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expected.h
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expected.h
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// Copyright 2022 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_TYPES_EXPECTED_H_
#define BASE_TYPES_EXPECTED_H_
#include <type_traits>
#include <utility>
#include "base/check.h"
#include "base/types/expected_internal.h" // IWYU pragma: export
#include "third_party/abseil-cpp/absl/utility/utility.h"
// Class template `expected<T, E>` is a vocabulary type which contains an
// expected value of type `T`, or an error `E`. The class skews towards behaving
// like a `T`, because its intended use is when the expected type is contained.
// When something unexpected occurs, more typing is required. When all is good,
// code mostly looks as if a `T` were being handled.
//
// Class template `expected<T, E>` contains either:
// * A value of type `T`, the expected value type; or
// * A value of type `E`, an error type used when an unexpected outcome
// occurred.
//
// The interface can be queried as to whether the underlying value is the
// expected value (of type `T`) or an unexpected value (of type `E`). The
// interface and the rational are based on `std::optional`. We consider
// `expected<T, E>` as a supplement to `optional<T>`, expressing why an expected
// value isn’t contained in the object.
//
// Example Usage:
//
// Before:
// bool ParseInt32(base::StringPiece input,
// int32_t* output,
// ParseIntError* error);
// ...
//
// int32_t output;
// ParseIntError error;
// if (ParseInt32("...". &output, &error)) {
// // process `output`
// } else {
// // process `error`
// }
//
// After:
//
// base::expected<int32_t, ParseIntError> ParseInt32(base::StringPiece input);
// ...
//
// if (auto parsed = ParseInt32("..."); parsed.has_value()) {
// // process `parsed.value()`
// } else {
// // process `parsed.error()`
// }
//
// References:
// * https://wg21.link/P0323
// * https://eel.is/c++draft/expected
namespace base {
// Note: base::unexpected and base::expected are C++17 compatible backports of
// C++23's std::unexpected and std::expected. They differ from the standard in
// the following ways:
//
// * Not all member functions can be used in a constexpr context. This is due to
// limitations in both the C++17 language and the Abseil library used for the
// implementation.
// * Since Chromium does not support exceptions, there is no bad_expected_access
// exception and the program will just terminate when the exception would have
// been thrown. Furthermore, all member functions are marked noexcept.
// * Calling operator* or operator-> on an unexpected value results in program
// termination, and not UB.
// * There is no operator bool due to bug-prone usage when the value type is
// convertible to bool, see e.g. https://abseil.io/tips/141.
// * Moving out of an expected object will put it into a moved-from state.
// Trying to use it before re-initializing it will result in program
// termination.
// * The expected<void> specialization is done via a defaulted boolean template
// parameter, due to the lack of requires clauses in C++17.
// * Since equality operators can not be defaulted in C++17, equality and
// inequality operators are specified explicitly.
// [expected.un.object], class template unexpected
// https://eel.is/c++draft/expected#un.object
template <typename E>
class unexpected {
public:
// [expected.un.ctor] Constructors
template <typename Err = E,
internal::EnableIfUnexpectedValueConstruction<E, Err> = 0>
constexpr explicit unexpected(Err&& err) noexcept
: value_(std::forward<Err>(err)) {}
template <typename... Args>
constexpr explicit unexpected(absl::in_place_t, Args&&... args) noexcept
: value_(std::forward<Args>(args)...) {}
template <typename U, typename... Args>
constexpr explicit unexpected(absl::in_place_t,
std::initializer_list<U> il,
Args&&... args) noexcept
: value_(il, std::forward<Args>(args)...) {}
// [expected.un.obs] Observers
constexpr E& value() & noexcept { return value_; }
constexpr const E& value() const& noexcept { return value_; }
constexpr E&& value() && noexcept { return std::move(value()); }
constexpr const E&& value() const&& noexcept { return std::move(value()); }
// [expected.un.swap] Swap
constexpr void swap(unexpected& other) noexcept {
using std::swap;
swap(value(), other.value());
}
friend constexpr void swap(unexpected& x, unexpected& y) noexcept {
x.swap(y);
}
private:
E value_;
};
// [expected.un.eq] Equality operator
template <typename E, typename G>
constexpr bool operator==(const unexpected<E>& lhs,
const unexpected<G>& rhs) noexcept {
return lhs.value() == rhs.value();
}
template <typename E, typename G>
constexpr bool operator!=(const unexpected<E>& lhs,
const unexpected<G>& rhs) noexcept {
return !(lhs == rhs);
}
template <typename E>
unexpected(E) -> unexpected<E>;
// in-place construction of unexpected values
struct unexpect_t {
explicit unexpect_t() = default;
};
inline constexpr unexpect_t unexpect{};
// [expected.expected], class template expected
// https://eel.is/c++draft/expected#expected
template <typename T, typename E>
class expected<T, E, /* is_void_v<T> = */ false> {
// Note: A partial specialization for void value types follows below.
static_assert(!std::is_void_v<T>, "Error: T must not be void");
public:
using value_type = T;
using error_type = E;
using unexpected_type = unexpected<E>;
// Alias template to explicitly opt into the std::pointer_traits machinery.
// See e.g. https://en.cppreference.com/w/cpp/memory/pointer_traits#Notes
template <typename U>
using rebind = expected<U, E>;
template <typename U, typename G, bool IsVoid>
friend class expected;
// [expected.object.ctor], constructors
constexpr expected() noexcept = default;
// Converting copy and move constructors. These constructors are explicit if
// either the value or error type is not implicitly convertible from `rhs`'s
// corresponding type.
template <typename U,
typename G,
internal::EnableIfExplicitConversion<T, E, const U&, const G&> = 0>
explicit constexpr expected(const expected<U, G>& rhs) noexcept
: impl_(rhs.impl_) {}
template <typename U,
typename G,
internal::EnableIfImplicitConversion<T, E, const U&, const G&> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
/* implicit */ constexpr expected(const expected<U, G>& rhs) noexcept
: impl_(rhs.impl_) {}
template <typename U,
typename G,
internal::EnableIfExplicitConversion<T, E, U, G> = 0>
explicit constexpr expected(expected<U, G>&& rhs) noexcept
: impl_(std::move(rhs.impl_)) {}
template <typename U,
typename G,
internal::EnableIfImplicitConversion<T, E, U, G> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
/* implicit */ constexpr expected(expected<U, G>&& rhs) noexcept
: impl_(std::move(rhs.impl_)) {}
template <typename U = T,
internal::EnableIfExplicitValueConstruction<T, E, U> = 0>
explicit constexpr expected(U&& v) noexcept
: impl_(kValTag, std::forward<U>(v)) {}
template <typename U = T,
internal::EnableIfImplicitValueConstruction<T, E, U> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
/* implicit */ constexpr expected(U&& v) noexcept
: impl_(kValTag, std::forward<U>(v)) {}
template <typename G,
internal::EnableIfExplicitUnexpectedConstruction<E, const G&> = 0>
explicit constexpr expected(const unexpected<G>& e) noexcept
: impl_(kErrTag, e.value()) {}
template <typename G,
internal::EnableIfImplicitUnexpectedConstruction<E, const G&> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
/* implicit */ constexpr expected(const unexpected<G>& e) noexcept
: impl_(kErrTag, e.value()) {}
template <typename G,
internal::EnableIfExplicitUnexpectedConstruction<E, G> = 0>
explicit constexpr expected(unexpected<G>&& e) noexcept
: impl_(kErrTag, std::move(e.value())) {}
template <typename G,
internal::EnableIfImplicitUnexpectedConstruction<E, G> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
/* implicit */ constexpr expected(unexpected<G>&& e) noexcept
: impl_(kErrTag, std::move(e.value())) {}
template <typename... Args>
constexpr explicit expected(absl::in_place_t, Args&&... args) noexcept
: impl_(kValTag, std::forward<Args>(args)...) {}
template <typename U, typename... Args>
constexpr explicit expected(absl::in_place_t,
std::initializer_list<U> il,
Args&&... args) noexcept
: impl_(kValTag, il, std::forward<Args>(args)...) {}
template <typename... Args>
constexpr explicit expected(unexpect_t, Args&&... args) noexcept
: impl_(kErrTag, std::forward<Args>(args)...) {}
template <typename U, typename... Args>
constexpr explicit expected(unexpect_t,
std::initializer_list<U> il,
Args&&... args) noexcept
: impl_(kErrTag, il, std::forward<Args>(args)...) {}
// [expected.object.assign], assignment
template <typename U = T, internal::EnableIfValueAssignment<T, E, U> = 0>
constexpr expected& operator=(U&& v) noexcept {
emplace(std::forward<U>(v));
return *this;
}
template <typename G>
constexpr expected& operator=(const unexpected<G>& e) noexcept {
impl_.emplace_error(e.value());
return *this;
}
template <typename G>
constexpr expected& operator=(unexpected<G>&& e) noexcept {
impl_.emplace_error(std::move(e.value()));
return *this;
}
template <typename... Args>
constexpr T& emplace(Args&&... args) noexcept {
return impl_.emplace_value(std::forward<Args>(args)...);
}
template <typename U, typename... Args>
constexpr T& emplace(std::initializer_list<U> il, Args&&... args) noexcept {
return impl_.emplace_value(il, std::forward<Args>(args)...);
}
// [expected.object.swap], swap
constexpr void swap(expected& rhs) noexcept { impl_.swap(rhs.impl_); }
friend constexpr void swap(expected& x, expected& y) noexcept { x.swap(y); }
// [expected.object.obs], observers
constexpr T* operator->() noexcept { return std::addressof(value()); }
constexpr const T* operator->() const noexcept {
return std::addressof(value());
}
constexpr T& operator*() & noexcept { return value(); }
constexpr const T& operator*() const& noexcept { return value(); }
constexpr T&& operator*() && noexcept { return std::move(value()); }
constexpr const T&& operator*() const&& noexcept {
return std::move(value());
}
// Note: Deviation from the Standard: No operator bool due to bug-prone
// patterns when the value type is convertible to bool, see e.g.
// https://abseil.io/tips/141.
constexpr bool has_value() const noexcept { return impl_.has_value(); }
constexpr T& value() & noexcept { return impl_.value(); }
constexpr const T& value() const& noexcept { return impl_.value(); }
constexpr T&& value() && noexcept { return std::move(value()); }
constexpr const T&& value() const&& noexcept { return std::move(value()); }
constexpr E& error() & noexcept { return impl_.error(); }
constexpr const E& error() const& noexcept { return impl_.error(); }
constexpr E&& error() && noexcept { return std::move(error()); }
constexpr const E&& error() const&& noexcept { return std::move(error()); }
template <typename U>
constexpr T value_or(U&& v) const& noexcept {
static_assert(std::is_copy_constructible_v<T>,
"expected<T, E>::value_or: T must be copy constructible");
static_assert(std::is_convertible_v<U&&, T>,
"expected<T, E>::value_or: U must be convertible to T");
return has_value() ? value() : static_cast<T>(std::forward<U>(v));
}
template <typename U>
constexpr T value_or(U&& v) && noexcept {
static_assert(std::is_move_constructible_v<T>,
"expected<T, E>::value_or: T must be move constructible");
static_assert(std::is_convertible_v<U&&, T>,
"expected<T, E>::value_or: U must be convertible to T");
return has_value() ? std::move(value())
: static_cast<T>(std::forward<U>(v));
}
private:
using Impl = internal::ExpectedImpl<T, E>;
static constexpr auto kValTag = Impl::kValTag;
static constexpr auto kErrTag = Impl::kErrTag;
Impl impl_;
};
// [expected.void], partial specialization of expected for void types
template <typename T, typename E>
class expected<T, E, /* is_void_v<T> = */ true> {
// Note: A partial specialization for non-void value types can be found above.
static_assert(std::is_void_v<T>, "Error: T must be void");
public:
using value_type = T;
using error_type = E;
using unexpected_type = unexpected<E>;
// Alias template to explicitly opt into the std::pointer_traits machinery.
// See e.g. https://en.cppreference.com/w/cpp/memory/pointer_traits#Notes
template <typename U>
using rebind = expected<U, E>;
template <typename U, typename G, bool IsVoid>
friend class expected;
// [expected.void.ctor], constructors
constexpr expected() noexcept = default;
// Converting copy and move constructors. These constructors are explicit if
// the error type is not implicitly convertible from `rhs`'s error type.
template <typename U,
typename G,
internal::EnableIfExplicitVoidConversion<E, U, const G&> = 0>
constexpr explicit expected(const expected<U, G>& rhs) noexcept
: impl_(rhs.impl_) {}
template <typename U,
typename G,
internal::EnableIfImplicitVoidConversion<E, U, const G&> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
constexpr /* implicit */ expected(const expected<U, G>& rhs) noexcept
: impl_(rhs.impl_) {}
template <typename U,
typename G,
internal::EnableIfExplicitVoidConversion<E, U, G> = 0>
constexpr explicit expected(expected<U, G>&& rhs) noexcept
: impl_(std::move(rhs.impl_)) {}
template <typename U,
typename G,
internal::EnableIfImplicitVoidConversion<E, U, G> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
constexpr /* implicit */ expected(expected<U, G>&& rhs) noexcept
: impl_(std::move(rhs.impl_)) {}
template <typename G,
internal::EnableIfExplicitUnexpectedConstruction<E, const G&> = 0>
explicit constexpr expected(const unexpected<G>& e) noexcept
: impl_(kErrTag, e.value()) {}
template <typename G,
internal::EnableIfImplicitUnexpectedConstruction<E, const G&> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
/* implicit */ constexpr expected(const unexpected<G>& e) noexcept
: impl_(kErrTag, e.value()) {}
template <typename G,
internal::EnableIfExplicitUnexpectedConstruction<E, G> = 0>
explicit constexpr expected(unexpected<G>&& e) noexcept
: impl_(kErrTag, std::move(e.value())) {}
template <typename G,
internal::EnableIfImplicitUnexpectedConstruction<E, G> = 0>
// NOLINTNEXTLINE(google-explicit-constructor)
/* implicit */ constexpr expected(unexpected<G>&& e) noexcept
: impl_(kErrTag, std::move(e.value())) {}
constexpr explicit expected(absl::in_place_t) noexcept {}
template <typename... Args>
constexpr explicit expected(unexpect_t, Args&&... args) noexcept
: impl_(kErrTag, std::forward<Args>(args)...) {}
template <typename U, typename... Args>
constexpr explicit expected(unexpect_t,
std::initializer_list<U> il,
Args&&... args) noexcept
: impl_(kErrTag, il, std::forward<Args>(args)...) {}
// [expected.void.assign], assignment
template <typename G>
constexpr expected& operator=(const unexpected<G>& e) noexcept {
impl_.emplace_error(e.value());
return *this;
}
template <typename G>
constexpr expected& operator=(unexpected<G>&& e) noexcept {
impl_.emplace_error(std::move(e.value()));
return *this;
}
constexpr void emplace() noexcept { impl_.emplace_value(); }
// [expected.void.swap], swap
constexpr void swap(expected& rhs) noexcept { impl_.swap(rhs.impl_); }
friend constexpr void swap(expected& x, expected& y) noexcept { x.swap(y); }
// [expected.void.obs], observers
// Note: Deviation from the Standard: No operator bool due to consistency with
// non-void expected types.
constexpr bool has_value() const noexcept { return impl_.has_value(); }
constexpr void operator*() const { CHECK(has_value()); }
constexpr void value() const { CHECK(has_value()); }
constexpr E& error() & { return impl_.error(); }
constexpr const E& error() const& { return impl_.error(); }
constexpr E&& error() && { return std::move(error()); }
constexpr const E&& error() const&& { return std::move(error()); }
private:
// Note: Since we can't store void types we use absl::monostate instead.
using Impl = internal::ExpectedImpl<absl::monostate, E>;
static constexpr auto kErrTag = Impl::kErrTag;
Impl impl_;
};
// [expected.object.eq], equality operators
// [expected.void.eq], equality operators
template <typename T, typename E, typename U, typename G, bool IsVoid>
constexpr bool operator==(const expected<T, E, IsVoid>& x,
const expected<U, G, IsVoid>& y) noexcept {
auto equal_values = [](const auto& x, const auto& y) {
// Values for expected void types always compare equal.
if constexpr (IsVoid) {
return true;
} else {
return x.value() == y.value();
}
};
return x.has_value() == y.has_value() &&
(x.has_value() ? equal_values(x, y) : x.error() == y.error());
}
template <typename T, typename E, typename U, internal::EnableIfNotVoid<T> = 0>
constexpr bool operator==(const expected<T, E>& x, const U& v) noexcept {
return x.has_value() && x.value() == v;
}
template <typename T, typename E, typename U, internal::EnableIfNotVoid<T> = 0>
constexpr bool operator==(const U& v, const expected<T, E>& x) noexcept {
return x == v;
}
template <typename T, typename E, typename G>
constexpr bool operator==(const expected<T, E>& x,
const unexpected<G>& e) noexcept {
return !x.has_value() && x.error() == e.value();
}
template <typename T, typename E, typename G>
constexpr bool operator==(const unexpected<G>& e,
const expected<T, E>& x) noexcept {
return x == e;
}
template <typename T, typename E, typename U, typename G, bool IsVoid>
constexpr bool operator!=(const expected<T, E, IsVoid>& x,
const expected<U, G, IsVoid>& y) noexcept {
return !(x == y);
}
template <typename T, typename E, typename U, internal::EnableIfNotVoid<T> = 0>
constexpr bool operator!=(const expected<T, E>& x, const U& v) noexcept {
return !(x == v);
}
template <typename T, typename E, typename U, internal::EnableIfNotVoid<T> = 0>
constexpr bool operator!=(const U& v, const expected<T, E>& x) noexcept {
return !(v == x);
}
template <typename T, typename E, typename G>
constexpr bool operator!=(const expected<T, E>& x,
const unexpected<G>& e) noexcept {
return !(x == e);
}
template <typename T, typename E, typename G>
constexpr bool operator!=(const unexpected<G>& e,
const expected<T, E>& x) noexcept {
return !(e == x);
}
} // namespace base
#endif // BASE_TYPES_EXPECTED_H_