281 lines
11 KiB
C++
281 lines
11 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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//
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// -----------------------------------------------------------------------------
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// File: flag.h
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// -----------------------------------------------------------------------------
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//
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// This header file defines the `absl::Flag<T>` type for holding command-line
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// flag data, and abstractions to create, get and set such flag data.
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//
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// It is important to note that this type is **unspecified** (an implementation
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// detail) and you do not construct or manipulate actual `absl::Flag<T>`
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// instances. Instead, you define and declare flags using the
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// `ABSL_FLAG()` and `ABSL_DECLARE_FLAG()` macros, and get and set flag values
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// using the `absl::GetFlag()` and `absl::SetFlag()` functions.
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#ifndef ABSL_FLAGS_FLAG_H_
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#define ABSL_FLAGS_FLAG_H_
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#include "absl/base/attributes.h"
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#include "absl/base/casts.h"
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#include "absl/flags/config.h"
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#include "absl/flags/declare.h"
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#include "absl/flags/internal/commandlineflag.h"
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#include "absl/flags/internal/flag.h"
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#include "absl/flags/marshalling.h"
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namespace absl {
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// Flag
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//
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// An `absl::Flag` holds a command-line flag value, providing a runtime
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// parameter to a binary. Such flags should be defined in the global namespace
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// and (preferably) in the module containing the binary's `main()` function.
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//
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// You should not construct and cannot use the `absl::Flag` type directly;
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// instead, you should declare flags using the `ABSL_DECLARE_FLAG()` macro
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// within a header file, and define your flag using `ABSL_FLAG()` within your
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// header's associated `.cc` file. Such flags will be named `FLAGS_name`.
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//
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// Example:
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//
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// .h file
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//
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// // Declares usage of a flag named "FLAGS_count"
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// ABSL_DECLARE_FLAG(int, count);
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//
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// .cc file
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//
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// // Defines a flag named "FLAGS_count" with a default `int` value of 0.
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// ABSL_FLAG(int, count, 0, "Count of items to process");
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//
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// No public methods of `absl::Flag<T>` are part of the Abseil Flags API.
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template <typename T>
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using Flag = flags_internal::Flag<T>;
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// GetFlag()
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//
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// Returns the value (of type `T`) of an `absl::Flag<T>` instance, by value. Do
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// not construct an `absl::Flag<T>` directly and call `absl::GetFlag()`;
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// instead, refer to flag's constructed variable name (e.g. `FLAGS_name`).
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// Because this function returns by value and not by reference, it is
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// thread-safe, but note that the operation may be expensive; as a result, avoid
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// `absl::GetFlag()` within any tight loops.
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//
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// Example:
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//
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// // FLAGS_count is a Flag of type `int`
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// int my_count = absl::GetFlag(FLAGS_count);
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//
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// // FLAGS_firstname is a Flag of type `std::string`
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// std::string first_name = absl::GetFlag(FLAGS_firstname);
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template <typename T>
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T GetFlag(const absl::Flag<T>& flag) {
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#define ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE(BIT) \
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static_assert( \
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!std::is_same<T, BIT>::value, \
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"Do not specify explicit template parameters to absl::GetFlag");
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ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE)
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#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE
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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 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 scope.
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// 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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flag.internal.Read(&u.value, &flags_internal::FlagOps<T>);
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return std::move(u.value);
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}
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// Overload for `GetFlag()` for types that support lock-free reads.
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#define ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT(T) \
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extern T GetFlag(const absl::Flag<T>& flag);
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ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT)
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#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT
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// SetFlag()
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//
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// Sets the value of an `absl::Flag` to the value `v`. Do not construct an
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// `absl::Flag<T>` directly and call `absl::SetFlag()`; instead, use the
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// flag's variable name (e.g. `FLAGS_name`). This function is
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// thread-safe, but is potentially expensive. Avoid setting flags in general,
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// but especially within performance-critical code.
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template <typename T>
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void SetFlag(absl::Flag<T>* flag, const T& v) {
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flag->internal.Write(&v, &flags_internal::FlagOps<T>);
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}
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// Overload of `SetFlag()` to allow callers to pass in a value that is
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// convertible to `T`. E.g., use this overload to pass a "const char*" when `T`
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// is `std::string`.
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template <typename T, typename V>
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void SetFlag(absl::Flag<T>* flag, const V& v) {
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T value(v);
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SetFlag<T>(flag, value);
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}
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} // namespace absl
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// ABSL_FLAG()
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//
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// This macro defines an `absl::Flag<T>` instance of a specified type `T`:
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//
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// ABSL_FLAG(T, name, default_value, help);
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//
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// where:
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//
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// * `T` is a supported flag type (see `marshalling.h`),
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// * `name` designates the name of the flag (as a global variable
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// `FLAGS_name`),
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// * `default_value` is an expression holding the default value for this flag
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// (which must be implicitly convertible to `T`),
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// * `help` is the help text, which can also be an expression.
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//
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// This macro expands to a flag named 'FLAGS_name' of type 'T':
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//
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// absl::Flag<T> FLAGS_name = ...;
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//
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// Note that all such instances are created as global variables.
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//
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// For `ABSL_FLAG()` values that you wish to expose to other translation units,
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// it is recommended to define those flags within the `.cc` file associated with
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// the header where the flag is declared.
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//
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// Note: do not construct objects of type `absl::Flag<T>` directly. Only use the
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// `ABSL_FLAG()` macro for such construction.
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#define ABSL_FLAG(Type, name, default_value, help) \
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ABSL_FLAG_IMPL(Type, name, default_value, help)
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// ABSL_FLAG().OnUpdate()
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//
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// Defines a flag of type `T` with a callback attached:
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//
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// ABSL_FLAG(T, name, default_value, help).OnUpdate(callback);
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//
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// After any setting of the flag value, the callback will be called at least
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// once. A rapid sequence of changes may be merged together into the same
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// callback. No concurrent calls to the callback will be made for the same
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// flag. Callbacks are allowed to read the current value of the flag but must
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// not mutate that flag.
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//
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// The update mechanism guarantees "eventual consistency"; if the callback
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// derives an auxiliary data structure from the flag value, it is guaranteed
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// that eventually the flag value and the derived data structure will be
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// consistent.
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//
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// Note: ABSL_FLAG.OnUpdate() does not have a public definition. Hence, this
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// comment serves as its API documentation.
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// -----------------------------------------------------------------------------
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// Implementation details below this section
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// -----------------------------------------------------------------------------
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// ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_NAMES
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#if ABSL_FLAGS_STRIP_NAMES
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#define ABSL_FLAG_IMPL_FLAGNAME(txt) ""
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#define ABSL_FLAG_IMPL_FILENAME() ""
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#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
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absl::flags_internal::FlagRegistrar<T, false>(&flag)
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#else
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#define ABSL_FLAG_IMPL_FLAGNAME(txt) txt
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#define ABSL_FLAG_IMPL_FILENAME() __FILE__
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#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
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absl::flags_internal::FlagRegistrar<T, true>(&flag)
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#endif
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// ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_HELP
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#if ABSL_FLAGS_STRIP_HELP
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#define ABSL_FLAG_IMPL_FLAGHELP(txt) absl::flags_internal::kStrippedFlagHelp
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#else
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#define ABSL_FLAG_IMPL_FLAGHELP(txt) txt
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#endif
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#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt) \
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static std::string AbslFlagsWrapHelp##name() { \
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return ABSL_FLAG_IMPL_FLAGHELP(txt); \
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}
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#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
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static void* AbslFlagsInitFlag##name() { \
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return absl::flags_internal::MakeFromDefaultValue<Type>(default_value); \
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}
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// ABSL_FLAG_IMPL
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//
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// Note: Name of registrar object is not arbitrary. It is used to "grab"
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// global name for FLAGS_no<flag_name> symbol, thus preventing the possibility
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// of defining two flags with names foo and nofoo.
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#define ABSL_FLAG_IMPL(Type, name, default_value, help) \
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namespace absl {} \
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ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
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ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help) \
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absl::Flag<Type> FLAGS_##name( \
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ABSL_FLAG_IMPL_FLAGNAME(#name), \
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&AbslFlagsWrapHelp##name, \
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ABSL_FLAG_IMPL_FILENAME(), \
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&absl::flags_internal::FlagMarshallingOps<Type>, \
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&AbslFlagsInitFlag##name); \
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extern bool FLAGS_no##name; \
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bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
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// ABSL_RETIRED_FLAG
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//
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// Designates the flag (which is usually pre-existing) as "retired." A retired
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// flag is a flag that is now unused by the program, but may still be passed on
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// the command line, usually by production scripts. A retired flag is ignored
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// and code can't access it at runtime.
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//
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// This macro registers a retired flag with given name and type, with a name
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// identical to the name of the original flag you are retiring. The retired
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// flag's type can change over time, so that you can retire code to support a
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// custom flag type.
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//
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// This macro has the same signature as `ABSL_FLAG`. To retire a flag, simply
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// replace an `ABSL_FLAG` definition with `ABSL_RETIRED_FLAG`, leaving the
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// arguments unchanged (unless of course you actually want to retire the flag
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// type at this time as well).
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//
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// `default_value` is only used as a double check on the type. `explanation` is
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// unused.
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// TODO(rogeeff): Return an anonymous struct instead of bool, and place it into
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// the unnamed namespace.
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#define ABSL_RETIRED_FLAG(type, flagname, default_value, explanation) \
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ABSL_ATTRIBUTE_UNUSED static const bool ignored_##flagname = \
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([] { return type(default_value); }, \
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absl::flags_internal::RetiredFlag<type>(#flagname))
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#endif // ABSL_FLAGS_FLAG_H_
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