shared_readable_ptr.hpp

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#pragma once
 
/// @file userver/utils/shared_readable_ptr.hpp
/// @brief @copybrief utils::SharedReadablePtr
 
#include <memory>
#include <type_traits>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils {
 
/// @ingroup userver_universal userver_containers
///
/// @brief `std::shared_ptr<const T>` wrapper that makes sure that the pointer
/// is stored before dereferencing. Protects from dangling references:
/// @code
///   // BAD! Result of `cache.Get()` may be destroyed after the invocation.
///   const auto& snapshot = *cache.Get();
/// @endcode
template <typename T>
class SharedReadablePtr final {
  static_assert(!std::is_const_v<T>,
                "SharedReadablePtr already adds `const` to `T`");
  static_assert(!std::is_reference_v<T>,
                "SharedReadablePtr does not work with references");
 
 public:
  using Base = std::shared_ptr<const T>;
  using MutableBase = std::shared_ptr<T>;
  using Weak = typename Base::weak_type;
  using Unique = std::unique_ptr<const T>;
  using element_type = T;
 
  SharedReadablePtr(const SharedReadablePtr& ptr) = default;
  SharedReadablePtr(SharedReadablePtr&& ptr) noexcept = default;
 
  constexpr SharedReadablePtr(std::nullptr_t) noexcept : base_(nullptr) {}
 
  SharedReadablePtr& operator=(const SharedReadablePtr& ptr) = default;
  SharedReadablePtr& operator=(SharedReadablePtr&& ptr) noexcept = default;
 
  SharedReadablePtr(const Base& ptr) noexcept : base_(ptr) {}
 
  SharedReadablePtr(Base&& ptr) noexcept : base_(std::move(ptr)) {}
 
  SharedReadablePtr(const MutableBase& ptr) noexcept : base_(ptr) {}
 
  SharedReadablePtr(MutableBase&& ptr) noexcept : base_(std::move(ptr)) {}
 
  SharedReadablePtr(Unique&& ptr) noexcept : base_(std::move(ptr)) {}
 
  SharedReadablePtr& operator=(const Base& ptr) {
    base_ = ptr;
    return *this;
  }
 
  SharedReadablePtr& operator=(Base&& ptr) noexcept {
    base_ = std::move(ptr);
    return *this;
  }
 
  SharedReadablePtr& operator=(const MutableBase& ptr) noexcept {
    base_ = ptr;
    return *this;
  }
 
  SharedReadablePtr& operator=(MutableBase&& ptr) noexcept {
    base_ = std::move(ptr);
    return *this;
  }
 
  SharedReadablePtr& operator=(std::nullptr_t) noexcept {
    Reset();
    return *this;
  }
 
  const T* Get() const& noexcept { return base_.get(); }
 
  const T& operator*() const& noexcept { return *base_; }
 
  const T& operator*() && { ReportMisuse(); }
 
  const T* operator->() const& noexcept { return base_.get(); }
 
  const T* operator->() && { ReportMisuse(); }
 
  operator const Base&() const& noexcept { return base_; }
 
  operator const Base&() && { ReportMisuse(); }
 
  operator Weak() const& noexcept { return base_; }
 
  operator Weak() && { ReportMisuse(); }
 
  explicit operator bool() const noexcept { return !!base_; }
 
  bool operator==(const SharedReadablePtr<T>& other) const {
    return base_ == other.base_;
  }
 
  bool operator!=(const SharedReadablePtr<T>& other) const {
    return !(*this == other);
  }
 
  void Reset() noexcept { base_.reset(); }
 
 private:
  [[noreturn]] static void ReportMisuse() {
    static_assert(!sizeof(T), "keep the pointer before using, please");
  }
 
  Base base_;
};
 
template <typename T>
bool operator==(const SharedReadablePtr<T>& ptr, std::nullptr_t) {
  return !ptr;
}
 
template <typename T>
bool operator==(std::nullptr_t, const SharedReadablePtr<T>& ptr) {
  return !ptr;
}
 
template <typename T>
bool operator!=(const SharedReadablePtr<T>& ptr, std::nullptr_t) {
  return !(ptr == nullptr);
}
 
template <typename T>
bool operator!=(std::nullptr_t, const SharedReadablePtr<T>& ptr) {
  return !(nullptr == ptr);
}
 
template <typename T, typename... Args>
SharedReadablePtr<T> MakeSharedReadable(Args&&... args) {
  return SharedReadablePtr<T>{std::make_shared<T>(std::forward<Args>(args)...)};
}
 
}  // namespace utils
 
USERVER_NAMESPACE_END