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#pragma once
#include <type_traits>
// A somewhat implementation of the expected monad, proposed here:
// https://isocpp.org/files/papers/n4015.pdf
// A class used to differentiate errors and values when they are of the same type.
template <typename Error>
class Unexpected
{
static_assert(!std::is_same<Error, void>::value, "Cannot have an Unexpected void");
public:
Unexpected() = delete;
constexpr explicit Unexpected(const Error &error) : mValue(error) {}
constexpr explicit Unexpected(Error &&error) : mValue(std::move(error)) {}
// For implicit conversions when doing makeUnexpected(other)
template <typename Other>
constexpr explicit Unexpected(const Unexpected<Other> &error) : mValue(error.value())
{
}
template <typename Other>
constexpr explicit Unexpected(Unexpected<Other> &&error) : mValue(std::move(error.value()))
{
}
constexpr const Error &value() const &
{
return mValue;
}
Error &value() &
{
return mValue;
}
constexpr const Error &&value() const &&
{
return std::move(mValue);
}
Error &&value() &&
{
return std::move(mValue);
}
private:
Error mValue;
};
template <class Error>
Unexpected<typename std::decay<Error>::type> makeUnexpected(Error &&e)
{
return Unexpected<typename std::decay<Error>::type>(std::forward<Error>(e));
}
template <typename Error>
bool operator==(const Unexpected<Error> &lhs, const Unexpected<Error> &rhs)
{
return lhs.value() == rhs.value();
}
template <typename Error>
bool operator!=(const Unexpected<Error> &lhs, const Unexpected<Error> &rhs)
{
return lhs.value() != rhs.value();
}
namespace detail {
namespace tags {
struct Expected
{};
struct Unexpected
{};
} // namespace tags
// Write functions here when storage related and when Type != void
template <typename Error, typename Type>
struct StorageBase
{
protected:
// Rule of 5 {{{
StorageBase(const StorageBase &other) : mIsValue(other.mIsValue)
{
// This is a constructor, you have to construct object, not assign them
// (hence the placement new)
if (mIsValue) {
new (std::addressof(mValue)) Type(other.mValue);
} else {
new (std::addressof(mError)) Unexpected<Error>(other.mError);
}
}
StorageBase(StorageBase &&other) : mIsValue(other.mIsValue)
{
// This is a constructor, you have to construct object, not assign them
// (hence the placement new)
if (mIsValue) {
new (std::addressof(mValue)) Type(std::move(other.mValue));
} else {
new (std::addressof(mError)) Unexpected<Error>(std::move(other.mError));
}
}
constexpr StorageBase &operator=(const StorageBase &other)
{
mIsValue = other.mIsValue;
if (mIsValue) {
mValue = other.mValue;
} else {
mError = other.mError;
}
return *this;
}
constexpr StorageBase &operator=(StorageBase &&other)
{
this->~StorageBase();
mIsValue = other.mIsValue;
if (mIsValue) {
mValue = std::move(other.mValue);
} else {
mError = std::move(other.mError);
}
return *this;
}
~StorageBase()
{
if (mIsValue) {
mValue.~Type();
} else {
mError.~Unexpected<Error>();
}
}
// }}}
template <typename... Args>
constexpr StorageBase(tags::Expected, Args &&... args)
: mValue(std::forward<Args>(args)...), mIsValue(true)
{
}
template <typename... Args>
constexpr StorageBase(tags::Unexpected, Args &&... args)
: mError(std::forward<Args>(args)...), mIsValue(false)
{
}
union
{
Unexpected<Error> mError;
Type mValue;
};
bool mIsValue;
};
// Write functions here when storage related and when Type == void
template <typename Error>
struct StorageBase<Error, void>
{
protected:
constexpr StorageBase(tags::Expected) : mIsValue(true) {}
template <typename... Args>
constexpr StorageBase(tags::Unexpected, Args &&... args)
: mError(std::forward<Args>(args)...), mIsValue(false)
{
}
Unexpected<Error> mError;
bool mIsValue;
};
// Write functions here when storage related, whether Type is void or not
template <typename Error, typename Type>
struct Storage : StorageBase<Error, Type>
{
protected:
// Forward the construction to StorageBase
using StorageBase<Error, Type>::StorageBase;
};
// Write functions here when dev API related and when Type != void
template <typename Error, typename Type>
struct ExpectedBase : detail::Storage<Error, Type>
{
constexpr ExpectedBase() : detail::Storage<Error, Type>(detail::tags::Expected{}) {}
template <typename OtherError>
constexpr ExpectedBase(const Unexpected<OtherError> &error)
: detail::Storage<Error, Type>(detail::tags::Unexpected{}, error)
{
}
template <typename OtherError>
constexpr ExpectedBase(Unexpected<OtherError> &&error)
: detail::Storage<Error, Type>(detail::tags::Unexpected{}, std::move(error))
{
}
constexpr ExpectedBase(const Type &value)
: detail::Storage<Error, Type>(detail::tags::Expected{}, value)
{
}
constexpr ExpectedBase(Type &&value)
: detail::Storage<Error, Type>(detail::tags::Expected{}, std::move(value))
{
}
// Warning: will crash if this is an error. You should always check this is
// an expected value before calling `.value()`
constexpr const Type &value() const &
{
Q_ASSERT(this->mIsValue);
return this->mValue;
}
Type &&value() &&
{
Q_ASSERT(this->mIsValue);
return std::move(this->mValue);
}
};
// Write functions here when dev API related and when Type == void
template <typename Error>
struct ExpectedBase<Error, void> : detail::Storage<Error, void>
{
// Rewrite constructors for unexpected because Expected doesn't have direct access to it.
template <typename OtherError>
constexpr ExpectedBase(const Unexpected<OtherError> &error)
: detail::Storage<Error, void>(detail::tags::Unexpected{}, error)
{
}
template <typename OtherError>
constexpr ExpectedBase(Unexpected<OtherError> &&error)
: detail::Storage<Error, void>(detail::tags::Unexpected{}, std::move(error))
{
}
};
} // namespace detail
// Write functions here when dev API related, whether Type is void or not
template <typename Error, typename Type = void>
class Expected : public detail::ExpectedBase<Error, Type>
{
static_assert(!std::is_same<Error, void>::value, "Expected with void Error is not implemented");
public:
using detail::ExpectedBase<Error, Type>::ExpectedBase;
constexpr const Error &error() const &
{
return this->mError.value();
}
constexpr bool isValue() const
{
return this->mIsValue;
}
constexpr explicit operator bool() const
{
return this->mIsValue;
}
};
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