Abseil Common Libraries (C++) (grcp 依赖) https://abseil.io/
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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
//
// 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 <cstring>
#include "absl/flags/internal/commandlineflag.h"
#include "absl/flags/internal/registry.h"
#include "absl/memory/memory.h"
#include "absl/strings/str_cat.h"
namespace absl {
namespace flags_internal {
constexpr int64_t AtomicInit() { return 0xababababababababll; }
template <typename T>
class Flag;
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_;
};
// Signature for the mutation callback used by watched Flags
// The callback is noexcept.
// TODO(rogeeff): add noexcept after C++17 support is added.
using FlagCallback = void (*)();
void InvokeCallback(absl::Mutex* primary_mu, absl::Mutex* callback_mu,
FlagCallback cb) ABSL_EXCLUSIVE_LOCKS_REQUIRED(primary_mu);
// This is "unspecified" implementation of absl::Flag<T> type.
template <typename T>
class Flag final : public flags_internal::CommandLineFlag {
public:
constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
const char* filename,
const flags_internal::FlagMarshallingOpFn marshalling_op,
const flags_internal::InitialValGenFunc initial_value_gen)
: flags_internal::CommandLineFlag(
name, flags_internal::HelpText::FromFunctionPointer(help_gen),
filename, &flags_internal::FlagOps<T>, marshalling_op,
initial_value_gen,
/*def=*/nullptr,
/*cur=*/nullptr),
atomic_(flags_internal::AtomicInit()),
callback_(nullptr) {}
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.
union U {
T value;
U() {}
~U() { value.~T(); }
};
U u;
Read(&u.value, &flags_internal::FlagOps<T>);
return std::move(u.value);
}
bool AtomicGet(T* v) const {
const int64_t r = atomic_.load(std::memory_order_acquire);
if (r != flags_internal::AtomicInit()) {
std::memcpy(v, &r, sizeof(T));
return true;
}
return false;
}
void Set(const T& v) { Write(&v, &flags_internal::FlagOps<T>); }
void SetCallback(const flags_internal::FlagCallback mutation_callback) {
absl::MutexLock l(InitFlagIfNecessary());
callback_ = mutation_callback;
InvokeCallback();
}
private:
friend class FlagState<T>;
void Destroy() const override {
// Values are heap allocated for Abseil Flags.
if (cur_) Delete(op_, cur_);
if (def_) Delete(op_, def_);
delete locks_;
}
void StoreAtomic() override {
if (sizeof(T) <= sizeof(int64_t)) {
int64_t t = 0;
std::memcpy(&t, cur_, (std::min)(sizeof(T), sizeof(int64_t)));
atomic_.store(t, std::memory_order_release);
}
}
// 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<flags_internal::FlagStateInterface> SaveState() override {
T curr_value = Get();
absl::MutexLock l(InitFlagIfNecessary());
return absl::make_unique<flags_internal::FlagState<T>>(
this, std::move(curr_value), modified_, on_command_line_, counter_);
}
// Restores the flag state to the supplied state object. If there is
// nothing to restore returns false. Otherwise returns true.
bool RestoreState(const flags_internal::FlagState<T>& flag_state) {
if (MutationCounter() == flag_state.counter_) return false;
Set(flag_state.cur_value_);
// Race condition here? This should disappear once we move the rest of the
// flag's data into Flag's internals.
absl::MutexLock l(InitFlagIfNecessary());
modified_ = flag_state.modified_;
on_command_line_ = flag_state.on_command_line_;
return true;
}
// Interfaces to overate on callbacks.
void InvokeCallback() override
ABSL_EXCLUSIVE_LOCKS_REQUIRED(locks_->primary_mu) {
flags_internal::InvokeCallback(&locks_->primary_mu, &locks_->callback_mu,
callback_);
}
// Flag's data
// For some types, a copy of the current value is kept in an atomically
// accessible field.
std::atomic<int64_t> atomic_;
FlagCallback callback_; // Mutation callback
};
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);
template <typename T, bool do_register>
class FlagRegistrar {
public:
explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) {
if (do_register) flags_internal::RegisterCommandLineFlag(flag_);
}
FlagRegistrar& OnUpdate(flags_internal::FlagCallback 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
} // namespace absl
#endif // ABSL_FLAGS_INTERNAL_FLAG_H_