about summary refs log tree commit diff
path: root/absl/flags/internal/flag.h
blob: 19119bbb61934a05167fbd94cc452017faa02d25 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
//
// Copyright 2019 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef ABSL_FLAGS_INTERNAL_FLAG_H_
#define ABSL_FLAGS_INTERNAL_FLAG_H_

#include <stdint.h>

#include <atomic>
#include <cstring>
#include <memory>
#include <string>
#include <type_traits>

#include "absl/base/call_once.h"
#include "absl/base/config.h"
#include "absl/base/thread_annotations.h"
#include "absl/flags/config.h"
#include "absl/flags/internal/commandlineflag.h"
#include "absl/flags/internal/registry.h"
#include "absl/memory/memory.h"
#include "absl/meta/type_traits.h"
#include "absl/strings/str_cat.h"
#include "absl/strings/string_view.h"
#include "absl/synchronization/mutex.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace flags_internal {

///////////////////////////////////////////////////////////////////////////////
// Flag value type operations, eg., parsing, copying, etc. are provided
// by function specific to that type with a signature matching FlagOpFn.

enum class FlagOp {
  kDelete,
  kClone,
  kCopy,
  kCopyConstruct,
  kSizeof,
  kStaticTypeId,
  kParse,
  kUnparse,
  kValueOffset,
};
using FlagOpFn = void* (*)(FlagOp, const void*, void*, void*);

// Forward declaration for Flag value specific operations.
template <typename T>
void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3);

// Deletes memory interpreting obj as flag value type pointer.
inline void Delete(FlagOpFn op, const void* obj) {
  op(FlagOp::kDelete, obj, nullptr, nullptr);
}
// Makes a copy of flag value pointed by obj.
inline void* Clone(FlagOpFn op, const void* obj) {
  return op(FlagOp::kClone, obj, nullptr, nullptr);
}
// Copies src to dst interpreting as flag value type pointers.
inline void Copy(FlagOpFn op, const void* src, void* dst) {
  op(FlagOp::kCopy, src, dst, nullptr);
}
// Construct a copy of flag value in a location pointed by dst
// based on src - pointer to the flag's value.
inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
  op(FlagOp::kCopyConstruct, src, dst, nullptr);
}
// Returns true if parsing of input text is successfull.
inline bool Parse(FlagOpFn op, absl::string_view text, void* dst,
                  std::string* error) {
  return op(FlagOp::kParse, &text, dst, error) != nullptr;
}
// Returns string representing supplied value.
inline std::string Unparse(FlagOpFn op, const void* val) {
  std::string result;
  op(FlagOp::kUnparse, val, &result, nullptr);
  return result;
}
// Returns size of flag value type.
inline size_t Sizeof(FlagOpFn op) {
  // This sequence of casts reverses the sequence from
  // `flags_internal::FlagOps()`
  return static_cast<size_t>(reinterpret_cast<intptr_t>(
      op(FlagOp::kSizeof, nullptr, nullptr, nullptr)));
}
// Returns static type id coresponding to the value type.
inline FlagStaticTypeId StaticTypeId(FlagOpFn op) {
  return reinterpret_cast<FlagStaticTypeId>(
      op(FlagOp::kStaticTypeId, nullptr, nullptr, nullptr));
}
// Returns offset of the field value_ from the field impl_ inside of
// absl::Flag<T> data. Given FlagImpl pointer p you can get the
// location of the corresponding value as:
//      reinterpret_cast<char*>(p) + ValueOffset().
inline ptrdiff_t ValueOffset(FlagOpFn op) {
  // This sequence of casts reverses the sequence from
  // `flags_internal::FlagOps()`
  return static_cast<ptrdiff_t>(reinterpret_cast<intptr_t>(
      op(FlagOp::kValueOffset, nullptr, nullptr, nullptr)));
}

///////////////////////////////////////////////////////////////////////////////
// Flag help auxiliary structs.

// This is help argument for absl::Flag encapsulating the string literal pointer
// or pointer to function generating it as well as enum descriminating two
// cases.
using HelpGenFunc = std::string (*)();

union FlagHelpMsg {
  constexpr explicit FlagHelpMsg(const char* help_msg) : literal(help_msg) {}
  constexpr explicit FlagHelpMsg(HelpGenFunc help_gen) : gen_func(help_gen) {}

  const char* literal;
  HelpGenFunc gen_func;
};

enum class FlagHelpKind : uint8_t { kLiteral = 0, kGenFunc = 1 };

struct FlagHelpArg {
  FlagHelpMsg source;
  FlagHelpKind kind;
};

extern const char kStrippedFlagHelp[];

// HelpConstexprWrap is used by struct AbslFlagHelpGenFor##name generated by
// ABSL_FLAG macro. It is only used to silence the compiler in the case where
// help message expression is not constexpr and does not have type const char*.
// If help message expression is indeed constexpr const char* HelpConstexprWrap
// is just a trivial identity function.
template <typename T>
const char* HelpConstexprWrap(const T&) {
  return nullptr;
}
constexpr const char* HelpConstexprWrap(const char* p) { return p; }
constexpr const char* HelpConstexprWrap(char* p) { return p; }

// These two HelpArg overloads allows us to select at compile time one of two
// way to pass Help argument to absl::Flag. We'll be passing
// AbslFlagHelpGenFor##name as T and integer 0 as a single argument to prefer
// first overload if possible. If T::Const is evaluatable on constexpr
// context (see non template int parameter below) we'll choose first overload.
// In this case the help message expression is immediately evaluated and is used
// to construct the absl::Flag. No additionl code is generated by ABSL_FLAG.
// Otherwise SFINAE kicks in and first overload is dropped from the
// consideration, in which case the second overload will be used. The second
// overload does not attempt to evaluate the help message expression
// immediately and instead delays the evaluation by returing the function
// pointer (&T::NonConst) genering the help message when necessary. This is
// evaluatable in constexpr context, but the cost is an extra function being
// generated in the ABSL_FLAG code.
template <typename T, int = (T::Const(), 1)>
constexpr FlagHelpArg HelpArg(int) {
  return {FlagHelpMsg(T::Const()), FlagHelpKind::kLiteral};
}

template <typename T>
constexpr FlagHelpArg HelpArg(char) {
  return {FlagHelpMsg(&T::NonConst), FlagHelpKind::kGenFunc};
}

///////////////////////////////////////////////////////////////////////////////
// Flag default value auxiliary structs.

// Signature for the function generating the initial flag value (usually
// based on default value supplied in flag's definition)
using FlagDfltGenFunc = void* (*)();

union FlagDefaultSrc {
  constexpr explicit FlagDefaultSrc(FlagDfltGenFunc gen_func_arg)
      : gen_func(gen_func_arg) {}

  void* dynamic_value;
  FlagDfltGenFunc gen_func;
};

enum class FlagDefaultKind : uint8_t { kDynamicValue = 0, kGenFunc = 1 };

///////////////////////////////////////////////////////////////////////////////
// Flag current value auxiliary structs.

constexpr int64_t UninitializedFlagValue() { return 0xababababababababll; }

template <typename T>
using FlagUseOneWordStorage = std::integral_constant<
    bool, absl::type_traits_internal::is_trivially_copyable<T>::value &&
              (sizeof(T) <= 8)>;

#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
// Clang does not always produce cmpxchg16b instruction when alignment of a 16
// bytes type is not 16.
struct alignas(16) AlignedTwoWords {
  int64_t first;
  int64_t second;

  bool IsInitialized() const {
    return first != flags_internal::UninitializedFlagValue();
  }
};

template <typename T>
using FlagUseTwoWordsStorage = std::integral_constant<
    bool, absl::type_traits_internal::is_trivially_copyable<T>::value &&
              (sizeof(T) > 8) && (sizeof(T) <= 16)>;
#else
// This is actually unused and only here to avoid ifdefs in other palces.
struct AlignedTwoWords {
  constexpr AlignedTwoWords() noexcept : dummy() {}
  constexpr AlignedTwoWords(int64_t, int64_t) noexcept : dummy() {}
  char dummy;

  bool IsInitialized() const {
    std::abort();
    return true;
  }
};

// This trait should be type dependent, otherwise SFINAE below will fail
template <typename T>
using FlagUseTwoWordsStorage =
    std::integral_constant<bool, sizeof(T) != sizeof(T)>;
#endif

template <typename T>
using FlagUseHeapStorage =
    std::integral_constant<bool, !FlagUseOneWordStorage<T>::value &&
                                     !FlagUseTwoWordsStorage<T>::value>;

enum class FlagValueStorageKind : uint8_t {
  kHeapAllocated = 0,
  kOneWordAtomic = 1,
  kTwoWordsAtomic = 2
};

template <typename T>
static constexpr FlagValueStorageKind StorageKind() {
  return FlagUseHeapStorage<T>::value
             ? FlagValueStorageKind::kHeapAllocated
             : FlagUseOneWordStorage<T>::value
                   ? FlagValueStorageKind::kOneWordAtomic
                   : FlagUseTwoWordsStorage<T>::value
                         ? FlagValueStorageKind::kTwoWordsAtomic
                         : FlagValueStorageKind::kHeapAllocated;
}

struct FlagHeapAllocatedValue {
  using value_type = void*;

  value_type value;
};

struct FlagOneWordValue {
  using value_type = std::atomic<int64_t>;
  constexpr FlagOneWordValue() : value(UninitializedFlagValue()) {}

  value_type value;
};

struct FlagTwoWordsValue {
  using value_type = std::atomic<AlignedTwoWords>;
  constexpr FlagTwoWordsValue()
      : value(AlignedTwoWords{UninitializedFlagValue(), 0}) {}

  value_type value;
};

template <typename T,
          FlagValueStorageKind Kind = flags_internal::StorageKind<T>()>
struct FlagValue;

template <typename T>
struct FlagValue<T, FlagValueStorageKind::kHeapAllocated>
    : FlagHeapAllocatedValue {
  bool Get(T*) const { return false; }
};

template <typename T>
struct FlagValue<T, FlagValueStorageKind::kOneWordAtomic> : FlagOneWordValue {
  bool Get(T* dst) const {
    int64_t one_word_val = value.load(std::memory_order_acquire);
    if (ABSL_PREDICT_FALSE(one_word_val == UninitializedFlagValue())) {
      return false;
    }
    std::memcpy(dst, static_cast<const void*>(&one_word_val), sizeof(T));
    return true;
  }
};

template <typename T>
struct FlagValue<T, FlagValueStorageKind::kTwoWordsAtomic> : FlagTwoWordsValue {
  bool Get(T* dst) const {
    AlignedTwoWords two_words_val = value.load(std::memory_order_acquire);
    if (ABSL_PREDICT_FALSE(!two_words_val.IsInitialized())) {
      return false;
    }
    std::memcpy(dst, static_cast<const void*>(&two_words_val), sizeof(T));
    return true;
  }
};

///////////////////////////////////////////////////////////////////////////////
// Flag callback auxiliary structs.

// Signature for the mutation callback used by watched Flags
// The callback is noexcept.
// TODO(rogeeff): add noexcept after C++17 support is added.
using FlagCallbackFunc = void (*)();

struct FlagCallback {
  FlagCallbackFunc func;
  absl::Mutex guard;  // Guard for concurrent callback invocations.
};

///////////////////////////////////////////////////////////////////////////////
// Flag implementation, which does not depend on flag value type.
// The class encapsulates the Flag's data and access to it.

struct DynValueDeleter {
  explicit DynValueDeleter(FlagOpFn op_arg = nullptr) : op(op_arg) {}
  void operator()(void* ptr) const {
    if (op != nullptr) flags_internal::Delete(op, ptr);
  }

  FlagOpFn op;
};

class FlagState;

class FlagImpl final : public flags_internal::CommandLineFlag {
 public:
  constexpr FlagImpl(const char* name, const char* filename, FlagOpFn op,
                     FlagHelpArg help, FlagValueStorageKind value_kind,
                     FlagDfltGenFunc default_value_gen)
      : name_(name),
        filename_(filename),
        op_(op),
        help_(help.source),
        help_source_kind_(static_cast<uint8_t>(help.kind)),
        value_storage_kind_(static_cast<uint8_t>(value_kind)),
        def_kind_(static_cast<uint8_t>(FlagDefaultKind::kGenFunc)),
        modified_(false),
        on_command_line_(false),
        counter_(0),
        callback_(nullptr),
        default_value_(default_value_gen),
        data_guard_{} {}

  // Constant access methods
  void Read(void* dst) const override ABSL_LOCKS_EXCLUDED(*DataGuard());

  // Mutating access methods
  void Write(const void* src) ABSL_LOCKS_EXCLUDED(*DataGuard());

  // Interfaces to operate on callbacks.
  void SetCallback(const FlagCallbackFunc mutation_callback)
      ABSL_LOCKS_EXCLUDED(*DataGuard());
  void InvokeCallback() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());

  // Used in read/write operations to validate source/target has correct type.
  // For example if flag is declared as absl::Flag<int> FLAGS_foo, a call to
  // absl::GetFlag(FLAGS_foo) validates that the type of FLAGS_foo is indeed
  // int. To do that we pass the "assumed" type id (which is deduced from type
  // int) as an argument `op`, which is in turn is validated against the type id
  // stored in flag object by flag definition statement.
  void AssertValidType(FlagStaticTypeId type_id) const;

 private:
  template <typename T>
  friend class Flag;
  friend class FlagState;

  // Ensures that `data_guard_` is initialized and returns it.
  absl::Mutex* DataGuard() const ABSL_LOCK_RETURNED((absl::Mutex*)&data_guard_);
  // Returns heap allocated value of type T initialized with default value.
  std::unique_ptr<void, DynValueDeleter> MakeInitValue() const
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
  // Flag initialization called via absl::call_once.
  void Init();

  // Offset value access methods. One per storage kind. These methods to not
  // respect const correctness, so be very carefull using them.

  // This is a shared helper routine which encapsulates most of the magic. Since
  // it is only used inside the three routines below, which are defined in
  // flag.cc, we can define it in that file as well.
  template <typename StorageT>
  typename StorageT::value_type& OffsetValue() const;
  // This is an accessor for a value stored in heap allocated storage.
  // Returns a mutable reference to a pointer to allow vlaue mutation.
  void*& HeapAllocatedValue() const;
  // This is an accessor for a value stored as one word atomic. Returns a
  // mutable reference to an atomic value.
  std::atomic<int64_t>& OneWordValue() const;
  // This is an accessor for a value stored as two words atomic. Returns a
  // mutable reference to an atomic value.
  std::atomic<AlignedTwoWords>& TwoWordsValue() const;

  // Attempts to parse supplied `value` string. If parsing is successful,
  // returns new value. Otherwise returns nullptr.
  std::unique_ptr<void, DynValueDeleter> TryParse(absl::string_view value,
                                                  std::string* err) const
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
  // Stores the flag value based on the pointer to the source.
  void StoreValue(const void* src) ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());

  FlagHelpKind HelpSourceKind() const {
    return static_cast<FlagHelpKind>(help_source_kind_);
  }
  FlagValueStorageKind ValueStorageKind() const {
    return static_cast<FlagValueStorageKind>(value_storage_kind_);
  }
  FlagDefaultKind DefaultKind() const
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard()) {
    return static_cast<FlagDefaultKind>(def_kind_);
  }

  // CommandLineFlag interface implementation
  absl::string_view Name() const override;
  std::string Filename() const override;
  absl::string_view Typename() const override;
  std::string Help() const override;
  FlagStaticTypeId TypeId() const override;
  bool IsModified() const override ABSL_LOCKS_EXCLUDED(*DataGuard());
  bool IsSpecifiedOnCommandLine() const override
      ABSL_LOCKS_EXCLUDED(*DataGuard());
  std::string DefaultValue() const override ABSL_LOCKS_EXCLUDED(*DataGuard());
  std::string CurrentValue() const override ABSL_LOCKS_EXCLUDED(*DataGuard());
  bool ValidateInputValue(absl::string_view value) const override
      ABSL_LOCKS_EXCLUDED(*DataGuard());
  void CheckDefaultValueParsingRoundtrip() const override
      ABSL_LOCKS_EXCLUDED(*DataGuard());

  // Interfaces to save and restore flags to/from persistent state.
  // Returns current flag state or nullptr if flag does not support
  // saving and restoring a state.
  std::unique_ptr<FlagStateInterface> SaveState() override
      ABSL_LOCKS_EXCLUDED(*DataGuard());

  // Restores the flag state to the supplied state object. If there is
  // nothing to restore returns false. Otherwise returns true.
  bool RestoreState(const FlagState& flag_state)
      ABSL_LOCKS_EXCLUDED(*DataGuard());

  bool ParseFrom(absl::string_view value, FlagSettingMode set_mode,
                 ValueSource source, std::string* error) override
      ABSL_LOCKS_EXCLUDED(*DataGuard());

  // Immutable flag's state.

  // Flags name passed to ABSL_FLAG as second arg.
  const char* const name_;
  // The file name where ABSL_FLAG resides.
  const char* const filename_;
  // Type-specific operations "vtable".
  const FlagOpFn op_;
  // Help message literal or function to generate it.
  const FlagHelpMsg help_;
  // Indicates if help message was supplied as literal or generator func.
  const uint8_t help_source_kind_ : 1;
  // Kind of storage this flag is using for the flag's value.
  const uint8_t value_storage_kind_ : 2;

  // ------------------------------------------------------------------------
  // The bytes containing the const bitfields must not be shared with bytes
  // containing the mutable bitfields.
  // ------------------------------------------------------------------------

  // Unique tag for absl::call_once call to initialize this flag.
  //
  // The placement of this variable between the immutable and mutable bitfields
  // is important as prevents them from occupying the same byte. If you remove
  // this variable, make sure to maintain this property.
  absl::once_flag init_control_;

  // Mutable flag's state (guarded by `data_guard_`).

  // If def_kind_ == kDynamicValue, default_value_ holds a dynamically allocated
  // value.
  uint8_t def_kind_ : 1 ABSL_GUARDED_BY(*DataGuard());
  // Has this flag's value been modified?
  bool modified_ : 1 ABSL_GUARDED_BY(*DataGuard());
  // Has this flag been specified on command line.
  bool on_command_line_ : 1 ABSL_GUARDED_BY(*DataGuard());

  // Mutation counter
  int64_t counter_ ABSL_GUARDED_BY(*DataGuard());
  // Optional flag's callback and absl::Mutex to guard the invocations.
  FlagCallback* callback_ ABSL_GUARDED_BY(*DataGuard());
  // Either a pointer to the function generating the default value based on the
  // value specified in ABSL_FLAG or pointer to the dynamically set default
  // value via SetCommandLineOptionWithMode. def_kind_ is used to distinguish
  // these two cases.
  FlagDefaultSrc default_value_;

  // This is reserved space for an absl::Mutex to guard flag data. It will be
  // initialized in FlagImpl::Init via placement new.
  // We can't use "absl::Mutex data_guard_", since this class is not literal.
  // We do not want to use "absl::Mutex* data_guard_", since this would require
  // heap allocation during initialization, which is both slows program startup
  // and can fail. Using reserved space + placement new allows us to avoid both
  // problems.
  alignas(absl::Mutex) mutable char data_guard_[sizeof(absl::Mutex)];
};

///////////////////////////////////////////////////////////////////////////////
// The Flag object parameterized by the flag's value type. This class implements
// flag reflection handle interface.

template <typename T>
class Flag {
 public:
  constexpr Flag(const char* name, const char* filename, const FlagHelpArg help,
                 const FlagDfltGenFunc default_value_gen)
      : impl_(name, filename, &FlagOps<T>, help,
              flags_internal::StorageKind<T>(), default_value_gen),
        value_() {}

  T Get() const {
    // See implementation notes in CommandLineFlag::Get().
    union U {
      T value;
      U() {}
      ~U() { value.~T(); }
    };
    U u;

#if !defined(NDEBUG)
    impl_.AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>);
#endif

    if (!value_.Get(&u.value)) impl_.Read(&u.value);
    return std::move(u.value);
  }
  void Set(const T& v) {
    impl_.AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>);
    impl_.Write(&v);
  }
  void SetCallback(const FlagCallbackFunc mutation_callback) {
    impl_.SetCallback(mutation_callback);
  }

  // CommandLineFlag interface
  absl::string_view Name() const { return impl_.Name(); }
  std::string Filename() const { return impl_.Filename(); }
  absl::string_view Typename() const { return ""; }
  std::string Help() const { return impl_.Help(); }
  bool IsModified() const { return impl_.IsModified(); }
  bool IsSpecifiedOnCommandLine() const {
    return impl_.IsSpecifiedOnCommandLine();
  }
  std::string DefaultValue() const { return impl_.DefaultValue(); }
  std::string CurrentValue() const { return impl_.CurrentValue(); }

 private:
  template <typename U, bool do_register>
  friend class FlagRegistrar;

  // Flag's data
  // The implementation depends on value_ field to be placed exactly after the
  // impl_ field, so that impl_ can figure out the offset to the value and
  // access it.
  FlagImpl impl_;
  FlagValue<T> value_;
};

///////////////////////////////////////////////////////////////////////////////
// Implementation of Flag value specific operations routine.
template <typename T>
void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) {
  switch (op) {
    case FlagOp::kDelete:
      delete static_cast<const T*>(v1);
      return nullptr;
    case FlagOp::kClone:
      return new T(*static_cast<const T*>(v1));
    case FlagOp::kCopy:
      *static_cast<T*>(v2) = *static_cast<const T*>(v1);
      return nullptr;
    case FlagOp::kCopyConstruct:
      new (v2) T(*static_cast<const T*>(v1));
      return nullptr;
    case FlagOp::kSizeof:
      return reinterpret_cast<void*>(static_cast<uintptr_t>(sizeof(T)));
    case FlagOp::kStaticTypeId:
      return reinterpret_cast<void*>(&FlagStaticTypeIdGen<T>);
    case FlagOp::kParse: {
      // Initialize the temporary instance of type T based on current value in
      // destination (which is going to be flag's default value).
      T temp(*static_cast<T*>(v2));
      if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp,
                              static_cast<std::string*>(v3))) {
        return nullptr;
      }
      *static_cast<T*>(v2) = std::move(temp);
      return v2;
    }
    case FlagOp::kUnparse:
      *static_cast<std::string*>(v2) =
          absl::UnparseFlag<T>(*static_cast<const T*>(v1));
      return nullptr;
    case FlagOp::kValueOffset: {
      // Round sizeof(FlagImp) to a multiple of alignof(FlagValue<T>) to get the
      // offset of the data.
      ptrdiff_t round_to = alignof(FlagValue<T>);
      ptrdiff_t offset =
          (sizeof(FlagImpl) + round_to - 1) / round_to * round_to;
      return reinterpret_cast<void*>(offset);
    }
  }
  return nullptr;
}

///////////////////////////////////////////////////////////////////////////////
// This class facilitates Flag object registration and tail expression-based
// flag definition, for example:
// ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
template <typename T, bool do_register>
class FlagRegistrar {
 public:
  explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) {
    if (do_register) flags_internal::RegisterCommandLineFlag(&flag_->impl_);
  }

  FlagRegistrar& OnUpdate(FlagCallbackFunc cb) && {
    flag_->SetCallback(cb);
    return *this;
  }

  // Make the registrar "die" gracefully as a bool on a line where registration
  // happens. Registrar objects are intended to live only as temporary.
  operator bool() const { return true; }  // NOLINT

 private:
  Flag<T>* flag_;  // Flag being registered (not owned).
};

// This struct and corresponding overload to MakeDefaultValue are used to
// facilitate usage of {} as default value in ABSL_FLAG macro.
struct EmptyBraces {};

template <typename T>
T* MakeFromDefaultValue(T t) {
  return new T(std::move(t));
}

template <typename T>
T* MakeFromDefaultValue(EmptyBraces) {
  return new T{};
}

}  // namespace flags_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_FLAGS_INTERNAL_FLAG_H_