c6c3c1b498
-- ed3a3431eee9e48e6553b0320e0308d2dde6725c by Derek Mauro <dmauro@google.com>: Project import generated by Copybara. PiperOrigin-RevId: 258631680 GitOrigin-RevId: ed3a3431eee9e48e6553b0320e0308d2dde6725c Change-Id: I1d7ae86a79783842092d29504605ba039c369603
121 lines
4 KiB
C++
121 lines
4 KiB
C++
//
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// Copyright 2019 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_FLAGS_INTERNAL_FLAG_H_
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#define ABSL_FLAGS_INTERNAL_FLAG_H_
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#include "absl/flags/internal/commandlineflag.h"
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#include "absl/flags/internal/registry.h"
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namespace absl {
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namespace flags_internal {
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// This is "unspecified" implementation of absl::Flag<T> type.
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template <typename T>
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class Flag : public flags_internal::CommandLineFlag {
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public:
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constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
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const char* filename,
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const flags_internal::FlagMarshallingOpFn marshalling_op_arg,
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const flags_internal::InitialValGenFunc initial_value_gen)
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: flags_internal::CommandLineFlag(
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name, flags_internal::HelpText::FromFunctionPointer(help_gen),
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filename, &flags_internal::FlagOps<T>, marshalling_op_arg,
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initial_value_gen,
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/*retired_arg=*/false, /*def_arg=*/nullptr,
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/*cur_arg=*/nullptr) {}
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T Get() const {
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// Implementation notes:
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//
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// We are wrapping a union around the value of `T` to serve three purposes:
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//
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// 1. `U.value` has correct size and alignment for a value of type `T`
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// 2. The `U.value` constructor is not invoked since U's constructor does
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// not
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// do it explicitly.
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// 3. The `U.value` destructor is invoked since U's destructor does it
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// explicitly. This makes `U` a kind of RAII wrapper around non default
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// constructible value of T, which is destructed when we leave the
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// scope. We do need to destroy U.value, which is constructed by
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// CommandLineFlag::Read even though we left it in a moved-from state
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// after std::move.
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//
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// All of this serves to avoid requiring `T` being default constructible.
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union U {
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T value;
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U() {}
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~U() { value.~T(); }
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};
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U u;
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this->Read(&u.value, &flags_internal::FlagOps<T>);
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return std::move(u.value);
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}
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bool AtomicGet(T* v) const {
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const int64_t r = this->atomic.load(std::memory_order_acquire);
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if (r != flags_internal::CommandLineFlag::kAtomicInit) {
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memcpy(v, &r, sizeof(T));
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return true;
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}
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return false;
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}
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void Set(const T& v) { this->Write(&v, &flags_internal::FlagOps<T>); }
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};
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// This class facilitates Flag object registration and tail expression-based
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// flag definition, for example:
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// ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
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template <typename T, bool do_register>
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class FlagRegistrar {
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public:
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explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) {
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if (do_register) flags_internal::RegisterCommandLineFlag(flag_);
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}
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FlagRegistrar& OnUpdate(flags_internal::FlagCallback cb) && {
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flag_->SetCallback(cb);
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return *this;
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}
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// Make the registrar "die" gracefully as a bool on a line where registration
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// happens. Registrar objects are intended to live only as temporary.
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operator bool() const { return true; } // NOLINT
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private:
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Flag<T>* flag_; // Flag being registered (not owned).
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};
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// This struct and corresponding overload to MakeDefaultValue are used to
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// facilitate usage of {} as default value in ABSL_FLAG macro.
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struct EmptyBraces {};
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template <typename T>
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T* MakeFromDefaultValue(T t) {
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return new T(std::move(t));
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}
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template <typename T>
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T* MakeFromDefaultValue(EmptyBraces) {
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return new T;
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}
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} // namespace flags_internal
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} // namespace absl
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#endif // ABSL_FLAGS_INTERNAL_FLAG_H_
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