gecko-dev/mfbt/Result.h

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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* A type suitable for returning either a value or an error from a function. */
#ifndef mozilla_Result_h
#define mozilla_Result_h
#include <algorithm>
#include <cstdint>
#include <cstring>
#include <type_traits>
#include "mozilla/Alignment.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/CompactPair.h"
#include "mozilla/Types.h"
#include "mozilla/Variant.h"
namespace mozilla {
/**
* Empty struct, indicating success for operations that have no return value.
* For example, if you declare another empty struct `struct OutOfMemory {};`,
* then `Result<Ok, OutOfMemory>` represents either success or OOM.
*/
struct Ok {};
template <typename E>
class GenericErrorResult;
template <typename V, typename E>
class Result;
namespace detail {
enum class PackingStrategy {
Variant,
NullIsOk,
LowBitTagIsError,
PackedVariant,
};
template <typename T>
struct UnusedZero;
template <typename V, typename E, PackingStrategy Strategy>
class ResultImplementation;
template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::Variant> {
mozilla::Variant<V, E> mStorage;
public:
ResultImplementation(ResultImplementation&&) = default;
ResultImplementation(const ResultImplementation&) = delete;
ResultImplementation& operator=(const ResultImplementation&) = delete;
ResultImplementation& operator=(ResultImplementation&&) = default;
explicit ResultImplementation(V&& aValue)
: mStorage(std::forward<V>(aValue)) {}
explicit ResultImplementation(const V& aValue) : mStorage(aValue) {}
template <typename... Args>
explicit ResultImplementation(std::in_place_t, Args&&... aArgs)
: mStorage(VariantType<V>{}, std::forward<Args>(aArgs)...) {}
explicit ResultImplementation(const E& aErrorValue) : mStorage(aErrorValue) {}
explicit ResultImplementation(E&& aErrorValue)
: mStorage(std::forward<E>(aErrorValue)) {}
bool isOk() const { return mStorage.template is<V>(); }
// The callers of these functions will assert isOk() has the proper value, so
// these functions (in all ResultImplementation specializations) don't need
// to do so.
V unwrap() { return std::move(mStorage.template as<V>()); }
const V& inspect() const { return mStorage.template as<V>(); }
E unwrapErr() { return std::move(mStorage.template as<E>()); }
const E& inspectErr() const { return mStorage.template as<E>(); }
};
// The purpose of EmptyWrapper is to make an empty class look like
// AlignedStorage2 for the purposes of the PackingStrategy::NullIsOk
// specializations of ResultImplementation below. We can't use AlignedStorage2
// itself with an empty class, since it would no longer be empty, and we want to
// avoid changing AlignedStorage2 just for this purpose.
template <typename V>
struct EmptyWrapper : V {
const V* addr() const { return this; }
V* addr() { return this; }
};
template <typename V>
using AlignedStorageOrEmpty =
std::conditional_t<std::is_empty_v<V>, EmptyWrapper<V>, AlignedStorage2<V>>;
template <typename V, typename E>
class ResultImplementationNullIsOkBase {
protected:
using ErrorStorageType = typename UnusedZero<E>::StorageType;
static constexpr auto kNullValue = UnusedZero<E>::nullValue;
static_assert(std::is_trivially_copyable_v<ErrorStorageType>);
// XXX This can't be statically asserted in general, if ErrorStorageType is
// not a basic type. With C++20 bit_cast, we could probably re-add such as
// assertion. static_assert(kNullValue == decltype(kNullValue)(0));
CompactPair<AlignedStorageOrEmpty<V>, ErrorStorageType> mValue;
public:
explicit ResultImplementationNullIsOkBase(const V& aSuccessValue)
: mValue(std::piecewise_construct, std::tuple<>(),
std::tuple(kNullValue)) {
if constexpr (!std::is_empty_v<V>) {
new (mValue.first().addr()) V(aSuccessValue);
}
}
explicit ResultImplementationNullIsOkBase(V&& aSuccessValue)
: mValue(std::piecewise_construct, std::tuple<>(),
std::tuple(kNullValue)) {
if constexpr (!std::is_empty_v<V>) {
new (mValue.first().addr()) V(std::move(aSuccessValue));
}
}
template <typename... Args>
explicit ResultImplementationNullIsOkBase(std::in_place_t, Args&&... aArgs)
: mValue(std::piecewise_construct, std::tuple<>(),
std::tuple(kNullValue)) {
if constexpr (!std::is_empty_v<V>) {
new (mValue.first().addr()) V(std::forward<Args>(aArgs)...);
}
}
explicit ResultImplementationNullIsOkBase(E aErrorValue)
: mValue(std::piecewise_construct, std::tuple<>(),
std::tuple(UnusedZero<E>::Store(std::move(aErrorValue)))) {
MOZ_ASSERT(mValue.second() != kNullValue);
}
ResultImplementationNullIsOkBase(ResultImplementationNullIsOkBase&& aOther)
: mValue(std::piecewise_construct, std::tuple<>(),
std::tuple(aOther.mValue.second())) {
if constexpr (!std::is_empty_v<V>) {
if (isOk()) {
new (mValue.first().addr()) V(std::move(*aOther.mValue.first().addr()));
}
}
}
ResultImplementationNullIsOkBase& operator=(
ResultImplementationNullIsOkBase&& aOther) {
if constexpr (!std::is_empty_v<V>) {
if (isOk()) {
mValue.first().addr()->~V();
}
}
mValue.second() = std::move(aOther.mValue.second());
if constexpr (!std::is_empty_v<V>) {
if (isOk()) {
new (mValue.first().addr()) V(std::move(*aOther.mValue.first().addr()));
}
}
return *this;
}
bool isOk() const { return mValue.second() == kNullValue; }
const V& inspect() const { return *mValue.first().addr(); }
V unwrap() { return std::move(*mValue.first().addr()); }
const E& inspectErr() const {
return UnusedZero<E>::Inspect(mValue.second());
}
E unwrapErr() { return UnusedZero<E>::Unwrap(mValue.second()); }
};
template <typename V, typename E,
bool IsVTriviallyDestructible = std::is_trivially_destructible_v<V>>
class ResultImplementationNullIsOk;
template <typename V, typename E>
class ResultImplementationNullIsOk<V, E, true>
: public ResultImplementationNullIsOkBase<V, E> {
public:
using ResultImplementationNullIsOkBase<V,
E>::ResultImplementationNullIsOkBase;
};
template <typename V, typename E>
class ResultImplementationNullIsOk<V, E, false>
: public ResultImplementationNullIsOkBase<V, E> {
public:
using ResultImplementationNullIsOkBase<V,
E>::ResultImplementationNullIsOkBase;
ResultImplementationNullIsOk(ResultImplementationNullIsOk&&) = default;
ResultImplementationNullIsOk& operator=(ResultImplementationNullIsOk&&) =
default;
~ResultImplementationNullIsOk() {
if (this->isOk()) {
this->mValue.first().addr()->~V();
}
}
};
/**
* Specialization for when the success type is default-constructible and the
* error type is a value type which can never have the value 0 (as determined by
* UnusedZero<>).
*/
template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::NullIsOk>
: public ResultImplementationNullIsOk<V, E> {
public:
using ResultImplementationNullIsOk<V, E>::ResultImplementationNullIsOk;
};
template <size_t S>
using UnsignedIntType = std::conditional_t<
S == 1, std::uint8_t,
std::conditional_t<
S == 2, std::uint16_t,
std::conditional_t<S == 3 || S == 4, std::uint32_t,
std::conditional_t<S <= 8, std::uint64_t, void>>>>;
/**
* Specialization for when alignment permits using the least significant bit
* as a tag bit.
*/
template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::LowBitTagIsError> {
static_assert(std::is_trivially_copyable_v<V> &&
std::is_trivially_destructible_v<V>);
static_assert(std::is_trivially_copyable_v<E> &&
std::is_trivially_destructible_v<E>);
static constexpr size_t kRequiredSize = std::max(sizeof(V), sizeof(E));
using StorageType = UnsignedIntType<kRequiredSize>;
#if defined(__clang__)
alignas(std::max(alignof(V), alignof(E))) StorageType mBits;
#else
// Some gcc versions choke on using std::max with alignas, see
// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929 (and this seems to have
// regressed in some gcc 9.x version before being fixed again) Keeping the
// code above since we would eventually drop this when we no longer support
// gcc versions with the bug.
alignas(alignof(V) > alignof(E) ? alignof(V) : alignof(E)) StorageType mBits;
#endif
public:
explicit ResultImplementation(V aValue) {
if constexpr (!std::is_empty_v<V>) {
std::memcpy(&mBits, &aValue, sizeof(V));
MOZ_ASSERT((mBits & 1) == 0);
} else {
(void)aValue;
mBits = 0;
}
}
explicit ResultImplementation(E aErrorValue) {
if constexpr (!std::is_empty_v<E>) {
std::memcpy(&mBits, &aErrorValue, sizeof(E));
MOZ_ASSERT((mBits & 1) == 0);
mBits |= 1;
} else {
(void)aErrorValue;
mBits = 1;
}
}
bool isOk() const { return (mBits & 1) == 0; }
V inspect() const {
V res;
std::memcpy(&res, &mBits, sizeof(V));
return res;
}
V unwrap() { return inspect(); }
E inspectErr() const {
const auto bits = mBits ^ 1;
E res;
std::memcpy(&res, &bits, sizeof(E));
return res;
}
E unwrapErr() { return inspectErr(); }
};
// Return true if any of the struct can fit in a word.
template <typename V, typename E>
struct IsPackableVariant {
struct VEbool {
V v;
E e;
bool ok;
};
struct EVbool {
E e;
V v;
bool ok;
};
using Impl =
std::conditional_t<sizeof(VEbool) <= sizeof(EVbool), VEbool, EVbool>;
static const bool value = sizeof(Impl) <= sizeof(uintptr_t);
};
/**
* Specialization for when both type are not using all the bytes, in order to
* use one byte as a tag.
*/
template <typename V, typename E>
class ResultImplementation<V, E, PackingStrategy::PackedVariant> {
using Impl = typename IsPackableVariant<V, E>::Impl;
Impl data;
public:
explicit ResultImplementation(V aValue) {
data.v = std::move(aValue);
data.ok = true;
}
explicit ResultImplementation(E aErrorValue) {
data.e = std::move(aErrorValue);
data.ok = false;
}
bool isOk() const { return data.ok; }
const V& inspect() const { return data.v; }
V unwrap() { return std::move(data.v); }
const E& inspectErr() const { return data.e; }
E unwrapErr() { return std::move(data.e); }
};
// To use nullptr as a special value, we need the counter part to exclude zero
// from its range of valid representations.
//
// By default assume that zero can be represented.
template <typename T>
struct UnusedZero {
static const bool value = false;
};
// A bit of help figuring out which of the above specializations to use.
//
// We begin by safely assuming types don't have a spare bit, unless they are
// empty.
template <typename T>
struct HasFreeLSB {
static const bool value = std::is_empty_v<T>;
};
// As an incomplete type, void* does not have a spare bit.
template <>
struct HasFreeLSB<void*> {
static const bool value = false;
};
// The lowest bit of a properly-aligned pointer is always zero if the pointee
// type is greater than byte-aligned. That bit is free to use if it's masked
// out of such pointers before they're dereferenced.
template <typename T>
struct HasFreeLSB<T*> {
static const bool value = (alignof(T) & 1) == 0;
};
// Select one of the previous result implementation based on the properties of
// the V and E types.
template <typename V, typename E>
struct SelectResultImpl {
static const PackingStrategy value =
(HasFreeLSB<V>::value && HasFreeLSB<E>::value)
? PackingStrategy::LowBitTagIsError
: (UnusedZero<E>::value && sizeof(E) <= sizeof(uintptr_t))
? PackingStrategy::NullIsOk
: (std::is_default_constructible_v<V> &&
std::is_default_constructible_v<E> &&
IsPackableVariant<V, E>::value)
? PackingStrategy::PackedVariant
: PackingStrategy::Variant;
using Type = ResultImplementation<V, E, value>;
};
template <typename T>
struct IsResult : std::false_type {};
template <typename V, typename E>
struct IsResult<Result<V, E>> : std::true_type {};
} // namespace detail
template <typename V, typename E>
auto ToResult(Result<V, E>&& aValue)
-> decltype(std::forward<Result<V, E>>(aValue)) {
return std::forward<Result<V, E>>(aValue);
}
/**
* Result<V, E> represents the outcome of an operation that can either succeed
* or fail. It contains either a success value of type V or an error value of
* type E.
*
* All Result methods are const, so results are basically immutable.
* This is just like Variant<V, E> but with a slightly different API, and the
* following cases are optimized so Result can be stored more efficiently:
*
* - If both the success and error types do not use their least significant bit,
* are trivially copyable and destructible, Result<V, E> is guaranteed to be as
* large as the larger type. This is determined via the HasFreeLSB trait. By
* default, empty classes (in particular Ok) and aligned pointer types are
* assumed to have a free LSB, but you can specialize this trait for other
* types. If the success type is empty, the representation is guaranteed to be
* all zero bits on success. Do not change this representation! There is JIT
* code that depends on it. (Implementation note: The lowest bit is used as a
* tag bit: 0 to indicate the Result's bits are a success value, 1 to indicate
* the Result's bits (with the 1 masked out) encode an error value)
*
* - Else, if the error type can't have a all-zero bits representation and is
* not larger than a pointer, a CompactPair is used to represent this rather
* than a Variant. This has shown to be better optimizable, and the template
* code is much simpler than that of Variant, so it should also compile faster.
* Whether an error type can't be all-zero bits, is determined via the
* UnusedZero trait. MFBT doesn't declare any public type UnusedZero, but
* nsresult is declared UnusedZero in XPCOM.
*
* The purpose of Result is to reduce the screwups caused by using `false` or
* `nullptr` to indicate errors.
* What screwups? See <https://bugzilla.mozilla.org/show_bug.cgi?id=912928> for
* a partial list.
*
* Result<const V, E> or Result<V, const E> are not meaningful. The success or
* error values in a Result instance are non-modifiable in-place anyway. This
* guarantee must also be maintained when evolving Result. They can be
* unwrap()ped, but this loses const qualification. However, Result<const V, E>
* or Result<V, const E> may be misleading and prevent movability. Just use
* Result<V, E>. (Result<const V*, E> may make sense though, just Result<const
* V* const, E> is not possible.)
*/
template <typename V, typename E>
class MOZ_MUST_USE_TYPE Result final {
// See class comment on Result<const V, E> and Result<V, const E>.
static_assert(!std::is_const_v<V>);
static_assert(!std::is_const_v<E>);
static_assert(!std::is_reference_v<V>);
static_assert(!std::is_reference_v<E>);
using Impl = typename detail::SelectResultImpl<V, E>::Type;
Impl mImpl;
public:
using ok_type = V;
using err_type = E;
/** Create a success result. */
MOZ_IMPLICIT Result(V&& aValue) : mImpl(std::forward<V>(aValue)) {
MOZ_ASSERT(isOk());
}
/** Create a success result. */
MOZ_IMPLICIT Result(const V& aValue) : mImpl(aValue) { MOZ_ASSERT(isOk()); }
/** Create a success result in-place. */
template <typename... Args>
explicit Result(std::in_place_t, Args&&... aArgs)
: mImpl(std::in_place, std::forward<Args>(aArgs)...) {
MOZ_ASSERT(isOk());
}
/** Create an error result. */
explicit Result(E aErrorValue) : mImpl(std::move(aErrorValue)) {
MOZ_ASSERT(isErr());
}
/**
* Create a (success/error) result from another (success/error) result with a
* different but convertible error type. */
template <typename E2,
typename = std::enable_if_t<std::is_convertible_v<E2, E>>>
MOZ_IMPLICIT Result(Result<V, E2>&& aOther)
: mImpl(aOther.isOk() ? Impl{aOther.unwrap()}
: Impl{aOther.unwrapErr()}) {}
/**
* Implementation detail of MOZ_TRY().
* Create an error result from another error result.
*/
template <typename E2>
MOZ_IMPLICIT Result(GenericErrorResult<E2>&& aErrorResult)
: mImpl(std::move(aErrorResult.mErrorValue)) {
static_assert(std::is_convertible_v<E2, E>, "E2 must be convertible to E");
MOZ_ASSERT(isErr());
}
/**
* Implementation detail of MOZ_TRY().
* Create an error result from another error result.
*/
template <typename E2>
MOZ_IMPLICIT Result(const GenericErrorResult<E2>& aErrorResult)
: mImpl(aErrorResult.mErrorValue) {
static_assert(std::is_convertible_v<E2, E>, "E2 must be convertible to E");
MOZ_ASSERT(isErr());
}
Result(const Result&) = delete;
Result(Result&&) = default;
Result& operator=(const Result&) = delete;
Result& operator=(Result&&) = default;
/** True if this Result is a success result. */
bool isOk() const { return mImpl.isOk(); }
/** True if this Result is an error result. */
bool isErr() const { return !mImpl.isOk(); }
/** Take the success value from this Result, which must be a success result.
*/
V unwrap() {
MOZ_ASSERT(isOk());
return mImpl.unwrap();
}
/**
* Take the success value from this Result, which must be a success result.
* If it is an error result, then return the aValue.
*/
V unwrapOr(V aValue) {
return MOZ_LIKELY(isOk()) ? mImpl.unwrap() : std::move(aValue);
}
/** Take the error value from this Result, which must be an error result. */
E unwrapErr() {
MOZ_ASSERT(isErr());
return mImpl.unwrapErr();
}
/** See the success value from this Result, which must be a success result. */
decltype(auto) inspect() const {
static_assert(!std::is_reference_v<
std::invoke_result_t<decltype(&Impl::inspect), Impl>> ||
std::is_const_v<std::remove_reference_t<
std::invoke_result_t<decltype(&Impl::inspect), Impl>>>);
MOZ_ASSERT(isOk());
return mImpl.inspect();
}
/** See the error value from this Result, which must be an error result. */
decltype(auto) inspectErr() const {
static_assert(
!std::is_reference_v<
std::invoke_result_t<decltype(&Impl::inspectErr), Impl>> ||
std::is_const_v<std::remove_reference_t<
std::invoke_result_t<decltype(&Impl::inspectErr), Impl>>>);
MOZ_ASSERT(isErr());
return mImpl.inspectErr();
}
/** Propagate the error value from this Result, which must be an error result.
*
* This can be used to propagate an error from a function call to the caller
* with a different value type, but the same error type:
*
* Result<T1, E> Func1() {
* Result<T2, E> res = Func2();
* if (res.isErr()) { return res.propagateErr(); }
* }
*/
GenericErrorResult<E> propagateErr() {
MOZ_ASSERT(isErr());
return GenericErrorResult<E>{mImpl.unwrapErr()};
}
/**
* Map a function V -> V2 over this result's success variant. If this result
* is an error, do not invoke the function and propagate the error.
*
* Mapping over success values invokes the function to produce a new success
* value:
*
* // Map Result<int, E> to another Result<int, E>
* Result<int, E> res(5);
* Result<int, E> res2 = res.map([](int x) { return x * x; });
* MOZ_ASSERT(res.isOk());
* MOZ_ASSERT(res2.unwrap() == 25);
*
* // Map Result<const char*, E> to Result<size_t, E>
* Result<const char*, E> res("hello, map!");
* Result<size_t, E> res2 = res.map(strlen);
* MOZ_ASSERT(res.isOk());
* MOZ_ASSERT(res2.unwrap() == 11);
*
* Mapping over an error does not invoke the function and propagates the
* error:
*
* Result<V, int> res(5);
* MOZ_ASSERT(res.isErr());
* Result<V2, int> res2 = res.map([](V v) { ... });
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 5);
*/
template <typename F>
auto map(F f) -> Result<std::result_of_t<F(V)>, E> {
using RetResult = Result<std::result_of_t<F(V)>, E>;
return MOZ_LIKELY(isOk()) ? RetResult(f(unwrap())) : RetResult(unwrapErr());
}
/**
* Map a function E -> E2 over this result's error variant. If this result is
* a success, do not invoke the function and move the success over.
*
* Mapping over error values invokes the function to produce a new error
* value:
*
* // Map Result<V, int> to another Result<V, int>
* Result<V, int> res(5);
* Result<V, int> res2 = res.mapErr([](int x) { return x * x; });
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 25);
*
* // Map Result<V, const char*> to Result<V, size_t>
* Result<V, const char*> res("hello, mapErr!");
* Result<V, size_t> res2 = res.mapErr(strlen);
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 14);
*
* Mapping over a success does not invoke the function and moves the success:
*
* Result<int, E> res(5);
* MOZ_ASSERT(res.isOk());
* Result<int, E2> res2 = res.mapErr([](E e) { ... });
* MOZ_ASSERT(res2.isOk());
* MOZ_ASSERT(res2.unwrap() == 5);
*/
template <typename F>
auto mapErr(F f) -> Result<V, std::result_of_t<F(E)>> {
using RetResult = Result<V, std::result_of_t<F(E)>>;
return MOZ_UNLIKELY(isErr()) ? RetResult(f(unwrapErr()))
: RetResult(unwrap());
}
/**
* Map a function E -> Result<V, E2> over this result's error variant. If
* this result is a success, do not invoke the function and move the success
* over.
*
* `orElse`ing over error values invokes the function to produce a new
* result:
*
* // `orElse` Result<V, int> error variant to another Result<V, int>
* // error variant or Result<V, int> success variant
* auto orElse = [](int x) -> Result<V, int> {
* if (x != 6) {
* return Err(x * x);
* }
* return V(...);
* };
*
* Result<V, int> res(5);
* auto res2 = res.orElse(orElse);
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 25);
*
* Result<V, int> res3(6);
* auto res4 = res3.orElse(orElse);
* MOZ_ASSERT(res4.isOk());
* MOZ_ASSERT(res4.unwrap() == ...);
*
* // `orElse` Result<V, const char*> error variant to Result<V, size_t>
* // error variant or Result<V, size_t> success variant
* auto orElse = [](const char* s) -> Result<V, size_t> {
* if (strcmp(s, "foo")) {
* return Err(strlen(s));
* }
* return V(...);
* };
*
* Result<V, const char*> res("hello, orElse!");
* auto res2 = res.orElse(orElse);
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res2.unwrapErr() == 14);
*
* Result<V, const char*> res3("foo");
* auto res4 = ress.orElse(orElse);
* MOZ_ASSERT(res4.isOk());
* MOZ_ASSERT(res4.unwrap() == ...);
*
* `orElse`ing over a success does not invoke the function and moves the
* success:
*
* Result<int, E> res(5);
* MOZ_ASSERT(res.isOk());
* Result<int, E2> res2 = res.orElse([](E e) { ... });
* MOZ_ASSERT(res2.isOk());
* MOZ_ASSERT(res2.unwrap() == 5);
*/
template <typename F>
auto orElse(F f) -> Result<V, typename std::result_of_t<F(E)>::err_type> {
return MOZ_UNLIKELY(isErr()) ? f(unwrapErr()) : unwrap();
}
/**
* Given a function V -> Result<V2, E>, apply it to this result's success
* value and return its result. If this result is an error value, it is
* propagated.
*
* This is sometimes called "flatMap" or ">>=" in other contexts.
*
* `andThen`ing over success values invokes the function to produce a new
* result:
*
* Result<const char*, Error> res("hello, andThen!");
* Result<HtmlFreeString, Error> res2 = res.andThen([](const char* s) {
* return containsHtmlTag(s)
* ? Result<HtmlFreeString, Error>(Error("Invalid: contains HTML"))
* : Result<HtmlFreeString, Error>(HtmlFreeString(s));
* }
* });
* MOZ_ASSERT(res2.isOk());
* MOZ_ASSERT(res2.unwrap() == HtmlFreeString("hello, andThen!");
*
* `andThen`ing over error results does not invoke the function, and just
* propagates the error result:
*
* Result<int, const char*> res("some error");
* auto res2 = res.andThen([](int x) { ... });
* MOZ_ASSERT(res2.isErr());
* MOZ_ASSERT(res.unwrapErr() == res2.unwrapErr());
*/
template <typename F, typename = std::enable_if_t<detail::IsResult<
std::invoke_result_t<F, V&&>>::value>>
auto andThen(F f) -> std::invoke_result_t<F, V&&> {
return MOZ_LIKELY(isOk()) ? f(unwrap()) : propagateErr();
}
};
/**
* A type that auto-converts to an error Result. This is like a Result without
* a success type. It's the best return type for functions that always return
* an error--functions designed to build and populate error objects. It's also
* useful in error-handling macros; see MOZ_TRY for an example.
*/
template <typename E>
class MOZ_MUST_USE_TYPE GenericErrorResult {
E mErrorValue;
template <typename V, typename E2>
friend class Result;
public:
explicit GenericErrorResult(const E& aErrorValue)
: mErrorValue(aErrorValue) {}
explicit GenericErrorResult(E&& aErrorValue)
: mErrorValue(std::move(aErrorValue)) {}
};
template <typename E>
inline auto Err(E&& aErrorValue) {
return GenericErrorResult<std::decay_t<E>>(std::forward<E>(aErrorValue));
}
} // namespace mozilla
/**
* MOZ_TRY(expr) is the C++ equivalent of Rust's `try!(expr);`. First, it
* evaluates expr, which must produce a Result value. On success, it
* discards the result altogether. On error, it immediately returns an error
* Result from the enclosing function.
*/
#define MOZ_TRY(expr) \
do { \
auto mozTryTempResult_ = ::mozilla::ToResult(expr); \
if (MOZ_UNLIKELY(mozTryTempResult_.isErr())) { \
return mozTryTempResult_.propagateErr(); \
} \
} while (0)
/**
* MOZ_TRY_VAR(target, expr) is the C++ equivalent of Rust's `target =
* try!(expr);`. First, it evaluates expr, which must produce a Result value. On
* success, the result's success value is assigned to target. On error,
* immediately returns the error result. |target| must be an lvalue.
*/
#define MOZ_TRY_VAR(target, expr) \
do { \
auto mozTryVarTempResult_ = (expr); \
if (MOZ_UNLIKELY(mozTryVarTempResult_.isErr())) { \
return mozTryVarTempResult_.propagateErr(); \
} \
(target) = mozTryVarTempResult_.unwrap(); \
} while (0)
#endif // mozilla_Result_h