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+//
+// 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 "absl/base/attributes.h"
+#include "absl/base/casts.h"
+#include "absl/flags/config.h"
+#include "absl/flags/declare.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/flag.h"
+#include "absl/flags/marshalling.h"
+
+namespace absl {
+
+// 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.
+template <typename T>
+using Flag = flags_internal::Flag<T>;
+
+// 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>
+T GetFlag(const absl::Flag<T>& flag) {
+#define ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE(BIT) \
+ static_assert( \
+ !std::is_same<T, BIT>::value, \
+ "Do not specify explicit template parameters to absl::GetFlag");
+ ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE)
+#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE
+
+ // 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;
+
+ flag.internal.Read(&u.value, &flags_internal::FlagOps<T>);
+ return std::move(u.value);
+}
+
+// Overload for `GetFlag()` for types that support lock-free reads.
+#define ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT(T) \
+ extern T GetFlag(const absl::Flag<T>& flag);
+ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT)
+#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT
+
+// 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->internal.Write(&v, &flags_internal::FlagOps<T>);
+}
+
+// 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);
+ SetFlag<T>(flag, value);
+}
+
+} // 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 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>(&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>(&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
+
+#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt) \
+ static std::string AbslFlagsWrapHelp##name() { \
+ return ABSL_FLAG_IMPL_FLAGHELP(txt); \
+ }
+
+#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
+ static void* AbslFlagsInitFlag##name() { \
+ return absl::flags_internal::MakeFromDefaultValue<Type>(default_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 {} \
+ ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
+ ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help) \
+ absl::Flag<Type> FLAGS_##name( \
+ ABSL_FLAG_IMPL_FLAGNAME(#name), \
+ &AbslFlagsWrapHelp##name, \
+ ABSL_FLAG_IMPL_FILENAME(), \
+ &absl::flags_internal::FlagMarshallingOps<Type>, \
+ &AbslFlagsInitFlag##name); \
+ extern bool FLAGS_no##name; \
+ bool 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_
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