************************
  template <size_t Index, typename... Types>
  struct tuple_element<Index, etl::tuple<Types...>>
  {
    using type = typename etl::nth_type_t<Index, Types...>;
  };
}

#endif
#endif