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