//

// 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 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;

};

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_FLAGS_COMMANDLINEFLAG_H_