389 lines
16 KiB
C++
389 lines
16 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 <string>
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#include <type_traits>
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#include "absl/base/attributes.h"
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#include "absl/base/config.h"
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#include "absl/base/optimization.h"
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#include "absl/flags/config.h"
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#include "absl/flags/internal/flag.h"
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#include "absl/flags/internal/registry.h"
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#include "absl/strings/string_view.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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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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#if !defined(_MSC_VER) || defined(__clang__)
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template <typename T>
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using Flag = flags_internal::Flag<T>;
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#else
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// MSVC debug builds do not implement initialization with constexpr constructors
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// correctly. To work around this we add a level of indirection, so that the
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// class `absl::Flag` contains an `internal::Flag*` (instead of being an alias
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// to that class) and dynamically allocates an instance when necessary. We also
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// forward all calls to internal::Flag methods via trampoline methods. In this
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// setup the `absl::Flag` class does not have constructor and virtual methods,
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// all the data members are public and thus MSVC is able to initialize it at
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// link time. To deal with multiple threads accessing the flag for the first
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// time concurrently we use an atomic boolean indicating if flag object is
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// initialized. We also employ the double-checked locking pattern where the
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// second level of protection is a global Mutex, so if two threads attempt to
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// construct the flag concurrently only one wins.
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// This solution is based on a recomendation here:
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// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html?childToView=648454#comment-648454
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namespace flags_internal {
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absl::Mutex* GetGlobalConstructionGuard();
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} // namespace flags_internal
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template <typename T>
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class Flag {
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public:
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// No constructor and destructor to ensure this is an aggregate type.
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// Visual Studio 2015 still requires the constructor for class to be
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// constexpr initializable.
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#if _MSC_VER <= 1900
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constexpr Flag(const char* name, const char* filename,
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const flags_internal::HelpGenFunc help_gen,
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const flags_internal::FlagDfltGenFunc default_value_gen)
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: name_(name),
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filename_(filename),
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help_gen_(help_gen),
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default_value_gen_(default_value_gen),
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inited_(false),
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impl_(nullptr) {}
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#endif
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flags_internal::Flag<T>& GetImpl() const {
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if (!inited_.load(std::memory_order_acquire)) {
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absl::MutexLock l(flags_internal::GetGlobalConstructionGuard());
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if (inited_.load(std::memory_order_acquire)) {
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return *impl_;
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}
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impl_ = new flags_internal::Flag<T>(
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name_, filename_,
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{flags_internal::FlagHelpMsg(help_gen_),
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flags_internal::FlagHelpKind::kGenFunc},
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{flags_internal::FlagDefaultSrc(default_value_gen_),
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flags_internal::FlagDefaultKind::kGenFunc});
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inited_.store(true, std::memory_order_release);
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}
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return *impl_;
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}
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// Public methods of `absl::Flag<T>` are NOT part of the Abseil Flags API.
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// See https://abseil.io/docs/cpp/guides/flags
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bool IsRetired() const { return GetImpl().IsRetired(); }
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absl::string_view Name() const { return GetImpl().Name(); }
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std::string Help() const { return GetImpl().Help(); }
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bool IsModified() const { return GetImpl().IsModified(); }
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bool IsSpecifiedOnCommandLine() const {
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return GetImpl().IsSpecifiedOnCommandLine();
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}
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std::string Filename() const { return GetImpl().Filename(); }
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std::string DefaultValue() const { return GetImpl().DefaultValue(); }
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std::string CurrentValue() const { return GetImpl().CurrentValue(); }
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template <typename U>
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inline bool IsOfType() const {
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return GetImpl().template IsOfType<U>();
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}
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T Get() const { return GetImpl().Get(); }
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void Set(const T& v) { GetImpl().Set(v); }
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void InvokeCallback() { GetImpl().InvokeCallback(); }
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const CommandLineFlag& Reflect() const { return GetImpl().Reflect(); }
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// The data members are logically private, but they need to be public for
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// this to be an aggregate type.
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const char* name_;
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const char* filename_;
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const flags_internal::HelpGenFunc help_gen_;
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const flags_internal::FlagDfltGenFunc default_value_gen_;
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mutable std::atomic<bool> inited_;
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mutable flags_internal::Flag<T>* impl_;
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};
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#endif
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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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ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
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return flag.Get();
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}
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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->Set(v);
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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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flag->Set(value);
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}
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// GetFlagReflectionHandle()
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//
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// Returns the reflection handle corresponding to specified Abseil Flag
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// instance. Use this handle to access flag's reflection information, like name,
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// location, default value etc.
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//
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// Example:
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//
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// std::string = absl::GetFlagReflectionHandle(FLAGS_count).DefaultValue();
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template <typename T>
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const CommandLineFlag& GetFlagReflectionHandle(const absl::Flag<T>& f) {
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return f.Reflect();
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}
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ABSL_NAMESPACE_END
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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 the list of types in `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 !defined(_MSC_VER) || defined(__clang__)
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#define ABSL_FLAG_IMPL_FLAG_PTR(flag) flag
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#define ABSL_FLAG_IMPL_HELP_ARG(name) \
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absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>( \
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FLAGS_help_storage_##name)
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#define ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name) \
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absl::flags_internal::DefaultArg<Type, AbslFlagDefaultGenFor##name>(0)
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#else
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#define ABSL_FLAG_IMPL_FLAG_PTR(flag) flag.GetImpl()
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#define ABSL_FLAG_IMPL_HELP_ARG(name) &AbslFlagHelpGenFor##name::NonConst
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#define ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name) &AbslFlagDefaultGenFor##name::Gen
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#endif
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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>(ABSL_FLAG_IMPL_FLAG_PTR(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>(ABSL_FLAG_IMPL_FLAG_PTR(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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// AbslFlagHelpGenFor##name is used to encapsulate both immediate (method Const)
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// and lazy (method NonConst) evaluation of help message expression. We choose
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// between the two via the call to HelpArg in absl::Flag instantiation below.
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// If help message expression is constexpr evaluable compiler will optimize
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// away this whole struct.
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// TODO(rogeeff): place these generated structs into local namespace and apply
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// ABSL_INTERNAL_UNIQUE_SHORT_NAME.
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// TODO(rogeeff): Apply __attribute__((nodebug)) to FLAGS_help_storage_##name
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#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt) \
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struct AbslFlagHelpGenFor##name { \
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/* The expression is run in the caller as part of the */ \
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/* default value argument. That keeps temporaries alive */ \
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/* long enough for NonConst to work correctly. */ \
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static constexpr absl::string_view Value( \
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absl::string_view v = ABSL_FLAG_IMPL_FLAGHELP(txt)) { \
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return v; \
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} \
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static std::string NonConst() { return std::string(Value()); } \
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}; \
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constexpr auto FLAGS_help_storage_##name ABSL_INTERNAL_UNIQUE_SMALL_NAME() \
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ABSL_ATTRIBUTE_SECTION_VARIABLE(flags_help_cold) = \
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absl::flags_internal::HelpStringAsArray<AbslFlagHelpGenFor##name>( \
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0);
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#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
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struct AbslFlagDefaultGenFor##name { \
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Type value = absl::flags_internal::InitDefaultValue<Type>(default_value); \
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static void Gen(void* p) { \
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new (p) Type(AbslFlagDefaultGenFor##name{}.value); \
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} \
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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 /* block flags in namespaces */ {} \
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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_CONST_INIT absl::Flag<Type> FLAGS_##name{ \
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ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_FILENAME(), \
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ABSL_FLAG_IMPL_HELP_ARG(name), ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name)}; \
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extern absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name; \
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absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name = \
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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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