| // |
| // 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 "absl/flags/internal/commandlineflag.h" |
| #include "absl/flags/internal/registry.h" |
| |
| namespace absl { |
| inline namespace lts_2019_08_08 { |
| namespace flags_internal { |
| |
| // This is "unspecified" implementation of absl::Flag<T> type. |
| template <typename T> |
| class Flag : 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_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) {} |
| |
| 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; |
| |
| this->Read(&u.value, &flags_internal::FlagOps<T>); |
| return std::move(u.value); |
| } |
| |
| 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; |
| } |
| |
| return false; |
| } |
| |
| void Set(const T& v) { this->Write(&v, &flags_internal::FlagOps<T>); } |
| }; |
| |
| // 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 |
| } // inline namespace lts_2019_08_08 |
| } // namespace absl |
| |
| #endif // ABSL_FLAGS_INTERNAL_FLAG_H_ |