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Diffstat (limited to 'third_party/abseil_cpp/absl/flags/flag.h')
| -rw-r--r-- | third_party/abseil_cpp/absl/flags/flag.h | 389 |
1 files changed, 389 insertions, 0 deletions
diff --git a/third_party/abseil_cpp/absl/flags/flag.h b/third_party/abseil_cpp/absl/flags/flag.h new file mode 100644 index 000000000000..90dc2894df54 --- /dev/null +++ b/third_party/abseil_cpp/absl/flags/flag.h @@ -0,0 +1,389 @@ +// +// 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. +// +// ----------------------------------------------------------------------------- +// File: flag.h +// ----------------------------------------------------------------------------- +// +// This header file defines the `absl::Flag<T>` type for holding command-line +// flag data, and abstractions to create, get and set such flag data. +// +// It is important to note that this type is **unspecified** (an implementation +// detail) and you do not construct or manipulate actual `absl::Flag<T>` +// instances. Instead, you define and declare flags using the +// `ABSL_FLAG()` and `ABSL_DECLARE_FLAG()` macros, and get and set flag values +// using the `absl::GetFlag()` and `absl::SetFlag()` functions. + +#ifndef ABSL_FLAGS_FLAG_H_ +#define ABSL_FLAGS_FLAG_H_ + +#include <string> +#include <type_traits> + +#include "absl/base/attributes.h" +#include "absl/base/config.h" +#include "absl/base/optimization.h" +#include "absl/flags/config.h" +#include "absl/flags/internal/flag.h" +#include "absl/flags/internal/registry.h" +#include "absl/strings/string_view.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN + +// Flag +// +// An `absl::Flag` holds a command-line flag value, providing a runtime +// parameter to a binary. Such flags should be defined in the global namespace +// and (preferably) in the module containing the binary's `main()` function. +// +// You should not construct and cannot use the `absl::Flag` type directly; +// instead, you should declare flags using the `ABSL_DECLARE_FLAG()` macro +// within a header file, and define your flag using `ABSL_FLAG()` within your +// header's associated `.cc` file. Such flags will be named `FLAGS_name`. +// +// Example: +// +// .h file +// +// // Declares usage of a flag named "FLAGS_count" +// ABSL_DECLARE_FLAG(int, count); +// +// .cc file +// +// // Defines a flag named "FLAGS_count" with a default `int` value of 0. +// ABSL_FLAG(int, count, 0, "Count of items to process"); +// +// No public methods of `absl::Flag<T>` are part of the Abseil Flags API. +#if !defined(_MSC_VER) || defined(__clang__) +template <typename T> +using Flag = flags_internal::Flag<T>; +#else +// 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 recomendation 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 + +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 GetImpl().Get(); } + void Set(const T& v) { GetImpl().Set(v); } + void InvokeCallback() { GetImpl().InvokeCallback(); } + + const CommandLineFlag& Reflect() const { return GetImpl().Reflect(); } + + // 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_; +}; +#endif + +// GetFlag() +// +// Returns the value (of type `T`) of an `absl::Flag<T>` instance, by value. Do +// not construct an `absl::Flag<T>` directly and call `absl::GetFlag()`; +// instead, refer to flag's constructed variable name (e.g. `FLAGS_name`). +// Because this function returns by value and not by reference, it is +// thread-safe, but note that the operation may be expensive; as a result, avoid +// `absl::GetFlag()` within any tight loops. +// +// Example: +// +// // FLAGS_count is a Flag of type `int` +// int my_count = absl::GetFlag(FLAGS_count); +// +// // FLAGS_firstname is a Flag of type `std::string` +// std::string first_name = absl::GetFlag(FLAGS_firstname); +template <typename T> +ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) { + return flag.Get(); +} + +// SetFlag() +// +// Sets the value of an `absl::Flag` to the value `v`. Do not construct an +// `absl::Flag<T>` directly and call `absl::SetFlag()`; instead, use the +// flag's variable name (e.g. `FLAGS_name`). This function is +// thread-safe, but is potentially expensive. Avoid setting flags in general, +// but especially within performance-critical code. +template <typename T> +void SetFlag(absl::Flag<T>* flag, const T& v) { + flag->Set(v); +} + +// Overload of `SetFlag()` to allow callers to pass in a value that is +// convertible to `T`. E.g., use this overload to pass a "const char*" when `T` +// is `std::string`. +template <typename T, typename V> +void SetFlag(absl::Flag<T>* flag, const V& v) { + T value(v); + flag->Set(value); +} + +// GetFlagReflectionHandle() +// +// Returns the reflection handle corresponding to specified Abseil Flag +// instance. Use this handle to access flag's reflection information, like name, +// location, default value etc. +// +// Example: +// +// std::string = absl::GetFlagReflectionHandle(FLAGS_count).DefaultValue(); + +template <typename T> +const CommandLineFlag& GetFlagReflectionHandle(const absl::Flag<T>& f) { + return f.Reflect(); +} + +ABSL_NAMESPACE_END +} // namespace absl + + +// ABSL_FLAG() +// +// This macro defines an `absl::Flag<T>` instance of a specified type `T`: +// +// ABSL_FLAG(T, name, default_value, help); +// +// where: +// +// * `T` is a supported flag type (see the list of types in `marshalling.h`), +// * `name` designates the name of the flag (as a global variable +// `FLAGS_name`), +// * `default_value` is an expression holding the default value for this flag +// (which must be implicitly convertible to `T`), +// * `help` is the help text, which can also be an expression. +// +// This macro expands to a flag named 'FLAGS_name' of type 'T': +// +// absl::Flag<T> FLAGS_name = ...; +// +// Note that all such instances are created as global variables. +// +// For `ABSL_FLAG()` values that you wish to expose to other translation units, +// it is recommended to define those flags within the `.cc` file associated with +// the header where the flag is declared. +// +// Note: do not construct objects of type `absl::Flag<T>` directly. Only use the +// `ABSL_FLAG()` macro for such construction. +#define ABSL_FLAG(Type, name, default_value, help) \ + ABSL_FLAG_IMPL(Type, name, default_value, help) + +// ABSL_FLAG().OnUpdate() +// +// Defines a flag of type `T` with a callback attached: +// +// ABSL_FLAG(T, name, default_value, help).OnUpdate(callback); +// +// After any setting of the flag value, the callback will be called at least +// once. A rapid sequence of changes may be merged together into the same +// callback. No concurrent calls to the callback will be made for the same +// flag. Callbacks are allowed to read the current value of the flag but must +// not mutate that flag. +// +// The update mechanism guarantees "eventual consistency"; if the callback +// derives an auxiliary data structure from the flag value, it is guaranteed +// that eventually the flag value and the derived data structure will be +// consistent. +// +// Note: ABSL_FLAG.OnUpdate() does not have a public definition. Hence, this +// comment serves as its API documentation. + + +// ----------------------------------------------------------------------------- +// Implementation details below this section +// ----------------------------------------------------------------------------- + +// ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_NAMES +#if !defined(_MSC_VER) || defined(__clang__) +#define ABSL_FLAG_IMPL_FLAG_PTR(flag) flag +#define ABSL_FLAG_IMPL_HELP_ARG(name) \ + absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>( \ + FLAGS_help_storage_##name) +#define ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name) \ + absl::flags_internal::DefaultArg<Type, AbslFlagDefaultGenFor##name>(0) +#else +#define ABSL_FLAG_IMPL_FLAG_PTR(flag) flag.GetImpl() +#define ABSL_FLAG_IMPL_HELP_ARG(name) &AbslFlagHelpGenFor##name::NonConst +#define ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name) &AbslFlagDefaultGenFor##name::Gen +#endif + +#if ABSL_FLAGS_STRIP_NAMES +#define ABSL_FLAG_IMPL_FLAGNAME(txt) "" +#define ABSL_FLAG_IMPL_FILENAME() "" +#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \ + absl::flags_internal::FlagRegistrar<T, false>(ABSL_FLAG_IMPL_FLAG_PTR(flag)) +#else +#define ABSL_FLAG_IMPL_FLAGNAME(txt) txt +#define ABSL_FLAG_IMPL_FILENAME() __FILE__ +#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \ + absl::flags_internal::FlagRegistrar<T, true>(ABSL_FLAG_IMPL_FLAG_PTR(flag)) +#endif + +// ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_HELP + +#if ABSL_FLAGS_STRIP_HELP +#define ABSL_FLAG_IMPL_FLAGHELP(txt) absl::flags_internal::kStrippedFlagHelp +#else +#define ABSL_FLAG_IMPL_FLAGHELP(txt) txt +#endif + +// AbslFlagHelpGenFor##name is used to encapsulate both immediate (method Const) +// and lazy (method NonConst) evaluation of help message expression. We choose +// between the two via the call to HelpArg in absl::Flag instantiation below. +// If help message expression is constexpr evaluable compiler will optimize +// away this whole struct. +// TODO(rogeeff): place these generated structs into local namespace and apply +// ABSL_INTERNAL_UNIQUE_SHORT_NAME. +// TODO(rogeeff): Apply __attribute__((nodebug)) to FLAGS_help_storage_##name +#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt) \ + struct AbslFlagHelpGenFor##name { \ + /* The expression is run in the caller as part of the */ \ + /* default value argument. That keeps temporaries alive */ \ + /* long enough for NonConst to work correctly. */ \ + static constexpr absl::string_view Value( \ + absl::string_view v = ABSL_FLAG_IMPL_FLAGHELP(txt)) { \ + return v; \ + } \ + static std::string NonConst() { return std::string(Value()); } \ + }; \ + constexpr auto FLAGS_help_storage_##name ABSL_INTERNAL_UNIQUE_SMALL_NAME() \ + ABSL_ATTRIBUTE_SECTION_VARIABLE(flags_help_cold) = \ + absl::flags_internal::HelpStringAsArray<AbslFlagHelpGenFor##name>( \ + 0); + +#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \ + struct AbslFlagDefaultGenFor##name { \ + Type value = absl::flags_internal::InitDefaultValue<Type>(default_value); \ + static void Gen(void* p) { \ + new (p) Type(AbslFlagDefaultGenFor##name{}.value); \ + } \ + }; + +// ABSL_FLAG_IMPL +// +// Note: Name of registrar object is not arbitrary. It is used to "grab" +// global name for FLAGS_no<flag_name> symbol, thus preventing the possibility +// of defining two flags with names foo and nofoo. +#define ABSL_FLAG_IMPL(Type, name, default_value, help) \ + namespace absl /* block flags in namespaces */ {} \ + ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \ + ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help) \ + ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{ \ + ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_FILENAME(), \ + ABSL_FLAG_IMPL_HELP_ARG(name), ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name)}; \ + extern absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name; \ + absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name = \ + ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name) + +// ABSL_RETIRED_FLAG +// +// Designates the flag (which is usually pre-existing) as "retired." A retired +// flag is a flag that is now unused by the program, but may still be passed on +// the command line, usually by production scripts. A retired flag is ignored +// and code can't access it at runtime. +// +// This macro registers a retired flag with given name and type, with a name +// identical to the name of the original flag you are retiring. The retired +// flag's type can change over time, so that you can retire code to support a +// custom flag type. +// +// This macro has the same signature as `ABSL_FLAG`. To retire a flag, simply +// replace an `ABSL_FLAG` definition with `ABSL_RETIRED_FLAG`, leaving the +// arguments unchanged (unless of course you actually want to retire the flag +// type at this time as well). +// +// `default_value` is only used as a double check on the type. `explanation` is +// unused. +// TODO(rogeeff): Return an anonymous struct instead of bool, and place it into +// the unnamed namespace. +#define ABSL_RETIRED_FLAG(type, flagname, default_value, explanation) \ + ABSL_ATTRIBUTE_UNUSED static const bool ignored_##flagname = \ + ([] { return type(default_value); }, \ + absl::flags_internal::RetiredFlag<type>(#flagname)) + +#endif // ABSL_FLAGS_FLAG_H_ |