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//
// Copyright 2021 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.
// Do not include this file directly.
// Include absl/flags/flag.h instead.
// MSVC debug builds do not implement initialization with constexpr constructors
// correctly. To work around this we add a level of indirection, so that the
// class `absl::Flag` contains an `internal::Flag*` (instead of being an alias
// to that class) and dynamically allocates an instance when necessary. We also
// forward all calls to internal::Flag methods via trampoline methods. In this
// setup the `absl::Flag` class does not have constructor and virtual methods,
// all the data members are public and thus MSVC is able to initialize it at
// link time. To deal with multiple threads accessing the flag for the first
// time concurrently we use an atomic boolean indicating if flag object is
// initialized. We also employ the double-checked locking pattern where the
// second level of protection is a global Mutex, so if two threads attempt to
// construct the flag concurrently only one wins.
//
// This solution is based on a recommendation here:
// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html?childToView=648454#comment-648454
namespace flags_internal {
absl::Mutex* GetGlobalConstructionGuard();
} // namespace flags_internal
// Public methods of `absl::Flag<T>` are NOT part of the Abseil Flags API.
// See https://abseil.io/docs/cpp/guides/flags
template <typename T>
class Flag {
public:
// No constructor and destructor to ensure this is an aggregate type.
// Visual Studio 2015 still requires the constructor for class to be
// constexpr initializable.
#if _MSC_VER <= 1900
constexpr Flag(const char* name, const char* filename,
const flags_internal::HelpGenFunc help_gen,
const flags_internal::FlagDfltGenFunc default_value_gen)
: name_(name),
filename_(filename),
help_gen_(help_gen),
default_value_gen_(default_value_gen),
inited_(false),
impl_(nullptr) {}
#endif
flags_internal::Flag<T>& GetImpl() const {
if (!inited_.load(std::memory_order_acquire)) {
absl::MutexLock l(flags_internal::GetGlobalConstructionGuard());
if (inited_.load(std::memory_order_acquire)) {
return *impl_;
}
impl_ = new flags_internal::Flag<T>(
name_, filename_,
{flags_internal::FlagHelpMsg(help_gen_),
flags_internal::FlagHelpKind::kGenFunc},
{flags_internal::FlagDefaultSrc(default_value_gen_),
flags_internal::FlagDefaultKind::kGenFunc});
inited_.store(true, std::memory_order_release);
}
return *impl_;
}
// Public methods of `absl::Flag<T>` are NOT part of the Abseil Flags API.
// See https://abseil.io/docs/cpp/guides/flags
bool IsRetired() const { return GetImpl().IsRetired(); }
absl::string_view Name() const { return GetImpl().Name(); }
std::string Help() const { return GetImpl().Help(); }
bool IsModified() const { return GetImpl().IsModified(); }
bool IsSpecifiedOnCommandLine() const {
return GetImpl().IsSpecifiedOnCommandLine();
}
std::string Filename() const { return GetImpl().Filename(); }
std::string DefaultValue() const { return GetImpl().DefaultValue(); }
std::string CurrentValue() const { return GetImpl().CurrentValue(); }
template <typename U>
inline bool IsOfType() const {
return GetImpl().template IsOfType<U>();
}
T Get() const {
return flags_internal::FlagImplPeer::InvokeGet<T>(GetImpl());
}
void Set(const T& v) {
flags_internal::FlagImplPeer::InvokeSet(GetImpl(), v);
}
void InvokeCallback() { GetImpl().InvokeCallback(); }
const CommandLineFlag& Reflect() const {
return flags_internal::FlagImplPeer::InvokeReflect(GetImpl());
}
// The data members are logically private, but they need to be public for
// this to be an aggregate type.
const char* name_;
const char* filename_;
const flags_internal::HelpGenFunc help_gen_;
const flags_internal::FlagDfltGenFunc default_value_gen_;
mutable std::atomic<bool> inited_;
mutable flags_internal::Flag<T>* impl_;
};
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