expected_ostream.h

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/*
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  it under the terms of the GNU General Public License, version 2.0,
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*/
 
#ifndef MYSQL_HARNESS_STDX_EXPECTED_OSTREAM_H_
#define MYSQL_HARNESS_STDX_EXPECTED_OSTREAM_H_
 
#include "mysql/harness/stdx/expected.h"
 
#include <ostream>
#include <type_traits>
 
// operator<< for std::expected
//
// the functions are kept in a separate header as it
//
// - isn't part of the std-proposal
// - includes <ostream> which isn't need for stdx::expected<> itself
 
namespace stdx {
namespace impl {
template <typename S, typename T, typename = void>
struct is_to_stream_writable : std::false_type {};
 
template <typename S, typename T>
struct is_to_stream_writable<
    S, T, std::void_t<decltype(std::declval<S &>() << std::declval<T>())>>
    : std::true_type {};
 
}  // namespace impl
 
/**
 * write stdx::expected<T, E> to std::ostream.
 *
 * T and E must be non-void.
 *
 * only takes part in overload-resolution if T and E support 'os << v'
 */
template <class T, class E>
inline std::ostream &operator<<(std::ostream &os,
                                const stdx::expected<T, E> &res)
  requires((impl::is_to_stream_writable<std::ostream, T>::value &&
            impl::is_to_stream_writable<std::ostream, E>::value))
{
  if (res)
    os << res.value();
  else
    os << res.error();
 
  return os;
}
 
/**
 * write stdx::expected<void, E> to std::ostream.
 *
 * only takes part in overload-resolution if E supports 'os << v'
 */
template <class E>
inline std::ostream &operator<<(std::ostream &os,
                                const stdx::expected<void, E> &res)  //
  requires(impl::is_to_stream_writable<std::ostream, E>::value)
{
  if (!res) os << res.error();
 
  return os;
}
 
/**
 * write stdx::unexpected<E> to std::ostream.
 *
 * only takes part in overload-resolution if E supports 'os << v'
 */
template <class E>
inline std::ostream &operator<<(std::ostream &os,
                                const stdx::unexpected<E> &res)  //
  requires(impl::is_to_stream_writable<std::ostream, E>::value)
{
  os << res.error();
 
  return os;
}
}  // namespace stdx
 
#endif