any_movable.hpp

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#pragma once
 
/// @file userver/utils/any_movable.hpp
/// @brief @copybrief utils::AnyMovable
 
#include <any>  // for std::bad_any_cast
#include <initializer_list>
#include <memory>
#include <type_traits>
#include <utility>
 
USERVER_NAMESPACE_BEGIN
 
/// Utilities
namespace utils {
 
/// @ingroup userver_universal userver_containers
///
/// @brief Replacement for `std::any` that is not copyable. It allows to store
/// non-copyable and even non-movable types.
///
/// Usage example:
/// @snippet universal/src/utils/any_movable_test.cpp  AnyMovable example usage
class AnyMovable final {
 public:
  /// Creates an empty `AnyMovable`
  constexpr AnyMovable() noexcept = default;
 
  /// `AnyMovable` is movable, but not copyable
  AnyMovable(AnyMovable&&) noexcept = default;
  AnyMovable& operator=(AnyMovable&&) noexcept = default;
 
  /// Copies or moves the provided object inside the `AnyMovable`. `const`,
  /// reference, arrays and function pointers are decayed.
  template <typename ValueType, typename = std::enable_if_t<!std::is_same_v<
                                    AnyMovable, std::decay_t<ValueType>>>>
  /*implicit*/ AnyMovable(ValueType&& value);
 
  /// In-place constructs an object of the specified type
  template <typename ValueType, typename... Args>
  explicit AnyMovable(std::in_place_type_t<ValueType> tag, Args&&... args);
 
  /// In-place constructs an object of the specified type
  template <typename ValueType, typename Item, typename... Args>
  explicit AnyMovable(std::in_place_type_t<ValueType> tag,
                      std::initializer_list<Item> list, Args&&... args);
 
  /// Copies or moves the provided object inside the `AnyMovable`. `const`,
  /// reference, arrays and function pointers are decayed.
  template <typename ValueType>
  AnyMovable& operator=(ValueType&& rhs);
 
  /// Check if the `AnyMovable` is empty
  bool HasValue() const noexcept;
 
  /// Destroy the old contents, making `*this` empty
  void Reset() noexcept;
 
  /// In-place constructs an object of the specified type
  template <typename ValueType, typename... Args>
  ValueType& Emplace(Args&&... args);
 
  /// In-place constructs an object of the specified type
  template <typename ValueType, typename Item, typename... Args>
  ValueType& Emplace(std::initializer_list<Item> list, Args&&... args);
 
 private:
  struct HolderBase;
 
  template <typename ValueType>
  struct Holder;
 
  struct HolderDeleter {
    void operator()(HolderBase* holder) noexcept;
  };
 
  template <typename ValueType>
  friend ValueType* AnyCast(AnyMovable*) noexcept;
 
  template <typename ValueType>
  friend const ValueType* AnyCast(const AnyMovable*) noexcept;
 
  std::unique_ptr<HolderBase, HolderDeleter> content_;
};
 
/// @brief The exception that is thrown when `AnyCast` fails
class BadAnyMovableCast final : public std::bad_any_cast {
 public:
  const char* what() const noexcept override;
};
 
/// @return nullptr if operand is nullptr or type of the data stored in operand
/// does not match ValueType
template <typename ValueType>
ValueType* AnyCast(AnyMovable* operand) noexcept;
 
/// @return nullptr if operand is nullptr or type of the data stored in operand
/// does not match ValueType
template <typename ValueType>
const ValueType* AnyCast(const AnyMovable* operand) noexcept;
 
/// @note Cast to a reference type to avoid extra copies
/// @throw BadAnyMovableCast if type of the data stored in operand
/// does not match ValueType
template <typename ValueType>
ValueType AnyCast(AnyMovable& operand);
 
/// @note Cast to a reference type to avoid extra copies
/// @throw BadAnyMovableCast if type of the data stored in operand
/// does not match ValueType
template <typename ValueType>
ValueType AnyCast(const AnyMovable& operand);
 
/// @note Cast to a reference type to avoid extra moves
/// @throw BadAnyMovableCast if type of the data stored in operand
/// does not match ValueType
template <typename ValueType>
ValueType AnyCast(AnyMovable&& operand);
 
struct AnyMovable::HolderBase {
  using DeleterType = void (*)(HolderBase&) noexcept;
 
  DeleterType deleter;
};
 
template <typename ValueType>
struct AnyMovable::Holder final : public HolderBase {
  static_assert(std::is_same_v<ValueType, std::decay_t<ValueType>>,
                "The requested type can't be stored in an AnyMovable");
 
  ValueType held;
 
  static void Deleter(HolderBase& holder) noexcept {
    delete &static_cast<Holder&>(holder);
  }
 
  template <typename... Args>
  static std::unique_ptr<HolderBase, HolderDeleter> Make(Args&&... args) {
    return std::unique_ptr<HolderBase, HolderDeleter>{
        // intended raw ctor call, sometimes casts
        // NOLINTNEXTLINE(google-readability-casting)
        new Holder{{&Deleter}, ValueType(std::forward<Args>(args)...)}};
  }
 
  static ValueType* GetIf(const AnyMovable* any) noexcept {
    return (any && any->content_ && any->content_->deleter == &Deleter)
               ? &static_cast<Holder&>(*any->content_).held
               : nullptr;
  }
};
 
template <typename ValueType, typename>
AnyMovable::AnyMovable(ValueType&& value)
    : content_(Holder<std::decay_t<ValueType>>::Make(
          std::forward<ValueType>(value))) {
  static_assert(
      !std::is_same_v<AnyMovable*, std::decay_t<ValueType>> &&
          !std::is_same_v<const AnyMovable*, std::decay_t<ValueType>>,
      "AnyMovable misuse detected: trying to wrap AnyMovable* in another "
      "AnyMovable. The pointer was probably meant to be dereferenced.");
}
 
template <typename ValueType, typename... Args>
AnyMovable::AnyMovable(std::in_place_type_t<ValueType> /*tag*/, Args&&... args)
    : content_(Holder<ValueType>::Make(std::forward<Args>(args)...)) {}
 
template <typename ValueType, typename Item, typename... Args>
AnyMovable::AnyMovable(std::in_place_type_t<ValueType> /*tag*/,
                       std::initializer_list<Item> list, Args&&... args)
    : content_(Holder<ValueType>::Make(list, std::forward<Args>(args)...)) {}
 
template <typename ValueType>
AnyMovable& AnyMovable::operator=(ValueType&& rhs) {
  *this = AnyMovable(std::forward<ValueType>(rhs));
  return *this;
}
 
template <typename ValueType, typename... Args>
ValueType& AnyMovable::Emplace(Args&&... args) {
  content_ = Holder<ValueType>::Make(std::forward<Args>(args)...);
  return static_cast<Holder<ValueType>&>(*content_).held;
}
 
template <typename ValueType, typename Item, typename... Args>
ValueType& AnyMovable::Emplace(std::initializer_list<Item> list,
                               Args&&... args) {
  content_ = Holder<ValueType>::Make(list, std::forward<Args>(args)...);
  return static_cast<Holder<ValueType>&>(*content_).held;
}
 
template <typename ValueType>
ValueType* AnyCast(AnyMovable* operand) noexcept {
  return AnyMovable::Holder<ValueType>::GetIf(operand);
}
 
template <typename ValueType>
const ValueType* AnyCast(const AnyMovable* operand) noexcept {
  return AnyMovable::Holder<ValueType>::GetIf(operand);
}
 
template <typename ValueType>
// might be requested by user
// NOLINTNEXTLINE(readability-const-return-type)
ValueType AnyCast(AnyMovable& operand) {
  using NonRef = std::remove_cv_t<std::remove_reference_t<ValueType>>;
  auto* result = AnyCast<NonRef>(&operand);
  if (!result) throw BadAnyMovableCast();
  return static_cast<ValueType>(*result);
}
 
template <typename ValueType>
// might be requested by user
// NOLINTNEXTLINE(readability-const-return-type)
ValueType AnyCast(const AnyMovable& operand) {
  using NonRef = std::remove_cv_t<std::remove_reference_t<ValueType>>;
  auto* result = AnyCast<NonRef>(&operand);
  if (!result) throw BadAnyMovableCast();
  return static_cast<ValueType>(*result);
}
 
template <typename ValueType>
ValueType AnyCast(AnyMovable&& operand) {
  using NonRef = std::remove_cv_t<std::remove_reference_t<ValueType>>;
  auto* result = AnyCast<NonRef>(&operand);
  if (!result) throw BadAnyMovableCast();
  return static_cast<ValueType>(std::move(*result));
}
 
}  // namespace utils
 
USERVER_NAMESPACE_END