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| 1 | +// |
| 2 | +// Copyright 2021 The Abseil Authors. |
| 3 | +// |
| 4 | +// Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | +// you may not use this file except in compliance with the License. |
| 6 | +// You may obtain a copy of the License at |
| 7 | +// |
| 8 | +// https://www.apache.org/licenses/LICENSE-2.0 |
| 9 | +// |
| 10 | +// Unless required by applicable law or agreed to in writing, software |
| 11 | +// distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | +// See the License for the specific language governing permissions and |
| 14 | +// limitations under the License. |
| 15 | + |
| 16 | +// Do not include this file directly. |
| 17 | +// Include absl/flags/flag.h instead. |
| 18 | + |
| 19 | +// MSVC debug builds do not implement initialization with constexpr constructors |
| 20 | +// correctly. To work around this we add a level of indirection, so that the |
| 21 | +// class `absl::Flag` contains an `internal::Flag*` (instead of being an alias |
| 22 | +// to that class) and dynamically allocates an instance when necessary. We also |
| 23 | +// forward all calls to internal::Flag methods via trampoline methods. In this |
| 24 | +// setup the `absl::Flag` class does not have constructor and virtual methods, |
| 25 | +// all the data members are public and thus MSVC is able to initialize it at |
| 26 | +// link time. To deal with multiple threads accessing the flag for the first |
| 27 | +// time concurrently we use an atomic boolean indicating if flag object is |
| 28 | +// initialized. We also employ the double-checked locking pattern where the |
| 29 | +// second level of protection is a global Mutex, so if two threads attempt to |
| 30 | +// construct the flag concurrently only one wins. |
| 31 | +// |
| 32 | +// This solution is based on a recomendation here: |
| 33 | +// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html?childToView=648454#comment-648454 |
| 34 | + |
| 35 | +namespace flags_internal { |
| 36 | +absl::Mutex* GetGlobalConstructionGuard(); |
| 37 | +} // namespace flags_internal |
| 38 | + |
| 39 | +// Public methods of `absl::Flag<T>` are NOT part of the Abseil Flags API. |
| 40 | +// See https://abseil.io/docs/cpp/guides/flags |
| 41 | +template <typename T> |
| 42 | +class Flag { |
| 43 | + public: |
| 44 | + // No constructor and destructor to ensure this is an aggregate type. |
| 45 | + // Visual Studio 2015 still requires the constructor for class to be |
| 46 | + // constexpr initializable. |
| 47 | +#if _MSC_VER <= 1900 |
| 48 | + constexpr Flag(const char* name, const char* filename, |
| 49 | + const flags_internal::HelpGenFunc help_gen, |
| 50 | + const flags_internal::FlagDfltGenFunc default_value_gen) |
| 51 | + : name_(name), |
| 52 | + filename_(filename), |
| 53 | + help_gen_(help_gen), |
| 54 | + default_value_gen_(default_value_gen), |
| 55 | + inited_(false), |
| 56 | + impl_(nullptr) {} |
| 57 | +#endif |
| 58 | + |
| 59 | + flags_internal::Flag<T>& GetImpl() const { |
| 60 | + if (!inited_.load(std::memory_order_acquire)) { |
| 61 | + absl::MutexLock l(flags_internal::GetGlobalConstructionGuard()); |
| 62 | + |
| 63 | + if (inited_.load(std::memory_order_acquire)) { |
| 64 | + return *impl_; |
| 65 | + } |
| 66 | + |
| 67 | + impl_ = new flags_internal::Flag<T>( |
| 68 | + name_, filename_, |
| 69 | + {flags_internal::FlagHelpMsg(help_gen_), |
| 70 | + flags_internal::FlagHelpKind::kGenFunc}, |
| 71 | + {flags_internal::FlagDefaultSrc(default_value_gen_), |
| 72 | + flags_internal::FlagDefaultKind::kGenFunc}); |
| 73 | + inited_.store(true, std::memory_order_release); |
| 74 | + } |
| 75 | + |
| 76 | + return *impl_; |
| 77 | + } |
| 78 | + |
| 79 | + // Public methods of `absl::Flag<T>` are NOT part of the Abseil Flags API. |
| 80 | + // See https://abseil.io/docs/cpp/guides/flags |
| 81 | + bool IsRetired() const { return GetImpl().IsRetired(); } |
| 82 | + absl::string_view Name() const { return GetImpl().Name(); } |
| 83 | + std::string Help() const { return GetImpl().Help(); } |
| 84 | + bool IsModified() const { return GetImpl().IsModified(); } |
| 85 | + bool IsSpecifiedOnCommandLine() const { |
| 86 | + return GetImpl().IsSpecifiedOnCommandLine(); |
| 87 | + } |
| 88 | + std::string Filename() const { return GetImpl().Filename(); } |
| 89 | + std::string DefaultValue() const { return GetImpl().DefaultValue(); } |
| 90 | + std::string CurrentValue() const { return GetImpl().CurrentValue(); } |
| 91 | + template <typename U> |
| 92 | + inline bool IsOfType() const { |
| 93 | + return GetImpl().template IsOfType<U>(); |
| 94 | + } |
| 95 | + T Get() const { |
| 96 | + return flags_internal::FlagImplPeer::InvokeGet<T>(GetImpl()); |
| 97 | + } |
| 98 | + void Set(const T& v) { |
| 99 | + flags_internal::FlagImplPeer::InvokeSet(GetImpl(), v); |
| 100 | + } |
| 101 | + void InvokeCallback() { GetImpl().InvokeCallback(); } |
| 102 | + |
| 103 | + const CommandLineFlag& Reflect() const { |
| 104 | + return flags_internal::FlagImplPeer::InvokeReflect(GetImpl()); |
| 105 | + } |
| 106 | + |
| 107 | + // The data members are logically private, but they need to be public for |
| 108 | + // this to be an aggregate type. |
| 109 | + const char* name_; |
| 110 | + const char* filename_; |
| 111 | + const flags_internal::HelpGenFunc help_gen_; |
| 112 | + const flags_internal::FlagDfltGenFunc default_value_gen_; |
| 113 | + |
| 114 | + mutable std::atomic<bool> inited_; |
| 115 | + mutable flags_internal::Flag<T>* impl_; |
| 116 | +}; |
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