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 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_