THUAI6/CAPI/cpp/grpc/include/absl/flags/commandlineflag.h

//
// 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
//
//      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: 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.
#ifndef ABSL_FLAGS_COMMANDLINEFLAG_H_
#define ABSL_FLAGS_COMMANDLINEFLAG_H_

#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
{
    ABSL_NAMESPACE_BEGIN
    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
    {
    public:
        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;

            Read(&u.value);
            // 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);

    protected:
        ~CommandLineFlag() = default;

    private:
        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 usign 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;
    };

    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_FLAGS_COMMANDLINEFLAG_H_