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// Copyright 2020 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// -----------------------------------------------------------------------------
// File: commandlineflag.h
// -----------------------------------------------------------------------------
// This header file defines the `CommandLineFlag`, which acts as a type-erased
// handle for accessing metadata about the Abseil Flag in question.
// Because an actual Abseil flag is of an unspecified type, you should not
// manipulate or interact directly with objects of that type. Instead, use the
// CommandLineFlag type as an intermediary.
#include <memory>
#include <string>
#include "absl/base/config.h"
#include "absl/base/internal/fast_type_id.h"
#include "absl/flags/internal/commandlineflag.h"
#include "absl/strings/string_view.h"
#include "absl/types/optional.h"
namespace absl {
namespace flags_internal {
class PrivateHandleAccessor;
} // namespace flags_internal
// CommandLineFlag
// This type acts as a type-erased handle for an instance of an Abseil Flag and
// holds reflection information pertaining to that flag. Use CommandLineFlag to
// access a flag's name, location, help string etc.
// To obtain an absl::CommandLineFlag, invoke `absl::FindCommandLineFlag()`
// passing it the flag name string.
// Example:
// // Obtain reflection handle for a flag named "flagname".
// const absl::CommandLineFlag* my_flag_data =
// absl::FindCommandLineFlag("flagname");
// // Now you can get flag info from that reflection handle.
// std::string flag_location = my_flag_data->Filename();
// ...
class CommandLineFlag {
constexpr CommandLineFlag() = default;
// Not copyable/assignable.
CommandLineFlag(const CommandLineFlag&) = delete;
CommandLineFlag& operator=(const CommandLineFlag&) = delete;
// absl::CommandLineFlag::IsOfType()
// Return true iff flag has type T.
template <typename T>
inline bool IsOfType() const {
return TypeId() == base_internal::FastTypeId<T>();
// absl::CommandLineFlag::TryGet()
// Attempts to retrieve the flag value. Returns value on success,
// absl::nullopt otherwise.
template <typename T>
absl::optional<T> TryGet() const {
if (IsRetired() || !IsOfType<T>()) {
return absl::nullopt;
// 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;
// allow retired flags to be "read", so we can report invalid access.
if (IsRetired()) {
return absl::nullopt;
return std::move(u.value);
// absl::CommandLineFlag::Name()
// Returns name of this flag.
virtual absl::string_view Name() const = 0;
// absl::CommandLineFlag::Filename()
// Returns name of the file where this flag is defined.
virtual std::string Filename() const = 0;
// absl::CommandLineFlag::Help()
// Returns help message associated with this flag.
virtual std::string Help() const = 0;
// absl::CommandLineFlag::IsRetired()
// Returns true iff this object corresponds to retired flag.
virtual bool IsRetired() const;
// absl::CommandLineFlag::DefaultValue()
// Returns the default value for this flag.
virtual std::string DefaultValue() const = 0;
// absl::CommandLineFlag::CurrentValue()
// Returns the current value for this flag.
virtual std::string CurrentValue() const = 0;
// absl::CommandLineFlag::ParseFrom()
// Sets the value of the flag based on specified string `value`. If the flag
// was successfully set to new value, it returns true. Otherwise, sets `error`
// to indicate the error, leaves the flag unchanged, and returns false.
bool ParseFrom(absl::string_view value, std::string* error);
~CommandLineFlag() = default;
friend class flags_internal::PrivateHandleAccessor;
// Sets the value of the flag based on specified string `value`. If the flag
// was successfully set to new value, it returns true. Otherwise, sets `error`
// to indicate the error, leaves the flag unchanged, and returns false. There
// are three ways to set the flag's value:
// * Update the current flag value
// * Update the flag's default value
// * Update the current flag value if it was never set before
// The mode is selected based on `set_mode` parameter.
virtual bool ParseFrom(absl::string_view value,
flags_internal::FlagSettingMode set_mode,
flags_internal::ValueSource source,
std::string& error) = 0;
// Returns id of the flag's value type.
virtual flags_internal::FlagFastTypeId TypeId() const = 0;
// Interface to save flag to some persistent state. Returns current flag state
// or nullptr if flag does not support saving and restoring a state.
virtual std::unique_ptr<flags_internal::FlagStateInterface> SaveState() = 0;
// Copy-construct a new value of the flag's type in a memory referenced by
// the dst based on the current flag's value.
virtual void Read(void* dst) const = 0;
// To be deleted. Used to return true if flag's current value originated from
// command line.
virtual bool IsSpecifiedOnCommandLine() const = 0;
// Validates supplied value using validator or parseflag routine
virtual bool ValidateInputValue(absl::string_view value) const = 0;
// Checks that flags default value can be converted to string and back to the
// flag's value type.
virtual void CheckDefaultValueParsingRoundtrip() const = 0;
} // namespace absl