Creation of LTS branch "lts_2020_02_25"20200225

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GitOrigin-RevId: 0033c9ea91a52ade7c6b725aa2ef3cbe15463421
Change-Id: I8a2b70063cb3ab40c6943a6db0fe40cae71ed8d7

1 files changed, 588 insertions, 40 deletions

diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index 9a5d9b4b..35a148cf 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -16,45 +16,547 @@
 #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/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-// This is "unspecified" implementation of absl::Flag<T> type.
 template <typename T>
-class Flag : public flags_internal::CommandLineFlag {
+class Flag;
+
+///////////////////////////////////////////////////////////////////////////////
+// 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,
+};
+using FlagOpFn = void* (*)(FlagOp, const void*, void*, void*);
+
+// 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*>(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;
+    default:
+      return nullptr;
+  }
+}
+
+// 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));
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Persistent state of the flag data.
+
+template <typename T>
+class FlagState : public flags_internal::FlagStateInterface {
+ public:
+  FlagState(Flag<T>* flag, T&& cur, bool modified, bool on_command_line,
+            int64_t counter)
+      : flag_(flag),
+        cur_value_(std::move(cur)),
+        modified_(modified),
+        on_command_line_(on_command_line),
+        counter_(counter) {}
+
+  ~FlagState() override = default;
+
+ private:
+  friend class Flag<T>;
+
+  // Restores the flag to the saved state.
+  void Restore() const override;
+
+  // Flag and saved flag data.
+  Flag<T>* flag_;
+  T cur_value_;
+  bool modified_;
+  bool on_command_line_;
+  int64_t counter_;
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// 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.
+
+// The minimum atomic size we believe to generate lock free code, i.e. all
+// trivially copyable types not bigger this size generate lock free code.
+static constexpr int kMinLockFreeAtomicSize = 8;
+
+// The same as kMinLockFreeAtomicSize but maximum atomic size. As double words
+// might use two registers, we want to dispatch the logic for them.
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+static constexpr int kMaxLockFreeAtomicSize = 16;
+#else
+static constexpr int kMaxLockFreeAtomicSize = 8;
+#endif
+
+// We can use atomic in cases when it fits in the register, trivially copyable
+// in order to make memcpy operations.
+template <typename T>
+struct IsAtomicFlagTypeTrait {
+  static constexpr bool value =
+      (sizeof(T) <= kMaxLockFreeAtomicSize &&
+       type_traits_internal::is_trivially_copyable<T>::value);
+};
+
+// Clang does not always produce cmpxchg16b instruction when alignment of a 16
+// bytes type is not 16.
+struct alignas(16) FlagsInternalTwoWordsType {
+  int64_t first;
+  int64_t second;
+};
+
+constexpr bool operator==(const FlagsInternalTwoWordsType& that,
+                          const FlagsInternalTwoWordsType& other) {
+  return that.first == other.first && that.second == other.second;
+}
+constexpr bool operator!=(const FlagsInternalTwoWordsType& that,
+                          const FlagsInternalTwoWordsType& other) {
+  return !(that == other);
+}
+
+constexpr int64_t SmallAtomicInit() { return 0xababababababababll; }
+
+template <typename T, typename S = void>
+struct BestAtomicType {
+  using type = int64_t;
+  static constexpr int64_t AtomicInit() { return SmallAtomicInit(); }
+};
+
+template <typename T>
+struct BestAtomicType<
+    T, typename std::enable_if<(kMinLockFreeAtomicSize < sizeof(T) &&
+                                sizeof(T) <= kMaxLockFreeAtomicSize),
+                               void>::type> {
+  using type = FlagsInternalTwoWordsType;
+  static constexpr FlagsInternalTwoWordsType AtomicInit() {
+    return {SmallAtomicInit(), SmallAtomicInit()};
+  }
+};
+
+struct FlagValue {
+  // Heap allocated value.
+  void* dynamic = nullptr;
+  // For some types, a copy of the current value is kept in an atomically
+  // accessible field.
+  union Atomics {
+    // Using small atomic for small types.
+    std::atomic<int64_t> small_atomic;
+    template <typename T,
+              typename K = typename std::enable_if<
+                  (sizeof(T) <= kMinLockFreeAtomicSize), void>::type>
+    int64_t load() const {
+      return small_atomic.load(std::memory_order_acquire);
+    }
+
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+    // Using big atomics for big types.
+    std::atomic<FlagsInternalTwoWordsType> big_atomic;
+    template <typename T, typename K = typename std::enable_if<
+                              (kMinLockFreeAtomicSize < sizeof(T) &&
+                               sizeof(T) <= kMaxLockFreeAtomicSize),
+                              void>::type>
+    FlagsInternalTwoWordsType load() const {
+      return big_atomic.load(std::memory_order_acquire);
+    }
+    constexpr Atomics()
+        : big_atomic{FlagsInternalTwoWordsType{SmallAtomicInit(),
+                                               SmallAtomicInit()}} {}
+#else
+    constexpr Atomics() : small_atomic{SmallAtomicInit()} {}
+#endif
+  };
+  Atomics atomics{};
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// 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) Delete(op, ptr);
+  }
+
+  FlagOpFn op;
+};
+
+class FlagImpl {
  public:
-  constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
-                 const char* filename,
-                 const flags_internal::FlagMarshallingOpFn marshalling_op_arg,
-                 const flags_internal::InitialValGenFunc initial_value_gen)
-      : flags_internal::CommandLineFlag(
-            name, flags_internal::HelpText::FromFunctionPointer(help_gen),
-            filename, &flags_internal::FlagOps<T>, marshalling_op_arg,
-            initial_value_gen,
-            /*retired_arg=*/false, /*def_arg=*/nullptr,
-            /*cur_arg=*/nullptr) {}
+  constexpr FlagImpl(const char* name, const char* filename, FlagOpFn op,
+                     FlagHelpArg help, FlagDfltGenFunc default_value_gen)
+      : name_(name),
+        filename_(filename),
+        op_(op),
+        help_(help.source),
+        help_source_kind_(static_cast<uint8_t>(help.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
+  absl::string_view Name() const;
+  std::string Filename() const;
+  std::string Help() const;
+  bool IsModified() const ABSL_LOCKS_EXCLUDED(*DataGuard());
+  bool IsSpecifiedOnCommandLine() const ABSL_LOCKS_EXCLUDED(*DataGuard());
+  std::string DefaultValue() const ABSL_LOCKS_EXCLUDED(*DataGuard());
+  std::string CurrentValue() const ABSL_LOCKS_EXCLUDED(*DataGuard());
+  void Read(void* dst) const ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  template <typename T, typename std::enable_if<
+                            !IsAtomicFlagTypeTrait<T>::value, int>::type = 0>
+  void Get(T* dst) const {
+    AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>);
+    Read(dst);
+  }
+  // Overload for `GetFlag()` for types that support lock-free reads.
+  template <typename T, typename std::enable_if<IsAtomicFlagTypeTrait<T>::value,
+                                                int>::type = 0>
+  void Get(T* dst) const {
+    // For flags of types which can be accessed "atomically" we want to avoid
+    // slowing down flag value access due to type validation. That's why
+    // this validation is hidden behind !NDEBUG
+#ifndef NDEBUG
+    AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>);
+#endif
+    using U = flags_internal::BestAtomicType<T>;
+    typename U::type r = value_.atomics.template load<T>();
+    if (r != U::AtomicInit()) {
+      std::memcpy(static_cast<void*>(dst), &r, sizeof(T));
+    } else {
+      Read(dst);
+    }
+  }
+  template <typename T>
+  void Set(const T& src) {
+    AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>);
+    Write(&src);
+  }
+
+  // Mutating access methods
+  void Write(const void* src) ABSL_LOCKS_EXCLUDED(*DataGuard());
+  bool SetFromString(absl::string_view value, FlagSettingMode set_mode,
+                     ValueSource source, std::string* err)
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  // If possible, updates copy of the Flag's value that is stored in an
+  // atomic word.
+  void StoreAtomic() ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+
+  // Interfaces to operate on callbacks.
+  void SetCallback(const FlagCallbackFunc mutation_callback)
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  void InvokeCallback() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+
+  // Interfaces to save/restore mutable flag data
+  template <typename T>
+  std::unique_ptr<FlagStateInterface> SaveState(Flag<T>* flag) const
+      ABSL_LOCKS_EXCLUDED(*DataGuard()) {
+    T&& cur_value = flag->Get();
+    absl::MutexLock l(DataGuard());
+
+    return absl::make_unique<FlagState<T>>(
+        flag, std::move(cur_value), modified_, on_command_line_, counter_);
+  }
+  bool RestoreState(const void* value, bool modified, bool on_command_line,
+                    int64_t counter) ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  // Value validation interfaces.
+  void CheckDefaultValueParsingRoundtrip() const
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  bool ValidateInputValue(absl::string_view value) const
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+ private:
+  // 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();
+  // Attempts to parse supplied `value` std::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_);
+  }
+  FlagDefaultKind DefaultKind() const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard()) {
+    return static_cast<FlagDefaultKind>(def_kind_);
+  }
+  // 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;
+
+  // 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;
+
+  // ------------------------------------------------------------------------
+  // 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_ ABSL_GUARDED_BY(*DataGuard());
+  // Current Flag Value
+  FlagValue 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 final : public flags_internal::CommandLineFlag {
+ public:
+  constexpr Flag(const char* name, const char* filename, const FlagHelpArg help,
+                 const FlagDfltGenFunc default_value_gen)
+      : impl_(name, filename, &FlagOps<T>, help, default_value_gen) {}
 
   T Get() const {
-    // Implementation notes:
-    //
-    // We are wrapping a union around the value of `T` to serve three purposes:
-    //
-    //  1. `U.value` has correct size and alignment for a value of type `T`
-    //  2. The `U.value` constructor is not invoked since U's constructor does
-    //  not
-    //     do it explicitly.
-    //  3. The `U.value` destructor is invoked since U's destructor does it
-    //     explicitly. This makes `U` a kind of RAII wrapper around non default
-    //     constructible value of T, which is destructed when we leave the
-    //     scope. We do need to destroy U.value, which is constructed by
-    //     CommandLineFlag::Read even though we left it in a moved-from state
-    //     after std::move.
-    //
-    // All of this serves to avoid requiring `T` being default constructible.
+    // See implementation notes in CommandLineFlag::Get().
     union U {
       T value;
       U() {}
@@ -62,23 +564,69 @@ class Flag : public flags_internal::CommandLineFlag {
     };
     U u;
 
-    this->Read(&u.value, &flags_internal::FlagOps<T>);
+    impl_.Get(&u.value);
     return std::move(u.value);
   }
+  void Set(const T& v) { impl_.Set(v); }
+  void SetCallback(const FlagCallbackFunc mutation_callback) {
+    impl_.SetCallback(mutation_callback);
+  }
 
-  bool AtomicGet(T* v) const {
-    const int64_t r = this->atomic.load(std::memory_order_acquire);
-    if (r != flags_internal::CommandLineFlag::kAtomicInit) {
-      memcpy(v, &r, sizeof(T));
-      return true;
-    }
+  // CommandLineFlag interface
+  absl::string_view Name() const override { return impl_.Name(); }
+  std::string Filename() const override { return impl_.Filename(); }
+  absl::string_view Typename() const override { return ""; }
+  std::string Help() const override { return impl_.Help(); }
+  bool IsModified() const override { return impl_.IsModified(); }
+  bool IsSpecifiedOnCommandLine() const override {
+    return impl_.IsSpecifiedOnCommandLine();
+  }
+  std::string DefaultValue() const override { return impl_.DefaultValue(); }
+  std::string CurrentValue() const override { return impl_.CurrentValue(); }
+  bool ValidateInputValue(absl::string_view value) const override {
+    return impl_.ValidateInputValue(value);
+  }
+
+  // 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 {
+    return impl_.SaveState(this);
+  }
 
-    return false;
+  // Restores the flag state to the supplied state object. If there is
+  // nothing to restore returns false. Otherwise returns true.
+  bool RestoreState(const FlagState<T>& flag_state) {
+    return impl_.RestoreState(&flag_state.cur_value_, flag_state.modified_,
+                              flag_state.on_command_line_, flag_state.counter_);
+  }
+  bool SetFromString(absl::string_view value, FlagSettingMode set_mode,
+                     ValueSource source, std::string* error) override {
+    return impl_.SetFromString(value, set_mode, source, error);
   }
+  void CheckDefaultValueParsingRoundtrip() const override {
+    impl_.CheckDefaultValueParsingRoundtrip();
+  }
+
+ private:
+  friend class FlagState<T>;
+
+  void Read(void* dst) const override { impl_.Read(dst); }
+  FlagStaticTypeId TypeId() const override { return &FlagStaticTypeIdGen<T>; }
 
-  void Set(const T& v) { this->Write(&v, &flags_internal::FlagOps<T>); }
+  // Flag's data
+  FlagImpl impl_;
 };
 
+template <typename T>
+inline void FlagState<T>::Restore() const {
+  if (flag_->RestoreState(*this)) {
+    ABSL_INTERNAL_LOG(INFO,
+                      absl::StrCat("Restore saved value of ", flag_->Name(),
+                                   " to: ", flag_->CurrentValue()));
+  }
+}
+
 // This class facilitates Flag object registration and tail expression-based
 // flag definition, for example:
 // ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
@@ -89,7 +637,7 @@ class FlagRegistrar {
     if (do_register) flags_internal::RegisterCommandLineFlag(flag_);
   }
 
-  FlagRegistrar& OnUpdate(flags_internal::FlagCallback cb) && {
+  FlagRegistrar& OnUpdate(FlagCallbackFunc cb) && {
     flag_->SetCallback(cb);
     return *this;
   }
@@ -117,7 +665,7 @@ T* MakeFromDefaultValue(EmptyBraces) {
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_FLAG_H_