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// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include <stddef.h>
#include <stdint.h>
#include <atomic>
#include <cstring>
#include <memory>
#include <new>
#include <string>
#include <type_traits>
#include <typeinfo>
#include "absl/base/attributes.h"
#include "absl/base/call_once.h"
#include "absl/base/config.h"
#include "absl/base/optimization.h"
#include "absl/base/thread_annotations.h"
#include "absl/flags/commandlineflag.h"
#include "absl/flags/config.h"
#include "absl/flags/internal/commandlineflag.h"
#include "absl/flags/internal/registry.h"
#include "absl/flags/marshalling.h"
#include "absl/meta/type_traits.h"
#include "absl/strings/string_view.h"
#include "absl/synchronization/mutex.h"
#include "absl/utility/utility.h"
namespace absl {
// Forward declaration of absl::Flag<T> public API.
namespace flags_internal {
template <typename T>
class Flag;
} // namespace flags_internal
#if defined(_MSC_VER) && !defined(__clang__)
template <typename T>
class Flag;
template <typename T>
using Flag = flags_internal::Flag<T>;
template <typename T>
ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag);
template <typename T>
void SetFlag(absl::Flag<T>* flag, const T& v);
template <typename T, typename V>
void SetFlag(absl::Flag<T>* flag, const V& v);
template <typename U>
const CommandLineFlag& GetFlagReflectionHandle(const absl::Flag<U>& f);
// Flag value type operations, eg., parsing, copying, etc. are provided
// by function specific to that type with a signature matching FlagOpFn.
namespace flags_internal {
enum class FlagOp {
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);
// Allocate aligned memory for a flag value.
inline void* Alloc(FlagOpFn op) {
return op(FlagOp::kAlloc, nullptr, nullptr, nullptr);
// Deletes memory interpreting obj as flag value type pointer.
inline void Delete(FlagOpFn op, void* obj) {
op(FlagOp::kDelete, nullptr, obj, 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);
// Makes a copy of flag value pointed by obj.
inline void* Clone(FlagOpFn op, const void* obj) {
void* res = flags_internal::Alloc(op);
flags_internal::CopyConstruct(op, obj, res);
return res;
// 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 fast type id coresponding to the value type.
inline FlagFastTypeId FastTypeId(FlagOpFn op) {
return reinterpret_cast<FlagFastTypeId>(
op(FlagOp::kFastTypeId, nullptr, nullptr, nullptr));
// Returns fast type id coresponding to the value type.
inline const std::type_info* RuntimeTypeId(FlagOpFn op) {
return reinterpret_cast<const std::type_info*>(
op(FlagOp::kRuntimeTypeId, 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)));
// Returns an address of RTTI's typeid(T).
template <typename T>
inline const std::type_info* GenRuntimeTypeId() {
return &typeid(T);
return 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 (*)();
template <size_t N>
struct FixedCharArray {
char value[N];
template <size_t... I>
static constexpr FixedCharArray<N> FromLiteralString(
absl::string_view str, absl::index_sequence<I...>) {
return (void)str, FixedCharArray<N>({{str[I]..., '\0'}});
template <typename Gen, size_t N = Gen::Value().size()>
constexpr FixedCharArray<N + 1> HelpStringAsArray(int) {
return FixedCharArray<N + 1>::FromLiteralString(
Gen::Value(), absl::make_index_sequence<N>{});
template <typename Gen>
constexpr std::false_type HelpStringAsArray(char) {
return std::false_type{};
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[];
// 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 Gen and integer 0 as a single argument to prefer
// first overload if possible. If help message is evaluatable on constexpr
// context We'll be able to make FixedCharArray out of it and 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 Gen, size_t N>
constexpr FlagHelpArg HelpArg(const FixedCharArray<N>& value) {
return {FlagHelpMsg(value.value), FlagHelpKind::kLiteral};
template <typename Gen>
constexpr FlagHelpArg HelpArg(std::false_type) {
return {FlagHelpMsg(&Gen::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 (*)(void*);
union FlagDefaultSrc {
constexpr explicit FlagDefaultSrc(FlagDfltGenFunc gen_func_arg)
: gen_func(gen_func_arg) {}
T name##_value; \
constexpr explicit FlagDefaultSrc(T value) : name##_value(value) {} // NOLINT
void* dynamic_value;
FlagDfltGenFunc gen_func;
enum class FlagDefaultKind : uint8_t {
kDynamicValue = 0,
kGenFunc = 1,
kOneWord = 2 // for default values UP to one word in size
struct FlagDefaultArg {
FlagDefaultSrc source;
FlagDefaultKind kind;
// This struct and corresponding overload to InitDefaultValue are used to
// facilitate usage of {} as default value in ABSL_FLAG macro.
// TODO(rogeeff): Fix handling types with explicit constructors.
struct EmptyBraces {};
template <typename T>
constexpr T InitDefaultValue(T t) {
return t;
template <typename T>
constexpr T InitDefaultValue(EmptyBraces) {
return T{};
template <typename ValueT, typename GenT,
typename std::enable_if<std::is_integral<ValueT>::value, int>::type =
(GenT{}, 0)>
constexpr FlagDefaultArg DefaultArg(int) {
return {FlagDefaultSrc(GenT{}.value), FlagDefaultKind::kOneWord};
template <typename ValueT, typename GenT>
constexpr FlagDefaultArg DefaultArg(char) {
return {FlagDefaultSrc(&GenT::Gen), FlagDefaultKind::kGenFunc};
// 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)>;
// 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)>;
// 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 {
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)>;
template <typename T>
using FlagUseBufferStorage =
std::integral_constant<bool, !FlagUseOneWordStorage<T>::value &&
enum class FlagValueStorageKind : uint8_t {
kAlignedBuffer = 0,
kOneWordAtomic = 1,
kTwoWordsAtomic = 2
template <typename T>
static constexpr FlagValueStorageKind StorageKind() {
return FlagUseBufferStorage<T>::value
? FlagValueStorageKind::kAlignedBuffer
: FlagUseOneWordStorage<T>::value
? FlagValueStorageKind::kOneWordAtomic
: FlagValueStorageKind::kTwoWordsAtomic;
struct FlagOneWordValue {
constexpr FlagOneWordValue() : value(UninitializedFlagValue()) {}
std::atomic<int64_t> value;
struct FlagTwoWordsValue {
constexpr FlagTwoWordsValue()
: value(AlignedTwoWords{UninitializedFlagValue(), 0}) {}
std::atomic<AlignedTwoWords> value;
template <typename T,
FlagValueStorageKind Kind = flags_internal::StorageKind<T>()>
struct FlagValue;
template <typename T>
struct FlagValue<T, FlagValueStorageKind::kAlignedBuffer> {
bool Get(T&) const { return false; }
alignas(T) char value[sizeof(T)];
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);
void operator()(void* ptr) const;
FlagOpFn op;
class FlagState;
class FlagImpl final : public CommandLineFlag {
constexpr FlagImpl(const char* name, const char* filename, FlagOpFn op,
FlagHelpArg help, FlagValueStorageKind value_kind,
FlagDefaultArg default_arg)
: name_(name),
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)
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 `type_id`, which is in turn is validated against the
// type id stored in flag object by flag definition statement.
void AssertValidType(FlagFastTypeId type_id,
const std::type_info* (*gen_rtti)()) const;
template <typename T>
friend class Flag;
friend class FlagState;
// Ensures that `data_guard_` is initialized and returns it.
absl::Mutex* DataGuard() const
// Returns heap allocated value of type T initialized with default value.
std::unique_ptr<void, DynValueDeleter> MakeInitValue() const
// 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
//, we can define it in that file as well.
template <typename StorageT>
StorageT* OffsetValue() const;
// This is an accessor for a value stored in an aligned buffer storage.
// Returns a mutable pointer to the start of a buffer.
void* AlignedBufferValue() 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
// 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
return static_cast<FlagDefaultKind>(def_kind_);
// CommandLineFlag interface implementation
absl::string_view Name() const override;
std::string Filename() const override;
std::string Help() const override;
FlagFastTypeId TypeId() const override;
bool IsSpecifiedOnCommandLine() const override
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
void CheckDefaultValueParsingRoundtrip() const override
// 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
// 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)
bool ParseFrom(absl::string_view value, FlagSettingMode set_mode,
ValueSource source, std::string& error) override
// 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;
uint8_t : 0; // The bytes containing the const bitfields must not be
// shared with bytes containing the mutable bitfields.
// Mutable flag's state (guarded by `data_guard_`).
// def_kind_ is not guard by DataGuard() since it is accessed in Init without
// locks.
uint8_t def_kind_ : 2;
// 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());
// Unique tag for absl::call_once call to initialize this flag.
absl::once_flag init_control_;
// 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 {
constexpr Flag(const char* name, const char* filename, FlagHelpArg help,
const FlagDefaultArg default_arg)
: impl_(name, filename, &FlagOps<T>, help,
flags_internal::StorageKind<T>(), default_arg),
value_() {}
// CommandLineFlag interface
absl::string_view Name() const { return impl_.Name(); }
std::string Filename() const { return impl_.Filename(); }
std::string Help() const { return impl_.Help(); }
// Do not use. To be removed.
bool IsSpecifiedOnCommandLine() const {
return impl_.IsSpecifiedOnCommandLine();
std::string DefaultValue() const { return impl_.DefaultValue(); }
std::string CurrentValue() const { return impl_.CurrentValue(); }
template <typename U, bool do_register>
friend class FlagRegistrar;
#if !defined(_MSC_VER) || defined(__clang__)
template <typename U>
friend U absl::GetFlag(const flags_internal::Flag<U>& flag);
template <typename U>
friend void absl::SetFlag(flags_internal::Flag<U>* flag, const U& v);
template <typename U, typename V>
friend void absl::SetFlag(flags_internal::Flag<U>* flag, const V& v);
template <typename U>
friend class absl::Flag;
T Get() const {
// See implementation notes in CommandLineFlag::Get().
union U {
T value;
U() {}
~U() { value.~T(); }
U u;
#if !defined(NDEBUG)
impl_.AssertValidType(base_internal::FastTypeId<T>(), &GenRuntimeTypeId<T>);
if (!value_.Get(u.value)) impl_.Read(&u.value);
return std::move(u.value);
void Set(const T& v) {
impl_.AssertValidType(base_internal::FastTypeId<T>(), &GenRuntimeTypeId<T>);
template <typename U>
friend const CommandLineFlag& absl::GetFlagReflectionHandle(
const absl::Flag<U>& f);
// Access to the reflection.
const CommandLineFlag& Reflect() const { return impl_; }
// 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::kAlloc: {
std::allocator<T> alloc;
return std::allocator_traits<std::allocator<T>>::allocate(alloc, 1);
case FlagOp::kDelete: {
T* p = static_cast<T*>(v2);
std::allocator<T> alloc;
std::allocator_traits<std::allocator<T>>::deallocate(alloc, p, 1);
return nullptr;
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::kFastTypeId:
return const_cast<void*>(base_internal::FastTypeId<T>());
case FlagOp::kRuntimeTypeId:
return const_cast<std::type_info*>(GenRuntimeTypeId<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);
struct FlagRegistrarEmpty {};
template <typename T, bool do_register>
class FlagRegistrar {
explicit FlagRegistrar(Flag<T>& flag) : flag_(flag) {
if (do_register) flags_internal::RegisterCommandLineFlag(flag_.impl_);
FlagRegistrar OnUpdate(FlagCallbackFunc cb) && {
return *this;
// Make the registrar "die" gracefully as an empty struct on a line where
// registration happens. Registrar objects are intended to live only as
// temporary.
operator FlagRegistrarEmpty() const { return {}; } // NOLINT
Flag<T>& flag_; // Flag being registered (not owned).
} // namespace flags_internal
} // namespace absl