src/snippets/expected_implementation.cpp

/* Documentation snippet
(C) 2017-2019 Niall Douglas <http://www.nedproductions.biz/> (7 commits)
File Created: Mar 2017


Boost Software License - Version 1.0 - August 17th, 2003

Permission is hereby granted, free of charge, to any person or organization
obtaining a copy of the software and accompanying documentation covered by
this license (the "Software") to use, reproduce, display, distribute,
execute, and transmit the Software, and to prepare derivative works of the
Software, and to permit third-parties to whom the Software is furnished to
do so, all subject to the following:

The copyright notices in the Software and this entire statement, including
the above license grant, this restriction and the following disclaimer,
must be included in all copies of the Software, in whole or in part, and
all derivative works of the Software, unless such copies or derivative
works are solely in the form of machine-executable object code generated by
a source language processor.

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, TITLE AND NON-INFRINGEMENT. IN NO EVENT
SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.
*/

#include "../../../include/boost/outcome/outcome.hpp"

#if(_HAS_CXX17 || __cplusplus >= 201700)
//! [expected_implementation]
/* Here is a fairly conforming implementation of P0323R3 `expected<T, E>` using `checked<T, E>`.
It passes the reference test suite for P0323R3 at
https://github.com/viboes/std-make/blob/master/test/expected/expected_pass.cpp with modifications
only to move the test much closer to the P0323R3 Expected, as the reference test suite is for a
much older proposed Expected.

Known differences from P0323R3 in this implementation:
- `T` and `E` cannot be the same type.
- `E` must be default constructible.
- No variant storage is implemented (note the Expected proposal does not actually require this).
*/

namespace detail
{
  template <class T, class E> using expected_result = BOOST_OUTCOME_V2_NAMESPACE::checked<T, E>;
  template <class T, class E> struct enable_default_constructor : public expected_result<T, E>
  {
    using base = expected_result<T, E>;
    using base::base;
    constexpr enable_default_constructor()
        : base{BOOST_OUTCOME_V2_NAMESPACE::in_place_type<T>}
    {
    }
  };
  template <class T, class E> using select_expected_base = std::conditional_t<std::is_default_constructible<T>::value, enable_default_constructor<T, E>, expected_result<T, E>>;
}
template <class T, class E> class expected : public detail::select_expected_base<T, E>
{
  static_assert(!std::is_same<T, E>::value, "T and E cannot be the same in this expected implementation");
  using base = detail::select_expected_base<T, E>;

public:
  // Inherit base's constructors
  using base::base;
  expected() = default;

  // Expected takes in_place not in_place_type
  template <class... Args>
  constexpr explicit expected(std::in_place_t /*unused*/, Args &&... args)
      : base{BOOST_OUTCOME_V2_NAMESPACE::in_place_type<T>, std::forward<Args>(args)...}
  {
  }

  // Expected always accepts a T even if ambiguous
  BOOST_OUTCOME_TEMPLATE(class U)
  BOOST_OUTCOME_TREQUIRES(BOOST_OUTCOME_TPRED(std::is_constructible<T, U>::value))
  constexpr expected(U &&v)
      : base{BOOST_OUTCOME_V2_NAMESPACE::in_place_type<T>, std::forward<U>(v)}
  {
  }

  // Expected has an emplace() modifier
  template <class... Args> void emplace(Args &&... args) { *static_cast<base *>(this) = base{BOOST_OUTCOME_V2_NAMESPACE::in_place_type<T>, std::forward<Args>(args)...}; }

  // Expected has a narrow operator* and operator->
  constexpr const T &operator*() const & { return base::assume_value(); }
  constexpr T &operator*() & { return base::assume_value(); }
  constexpr const T &&operator*() const && { return base::assume_value(); }
  constexpr T &&operator*() && { return base::assume_value(); }
  constexpr const T *operator->() const { return &base::assume_value(); }
  constexpr T *operator->() { return &base::assume_value(); }

  // Expected has a narrow error() observer
  constexpr const E &error() const & { return base::assume_error(); }
  constexpr E &error() & { return base::assume_error(); }
  constexpr const E &&error() const && { return base::assume_error(); }
  constexpr E &error() && { return base::assume_error(); }
};
template <class E> class expected<void, E> : public BOOST_OUTCOME_V2_NAMESPACE::result<void, E, BOOST_OUTCOME_V2_NAMESPACE::policy::throw_bad_result_access<E, void>>
{
  using base = BOOST_OUTCOME_V2_NAMESPACE::result<void, E, BOOST_OUTCOME_V2_NAMESPACE::policy::throw_bad_result_access<E, void>>;

public:
  // Inherit base constructors
  using base::base;

  // Expected has a narrow operator* and operator->
  constexpr void operator*() const { base::assume_value(); }
  constexpr void operator->() const { base::assume_value(); }
};
template <class E> using unexpected = BOOST_OUTCOME_V2_NAMESPACE::failure_type<E>;
template <class E> unexpected<E> make_unexpected(E &&arg)
{
  return BOOST_OUTCOME_V2_NAMESPACE::failure<E>(std::forward<E>(arg));
}
template <class E, class... Args> unexpected<E> make_unexpected(Args &&... args)
{
  return BOOST_OUTCOME_V2_NAMESPACE::failure<E>(std::forward<Args>(args)...);
}
template <class E> using bad_expected_access = BOOST_OUTCOME_V2_NAMESPACE::bad_result_access_with<E>;
//! [expected_implementation]
#endif

int main()
{
  return 0;
}