mfbt/Result.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   2  * vim: set ts=8 sts=2 et sw=2 tw=80:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* A type suitable for returning either a value or an error from a function. */
   8
   9 #ifndef mozilla_Result_h
  10 #define mozilla_Result_h
  11
  12 #include <type_traits>
  13 #include "mozilla/Alignment.h"
  14 #include "mozilla/Assertions.h"
  15 #include "mozilla/Attributes.h"
  16 #include "mozilla/Types.h"
  17 #include "mozilla/TypeTraits.h"
  18 #include "mozilla/Variant.h"
  19
  20 namespace mozilla {
  21
  22 /**
  23  * Empty struct, indicating success for operations that have no return value.
  24  * For example, if you declare another empty struct `struct OutOfMemory {};`,
  25  * then `Result<Ok, OutOfMemory>` represents either success or OOM.
  26  */
  27 struct Ok {};
  28
  29 template <typename E>
  30 class GenericErrorResult;
  31 template <typename V, typename E>
  32 class Result;
  33
  34 namespace detail {
  35
  36 enum class PackingStrategy {
  37   Variant,
  38   NullIsOk,
  39   LowBitTagIsError,
  40   PackedVariant,
  41 };
  42
  43 template <typename V, typename E, PackingStrategy Strategy>
  44 class ResultImplementation;
  45
  46 template <typename V, typename E>
  47 class ResultImplementation<V, E, PackingStrategy::Variant> {
  48   mozilla::Variant<V, E> mStorage;
  49
  50  public:
  51   ResultImplementation(ResultImplementation&&) = default;
  52   ResultImplementation(const ResultImplementation&) = default;
  53   ResultImplementation& operator=(const ResultImplementation&) = default;
  54   ResultImplementation& operator=(ResultImplementation&&) = default;
  55
  56   explicit ResultImplementation(V&& aValue)
  57       : mStorage(std::forward<V>(aValue)) {}
  58   explicit ResultImplementation(const V& aValue) : mStorage(aValue) {}
  59   explicit ResultImplementation(E aErrorValue)
  60       : mStorage(std::forward<E>(aErrorValue)) {}
  61
  62   bool isOk() const { return mStorage.template is<V>(); }
  63
  64   // The callers of these functions will assert isOk() has the proper value, so
  65   // these functions (in all ResultImplementation specializations) don't need
  66   // to do so.
  67   V unwrap() { return std::move(mStorage.template as<V>()); }
  68   const V& inspect() const { return mStorage.template as<V>(); }
  69
  70   E unwrapErr() { return std::move(mStorage.template as<E>()); }
  71   const E& inspectErr() const { return mStorage.template as<E>(); }
  72 };
  73
  74 /**
  75  * mozilla::Variant doesn't like storing a reference. This is a specialization
  76  * to store E as pointer if it's a reference.
  77  */
  78 template <typename V, typename E>
  79 class ResultImplementation<V, E&, PackingStrategy::Variant> {
  80   mozilla::Variant<V, E*> mStorage;
  81
  82  public:
  83   explicit ResultImplementation(V&& aValue)
  84       : mStorage(std::forward<V>(aValue)) {}
  85   explicit ResultImplementation(const V& aValue) : mStorage(aValue) {}
  86   explicit ResultImplementation(E& aErrorValue) : mStorage(&aErrorValue) {}
  87
  88   bool isOk() const { return mStorage.template is<V>(); }
  89
  90   const V& inspect() const { return mStorage.template as<V>(); }
  91   V unwrap() { return std::move(mStorage.template as<V>()); }
  92
  93   E& unwrapErr() { return *mStorage.template as<E*>(); }
  94   const E& inspectErr() const { return *mStorage.template as<E*>(); }
  95 };
  96
  97 /**
  98  * Specialization for when the success type is Ok (or another empty class) and
  99  * the error type is a reference.
 100  */
 101 template <typename V, typename E>
 102 class ResultImplementation<V, E&, PackingStrategy::NullIsOk> {
 103   E* mErrorValue;
 104
 105  public:
 106   explicit ResultImplementation(V) : mErrorValue(nullptr) {}
 107   explicit ResultImplementation(E& aErrorValue) : mErrorValue(&aErrorValue) {}
 108
 109   bool isOk() const { return mErrorValue == nullptr; }
 110
 111   const V& inspect() const = delete;
 112   V unwrap() { return V(); }
 113
 114   const E& inspectErr() const { return *mErrorValue; }
 115   E& unwrapErr() { return *mErrorValue; }
 116 };
 117
 118 /**
 119  * Specialization for when the success type is Ok (or another empty class) and
 120  * the error type is a value type which can never have the value 0 (as
 121  * determined by UnusedZero<>).
 122  */
 123 template <typename V, typename E>
 124 class ResultImplementation<V, E, PackingStrategy::NullIsOk> {
 125   static constexpr E NullValue = E(0);
 126
 127   E mErrorValue;
 128
 129  public:
 130   explicit ResultImplementation(V) : mErrorValue(NullValue) {}
 131   explicit ResultImplementation(E aErrorValue) : mErrorValue(aErrorValue) {
 132     MOZ_ASSERT(aErrorValue != NullValue);
 133   }
 134
 135   bool isOk() const { return mErrorValue == NullValue; }
 136
 137   const V& inspect() const = delete;
 138   V unwrap() { return V(); }
 139
 140   const E& inspectErr() const { return mErrorValue; }
 141   E unwrapErr() { return std::move(mErrorValue); }
 142 };
 143
 144 /**
 145  * Specialization for when alignment permits using the least significant bit as
 146  * a tag bit.
 147  */
 148 template <typename V, typename E>
 149 class ResultImplementation<V*, E&, PackingStrategy::LowBitTagIsError> {
 150   uintptr_t mBits;
 151
 152  public:
 153   explicit ResultImplementation(V* aValue)
 154       : mBits(reinterpret_cast<uintptr_t>(aValue)) {
 155     MOZ_ASSERT((uintptr_t(aValue) % MOZ_ALIGNOF(V)) == 0,
 156                "Result value pointers must not be misaligned");
 157   }
 158   explicit ResultImplementation(E& aErrorValue)
 159       : mBits(reinterpret_cast<uintptr_t>(&aErrorValue) | 1) {
 160     MOZ_ASSERT((uintptr_t(&aErrorValue) % MOZ_ALIGNOF(E)) == 0,
 161                "Result errors must not be misaligned");
 162   }
 163
 164   bool isOk() const { return (mBits & 1) == 0; }
 165
 166   V* inspect() const { return reinterpret_cast<V*>(mBits); }
 167   V* unwrap() { return inspect(); }
 168
 169   E& inspectErr() const { return *reinterpret_cast<E*>(mBits ^ 1); }
 170   E& unwrapErr() { return inspectErr(); }
 171 };
 172
 173 // Return true if any of the struct can fit in a word.
 174 template <typename V, typename E>
 175 struct IsPackableVariant {
 176   struct VEbool {
 177     V v;
 178     E e;
 179     bool ok;
 180   };
 181   struct EVbool {
 182     E e;
 183     V v;
 184     bool ok;
 185   };
 186
 187   using Impl = typename Conditional<sizeof(VEbool) <= sizeof(EVbool), VEbool,
 188                                     EVbool>::Type;
 189
 190   static const bool value = sizeof(Impl) <= sizeof(uintptr_t);
 191 };
 192
 193 /**
 194  * Specialization for when both type are not using all the bytes, in order to
 195  * use one byte as a tag.
 196  */
 197 template <typename V, typename E>
 198 class ResultImplementation<V, E, PackingStrategy::PackedVariant> {
 199   using Impl = typename IsPackableVariant<V, E>::Impl;
 200   Impl data;
 201
 202  public:
 203   explicit ResultImplementation(V aValue) {
 204     data.v = std::move(aValue);
 205     data.ok = true;
 206   }
 207   explicit ResultImplementation(E aErrorValue) {
 208     data.e = std::move(aErrorValue);
 209     data.ok = false;
 210   }
 211
 212   bool isOk() const { return data.ok; }
 213
 214   const V& inspect() const { return data.v; }
 215   V unwrap() { return std::move(data.v); }
 216
 217   const E& inspectErr() const { return data.e; }
 218   E unwrapErr() { return std::move(data.e); }
 219 };
 220
 221 // To use nullptr as a special value, we need the counter part to exclude zero
 222 // from its range of valid representations.
 223 //
 224 // By default assume that zero can be represented.
 225 template <typename T>
 226 struct UnusedZero {
 227   static const bool value = false;
 228 };
 229
 230 // References can't be null.
 231 template <typename T>
 232 struct UnusedZero<T&> {
 233   static const bool value = true;
 234 };
 235
 236 // A bit of help figuring out which of the above specializations to use.
 237 //
 238 // We begin by safely assuming types don't have a spare bit.
 239 template <typename T>
 240 struct HasFreeLSB {
 241   static const bool value = false;
 242 };
 243
 244 // As an incomplete type, void* does not have a spare bit.
 245 template <>
 246 struct HasFreeLSB<void*> {
 247   static const bool value = false;
 248 };
 249
 250 // The lowest bit of a properly-aligned pointer is always zero if the pointee
 251 // type is greater than byte-aligned. That bit is free to use if it's masked
 252 // out of such pointers before they're dereferenced.
 253 template <typename T>
 254 struct HasFreeLSB<T*> {
 255   static const bool value = (alignof(T) & 1) == 0;
 256 };
 257
 258 // We store references as pointers, so they have a free bit if a pointer would
 259 // have one.
 260 template <typename T>
 261 struct HasFreeLSB<T&> {
 262   static const bool value = HasFreeLSB<T*>::value;
 263 };
 264
 265 // Select one of the previous result implementation based on the properties of
 266 // the V and E types.
 267 template <typename V, typename E>
 268 struct SelectResultImpl {
 269   static const PackingStrategy value =
 270       (IsEmpty<V>::value && UnusedZero<E>::value)
 271           ? PackingStrategy::NullIsOk
 272           : (detail::HasFreeLSB<V>::value && detail::HasFreeLSB<E>::value)
 273                 ? PackingStrategy::LowBitTagIsError
 274                 : (IsDefaultConstructible<V>::value &&
 275                    IsDefaultConstructible<E>::value &&
 276                    IsPackableVariant<V, E>::value)
 277                       ? PackingStrategy::PackedVariant
 278                       : PackingStrategy::Variant;
 279
 280   using Type = detail::ResultImplementation<V, E, value>;
 281 };
 282
 283 template <typename T>
 284 struct IsResult : FalseType {};
 285
 286 template <typename V, typename E>
 287 struct IsResult<Result<V, E>> : TrueType {};
 288
 289 }  // namespace detail
 290
 291 template <typename V, typename E>
 292 auto ToResult(Result<V, E>&& aValue)
 293     -> decltype(std::forward<Result<V, E>>(aValue)) {
 294   return std::forward<Result<V, E>>(aValue);
 295 }
 296
 297 /**
 298  * Result<V, E> represents the outcome of an operation that can either succeed
 299  * or fail. It contains either a success value of type V or an error value of
 300  * type E.
 301  *
 302  * All Result methods are const, so results are basically immutable.
 303  * This is just like Variant<V, E> but with a slightly different API, and the
 304  * following cases are optimized so Result can be stored more efficiently:
 305  *
 306  * - If the success type is Ok (or another empty class) and the error type is a
 307  *   reference, Result<V, E&> is guaranteed to be pointer-sized and all zero
 308  *   bits on success. Do not change this representation! There is JIT code that
 309  *   depends on it.
 310  *
 311  * - If the success type is a pointer type and the error type is a reference
 312  *   type, and the least significant bit is unused for both types when stored
 313  *   as a pointer (due to alignment rules), Result<V*, E&> is guaranteed to be
 314  *   pointer-sized. In this case, we use the lowest bit as tag bit: 0 to
 315  *   indicate the Result's bits are a V, 1 to indicate the Result's bits (with
 316  *   the 1 masked out) encode an E*.
 317  *
 318  * The purpose of Result is to reduce the screwups caused by using `false` or
 319  * `nullptr` to indicate errors.
 320  * What screwups? See <https://bugzilla.mozilla.org/show_bug.cgi?id=912928> for
 321  * a partial list.
 322  */
 323 template <typename V, typename E>
 324 class MOZ_MUST_USE_TYPE Result final {
 325   using Impl = typename detail::SelectResultImpl<V, E>::Type;
 326
 327   Impl mImpl;
 328
 329  public:
 330   /** Create a success result. */
 331   MOZ_IMPLICIT Result(V&& aValue) : mImpl(std::forward<V>(aValue)) {
 332     MOZ_ASSERT(isOk());
 333   }
 334
 335   /** Create a success result. */
 336   MOZ_IMPLICIT Result(const V& aValue) : mImpl(aValue) { MOZ_ASSERT(isOk()); }
 337
 338   /** Create an error result. */
 339   explicit Result(E aErrorValue) : mImpl(std::forward<E>(aErrorValue)) {
 340     MOZ_ASSERT(isErr());
 341   }
 342
 343   /**
 344    * Implementation detail of MOZ_TRY().
 345    * Create an error result from another error result.
 346    */
 347   template <typename E2>
 348   MOZ_IMPLICIT Result(GenericErrorResult<E2>&& aErrorResult)
 349       : mImpl(std::forward<E2>(aErrorResult.mErrorValue)) {
 350     static_assert(mozilla::IsConvertible<E2, E>::value,
 351                   "E2 must be convertible to E");
 352     MOZ_ASSERT(isErr());
 353   }
 354
 355   /**
 356    * Implementation detail of MOZ_TRY().
 357    * Create an error result from another error result.
 358    */
 359   template <typename E2>
 360   MOZ_IMPLICIT Result(const GenericErrorResult<E2>& aErrorResult)
 361       : mImpl(aErrorResult.mErrorValue) {
 362     static_assert(mozilla::IsConvertible<E2, E>::value,
 363                   "E2 must be convertible to E");
 364     MOZ_ASSERT(isErr());
 365   }
 366
 367   Result(const Result&) = default;
 368   Result(Result&&) = default;
 369   Result& operator=(const Result&) = default;
 370   Result& operator=(Result&&) = default;
 371
 372   /** True if this Result is a success result. */
 373   bool isOk() const { return mImpl.isOk(); }
 374
 375   /** True if this Result is an error result. */
 376   bool isErr() const { return !mImpl.isOk(); }
 377
 378   /** Take the success value from this Result, which must be a success result.
 379    */
 380   V unwrap() {
 381     MOZ_ASSERT(isOk());
 382     return mImpl.unwrap();
 383   }
 384
 385   /**
 386    * Take the success value from this Result, which must be a success result.
 387    * If it is an error result, then return the aValue.
 388    */
 389   V unwrapOr(V aValue) {
 390     return MOZ_LIKELY(isOk()) ? mImpl.unwrap() : std::move(aValue);
 391   }
 392
 393   /** Take the error value from this Result, which must be an error result. */
 394   E unwrapErr() {
 395     MOZ_ASSERT(isErr());
 396     return mImpl.unwrapErr();
 397   }
 398
 399   /** See the success value from this Result, which must be a success result. */
 400   const V& inspect() const { return mImpl.inspect(); }
 401
 402   /** See the error value from this Result, which must be an error result. */
 403   const E& inspectErr() const {
 404     MOZ_ASSERT(isErr());
 405     return mImpl.inspectErr();
 406   }
 407
 408   /**
 409    * Map a function V -> W over this result's success variant. If this result is
 410    * an error, do not invoke the function and return a copy of the error.
 411    *
 412    * Mapping over success values invokes the function to produce a new success
 413    * value:
 414    *
 415    *     // Map Result<int, E> to another Result<int, E>
 416    *     Result<int, E> res(5);
 417    *     Result<int, E> res2 = res.map([](int x) { return x * x; });
 418    *     MOZ_ASSERT(res2.unwrap() == 25);
 419    *
 420    *     // Map Result<const char*, E> to Result<size_t, E>
 421    *     Result<const char*, E> res("hello, map!");
 422    *     Result<size_t, E> res2 = res.map(strlen);
 423    *     MOZ_ASSERT(res2.unwrap() == 11);
 424    *
 425    * Mapping over an error does not invoke the function and copies the error:
 426    *
 427    *     Result<V, int> res(5);
 428    *     MOZ_ASSERT(res.isErr());
 429    *     Result<W, int> res2 = res.map([](V v) { ... });
 430    *     MOZ_ASSERT(res2.isErr());
 431    *     MOZ_ASSERT(res2.unwrapErr() == 5);
 432    */
 433   template <typename F>
 434   auto map(F f) -> Result<decltype(f(*((V*)nullptr))), E> {
 435     using RetResult = Result<decltype(f(*((V*)nullptr))), E>;
 436     return MOZ_LIKELY(isOk()) ? RetResult(f(unwrap())) : RetResult(unwrapErr());
 437   }
 438
 439   /**
 440    * Map a function V -> W over this result's error variant. If this result is
 441    * a success, do not invoke the function and move the success over.
 442    *
 443    * Mapping over error values invokes the function to produce a new error
 444    * value:
 445    *
 446    *     // Map Result<V, int> to another Result<V, int>
 447    *     Result<V, int> res(5);
 448    *     Result<V, int> res2 = res.mapErr([](int x) { return x * x; });
 449    *     MOZ_ASSERT(res2.unwrapErr() == 25);
 450    *
 451    *     // Map Result<V, const char*> to Result<V, size_t>
 452    *     Result<V, const char*> res("hello, map!");
 453    *     Result<size_t, E> res2 = res.mapErr(strlen);
 454    *     MOZ_ASSERT(res2.unwrapErr() == 11);
 455    *
 456    * Mapping over a success does not invoke the function and copies the error:
 457    *
 458    *     Result<int, V> res(5);
 459    *     MOZ_ASSERT(res.isOk());
 460    *     Result<int, W> res2 = res.mapErr([](V v) { ... });
 461    *     MOZ_ASSERT(res2.isOk());
 462    *     MOZ_ASSERT(res2.unwrap() == 5);
 463    */
 464   template <typename F>
 465   auto mapErr(F f) -> Result<V, std::result_of_t<F(E)>> {
 466     using RetResult = Result<V, std::result_of_t<F(E)>>;
 467     return isOk() ? RetResult(unwrap()) : RetResult(f(unwrapErr()));
 468   }
 469
 470   /**
 471    * Given a function V -> Result<W, E>, apply it to this result's success value
 472    * and return its result. If this result is an error value, then return a
 473    * copy.
 474    *
 475    * This is sometimes called "flatMap" or ">>=" in other contexts.
 476    *
 477    * `andThen`ing over success values invokes the function to produce a new
 478    * result:
 479    *
 480    *     Result<const char*, Error> res("hello, andThen!");
 481    *     Result<HtmlFreeString, Error> res2 = res.andThen([](const char* s) {
 482    *       return containsHtmlTag(s)
 483    *         ? Result<HtmlFreeString, Error>(Error("Invalid: contains HTML"))
 484    *         : Result<HtmlFreeString, Error>(HtmlFreeString(s));
 485    *       }
 486    *     });
 487    *     MOZ_ASSERT(res2.isOk());
 488    *     MOZ_ASSERT(res2.unwrap() == HtmlFreeString("hello, andThen!");
 489    *
 490    * `andThen`ing over error results does not invoke the function, and just
 491    * produces a new copy of the error result:
 492    *
 493    *     Result<int, const char*> res("some error");
 494    *     auto res2 = res.andThen([](int x) { ... });
 495    *     MOZ_ASSERT(res2.isErr());
 496    *     MOZ_ASSERT(res.unwrapErr() == res2.unwrapErr());
 497    */
 498   template <typename F, typename = typename EnableIf<detail::IsResult<decltype(
 499                             (*((F*)nullptr))(*((V*)nullptr)))>::value>::Type>
 500   auto andThen(F f) -> decltype(f(*((V*)nullptr))) {
 501     return MOZ_LIKELY(isOk()) ? f(unwrap())
 502                               : GenericErrorResult<E>(unwrapErr());
 503   }
 504 };
 505
 506 /**
 507  * A type that auto-converts to an error Result. This is like a Result without
 508  * a success type. It's the best return type for functions that always return
 509  * an error--functions designed to build and populate error objects. It's also
 510  * useful in error-handling macros; see MOZ_TRY for an example.
 511  */
 512 template <typename E>
 513 class MOZ_MUST_USE_TYPE GenericErrorResult {
 514   E mErrorValue;
 515
 516   template <typename V, typename E2>
 517   friend class Result;
 518
 519  public:
 520   explicit GenericErrorResult(E aErrorValue)
 521       : mErrorValue(std::forward<E>(aErrorValue)) {}
 522 };
 523
 524 template <typename E>
 525 inline GenericErrorResult<E> Err(E&& aErrorValue) {
 526   return GenericErrorResult<E>(std::forward<E>(aErrorValue));
 527 }
 528
 529 }  // namespace mozilla
 530
 531 /**
 532  * MOZ_TRY(expr) is the C++ equivalent of Rust's `try!(expr);`. First, it
 533  * evaluates expr, which must produce a Result value. On success, it
 534  * discards the result altogether. On error, it immediately returns an error
 535  * Result from the enclosing function.
 536  */
 537 #define MOZ_TRY(expr)                                       \
 538   do {                                                      \
 539     auto mozTryTempResult_ = ::mozilla::ToResult(expr);     \
 540     if (MOZ_UNLIKELY(mozTryTempResult_.isErr())) {          \
 541       return ::mozilla::Err(mozTryTempResult_.unwrapErr()); \
 542     }                                                       \
 543   } while (0)
 544
 545 /**
 546  * MOZ_TRY_VAR(target, expr) is the C++ equivalent of Rust's `target =
 547  * try!(expr);`. First, it evaluates expr, which must produce a Result value. On
 548  * success, the result's success value is assigned to target. On error,
 549  * immediately returns the error result. |target| must evaluate to a reference
 550  * without any side effects.
 551  */
 552 #define MOZ_TRY_VAR(target, expr)                              \
 553   do {                                                         \
 554     auto mozTryVarTempResult_ = (expr);                        \
 555     if (MOZ_UNLIKELY(mozTryVarTempResult_.isErr())) {          \
 556       return ::mozilla::Err(mozTryVarTempResult_.unwrapErr()); \
 557     }                                                          \
 558     (target) = mozTryVarTempResult_.unwrap();                  \
 559   } while (0)
 560
 561 #endif  // mozilla_Result_h