tvl-depot/absl/flags/flag.h
Abseil Team 1de0166368 Export of internal Abseil changes
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330051e00cd57ee516b4eaf656965656ffbcd0bc by Abseil Team <absl-team@google.com>:

Fix indentation in comment.

PiperOrigin-RevId: 287997504

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35fb1a893e708031ba4fc0db460875eb0d31820e by Abseil Team <absl-team@google.com>:

Enable compile-time enforcement that absl::Substitute patterns to not contain unescaped $ symbols.

absl::Substitute already considers unescaped $ symbols undefined behavior and crashes when it's passed them in debug builds.  Some code isn't ever built in debug mode, though, and inadvertently used some unescaped $ symbols, which led to surprising results.  This change will prevent that problem from happening in the future.
PiperOrigin-RevId: 287906643

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c5762833ebde6d7110bf68041a823b571c238e9e by Gennadiy Rozental <rogeeff@google.com>:

Move all the flag data into a single place instead of being split between handle and flag object.

After this change CommandLineFlag will not hold any data anymore. And we also do not need to pass the CommandLineFlag around in Abseil Flag implementation to report flag name and location.

PiperOrigin-RevId: 287899076

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8b5fb644f1e3d9267b7a75106fe9a72c886db786 by Derek Mauro <dmauro@google.com>:

Upgrade CI testing to Bazel 2.0.0 and Clang 407ac2eb5f13

-fno-sanitize-blacklist is to workaround
https://github.com/bazelbuild/bazel/issues/10510

PiperOrigin-RevId: 287875363

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a20cc1d58895de2babc3748a6c79d1d6813734ef by Abseil Team <absl-team@google.com>:

Make ABSL_RETIRED_FLAG behave consistently with ABSL_FLAG.

Before the change:
ABSL_RETIRED_FLAG does not compile when there are competing ctors in the type, even when ABSL_FLAG does.

After the change:
ABSL_RETIRED_FLAG compiles when ABSL_FLAG does.

PiperOrigin-RevId: 286483183

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1cff7e67329d2be9e50bee1f2e76ef9ffd2edde5 by Abseil Team <absl-team@google.com>:

Support C++20 erase_if API in unordered associative containers

See [unord.set.erasure]: https://eel.is/c++draft/unord.set.erasure
See [unord.map.erasure]: https://eel.is/c++draft/unord.map.erasure

PiperOrigin-RevId: 286461140
GitOrigin-RevId: 330051e00cd57ee516b4eaf656965656ffbcd0bc
Change-Id: I5513110b41c2af08a44da54612cff341ac5c6607
2020-01-03 13:41:32 -05:00

392 lines
16 KiB
C++

//
// 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 {
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::FlagMarshallingOpFn marshalling_op,
const flags_internal::HelpGenFunc help_gen,
const flags_internal::FlagDfltGenFunc default_value_gen)
: name_(name),
filename_(filename),
marshalling_op_(marshalling_op),
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_, marshalling_op_,
{flags_internal::FlagHelpSrc(help_gen_),
flags_internal::FlagHelpSrcKind::kGenFunc},
default_value_gen_);
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(); }
bool IsAbseilFlag() const { return GetImpl()->IsAbseilFlag(); }
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();
}
absl::string_view Typename() const { return GetImpl()->Typename(); }
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(); }
bool AtomicGet(T* v) const { return GetImpl()->AtomicGet(v); }
void Set(const T& v) { GetImpl()->Set(v); }
void SetCallback(const flags_internal::FlagCallback mutation_callback) {
GetImpl()->SetCallback(mutation_callback);
}
void InvokeCallback() { GetImpl()->InvokeCallback(); }
// 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::FlagMarshallingOpFn marshalling_op_;
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) {
#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
return flag.Get();
}
// Overload for `GetFlag()` for types that support lock-free reads.
#define ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT(T) \
ABSL_MUST_USE_RESULT 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->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);
}
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 ABSL_FLAGS_STRIP_NAMES
#define ABSL_FLAG_IMPL_FLAGNAME(txt) ""
#define ABSL_FLAG_IMPL_FILENAME() ""
#if !defined(_MSC_VER) || defined(__clang__)
#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
absl::flags_internal::FlagRegistrar<T, false>(&flag)
#else
#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
absl::flags_internal::FlagRegistrar<T, false>(flag.GetImpl())
#endif
#else
#define ABSL_FLAG_IMPL_FLAGNAME(txt) txt
#define ABSL_FLAG_IMPL_FILENAME() __FILE__
#if !defined(_MSC_VER) || defined(__clang__)
#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
absl::flags_internal::FlagRegistrar<T, true>(&flag)
#else
#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
absl::flags_internal::FlagRegistrar<T, true>(flag.GetImpl())
#endif
#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.
#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt) \
struct AbslFlagHelpGenFor##name { \
template <typename T = void> \
static constexpr const char* Const() { \
return absl::flags_internal::HelpConstexprWrap( \
ABSL_FLAG_IMPL_FLAGHELP(txt)); \
} \
static std::string NonConst() { 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.
#if !defined(_MSC_VER) || defined(__clang__)
#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::flags_internal::FlagMarshallingOps<Type>, \
absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>(0), \
&AbslFlagsInitFlag##name}; \
extern bool FLAGS_no##name; \
bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
#else
// MSVC version uses aggregate initialization. We also do not try to
// optimize away help wrapper.
#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::flags_internal::FlagMarshallingOps<Type>, \
&AbslFlagHelpGenFor##name::NonConst, &AbslFlagsInitFlag##name}; \
extern bool FLAGS_no##name; \
bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
#endif
// 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_