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#pragma once
#include <variant>
namespace Quotient {
//! \brief A minimal subset of std::expected from C++23
template <typename T, typename E,
std::enable_if_t<!std::is_same_v<T, E>, bool> = true>
class Expected {
private:
template <typename X>
using enable_if_constructible_t = std::enable_if_t<
std::is_constructible_v<T, X> || std::is_constructible_v<E, X>>;
public:
using value_type = T;
using error_type = E;
Expected() = default;
explicit Expected(const Expected&) = default;
explicit Expected(Expected&&) noexcept = default;
template <typename X, typename = enable_if_constructible_t<X>>
Expected(X&& x)
: data(std::forward<X>(x))
{}
Expected& operator=(const Expected&) = default;
Expected& operator=(Expected&&) noexcept = default;
template <typename X, typename = enable_if_constructible_t<X>>
Expected& operator=(X&& x)
{
data = std::forward<X>(x);
return *this;
}
bool has_value() const { return std::holds_alternative<T>(data); }
explicit operator bool() const { return has_value(); }
const value_type& value() const& { return std::get<T>(data); }
value_type& value() & { return std::get<T>(data); }
value_type value() && { return std::get<T>(std::move(data)); }
const value_type& operator*() const& { return value(); }
value_type& operator*() & { return value(); }
const value_type* operator->() const& { return std::get_if<T>(&data); }
value_type* operator->() & { return std::get_if<T>(&data); }
template <class U>
T value_or(U&& fallback) const&
{
if (has_value())
return value();
return std::forward<U>(fallback);
}
template <class U>
T value_or(U&& fallback) &&
{
if (has_value())
return value();
return std::forward<U>(fallback);
}
const E& error() const& { return std::get<E>(data); }
E& error() & { return std::get<E>(data); }
private:
std::variant<T, E> data;
};
} // namespace Quotient
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