observer_ptr.hpp

// SPDX-License-Identifier: MIT
#ifndef LIBSBX_MEMORY_OBSERVER_PTR_HPP_
#define LIBSBX_MEMORY_OBSERVER_PTR_HPP_
 
#include <memory>
#include <utility>
 
#include <libsbx/utility/assert.hpp>
 
namespace sbx::memory {
 
template<typename Type, typename Value>
concept smart_pointer = requires(Type instance) {
  typename Type::element_type;
  requires std::same_as<typename Type::element_type, Value>;
  { instance.get() } -> std::same_as<Value*>;
}; // concept smart_pointer
 
template<typename Type>
class observer_ptr {
 
public:
 
  using value_type = Type;
  using pointer = value_type*;
  using const_pointer = const value_type*;
 
  constexpr observer_ptr() noexcept = default;
 
  constexpr observer_ptr(std::nullptr_t) noexcept
  : _value{nullptr} { }
 
  constexpr observer_ptr(pointer value) noexcept
  : _value{value} { }
 
  template<smart_pointer<value_type> Pointer>
  constexpr observer_ptr(const Pointer& value) noexcept
  : _value{value.get()} { }
 
  template<typename Other>
  requires (std::is_convertible_v<Other*, pointer>)
  constexpr observer_ptr(observer_ptr<Other>& other) noexcept
  : _value{reinterpret_cast<pointer>(other.get())} { }
 
  constexpr observer_ptr(const observer_ptr&) noexcept = default;
 
  constexpr observer_ptr(observer_ptr&&) noexcept = default;
 
  constexpr ~observer_ptr() noexcept = default;
 
  constexpr auto operator=(const observer_ptr&) noexcept -> observer_ptr& = default;
 
  constexpr auto operator=(observer_ptr&&) noexcept -> observer_ptr& = default;
 
  constexpr auto operator=(std::nullptr_t) noexcept -> observer_ptr& {
    _value = nullptr;
    return *this;
  }
 
  constexpr auto operator=(pointer value) noexcept -> observer_ptr& {
    _value = value;
    return *this;
  }
 
  constexpr auto release() noexcept -> pointer {
    auto value = _value;
    _value = nullptr;
    return value;
  }
 
  constexpr auto reset(pointer value = nullptr) noexcept -> void {
    _value = value;
  }
 
  constexpr auto swap(observer_ptr& other) noexcept -> void {
    std::swap(_value, other._value);
  }
 
  constexpr auto is_valid() const noexcept -> bool {
    return _value != nullptr;
  }
 
  constexpr operator bool() const noexcept {
    return is_valid();
  }
 
  constexpr auto operator->() const noexcept -> const_pointer {
    utility::assert_that(is_valid(), "Cannot dereference a null pointer.");
    return _value;
  }
 
  constexpr auto operator->() noexcept -> pointer {
    utility::assert_that(is_valid(), "Cannot dereference a null pointer.");
    return _value;
  }
 
  constexpr auto operator*() const noexcept(noexcept(*std::declval<pointer>())) -> std::add_const_t<std::add_lvalue_reference_t<value_type>> {
    utility::assert_that(is_valid(), "Cannot dereference a null pointer.");
    return *_value;
  }
 
  constexpr auto operator*() noexcept(noexcept(*std::declval<pointer>())) -> std::add_lvalue_reference_t<value_type> {
    utility::assert_that(is_valid(), "Cannot dereference a null pointer.");
    return *_value;
  }
 
  constexpr auto get() const noexcept -> const_pointer {
    return _value;
  }
 
  constexpr auto get() noexcept -> pointer {
    return _value;
  }
 
private:
 
  pointer _value{};
 
}; // class observer_ptr
 
template<typename Type>
constexpr auto operator==(const observer_ptr<Type>& lhs, const observer_ptr<Type>& rhs) noexcept -> bool {
  return lhs.get() == rhs.get();
}
 
template<typename Type>
constexpr auto operator==(const observer_ptr<Type>& lhs, std::nullptr_t) noexcept -> bool {
  return lhs.get() == nullptr;
}
 
template<typename Type, smart_pointer<Type> Pointer>
constexpr auto operator==(const observer_ptr<Type>& lhs, const Pointer& rhs) noexcept -> bool {
  return lhs.get() == rhs.get();
}
 
 
template<typename Type>
constexpr auto make_observer(Type* value) noexcept -> observer_ptr<Type> {
  return observer_ptr<Type>{value};
}
 
template<typename Type>
constexpr auto make_observer(Type& value) noexcept -> observer_ptr<Type> {
  return observer_ptr<Type>{std::addressof(value)};
}
 
template<typename Type, smart_pointer<Type> Pointer>
constexpr auto make_observer(Pointer& value) noexcept -> observer_ptr<Type> {
  return observer_ptr<Type>{value.get()};
}
 
} // namespace sbx::memory
 
template<typename Type>
struct std::hash<sbx::memory::observer_ptr<Type>> {
  constexpr auto operator()(const sbx::memory::observer_ptr<Type>& value) const noexcept -> std::size_t {
    return std::hash<std::uintptr_t>{}(reinterpret_cast<std::uintptr_t>(value.get()));
  }
};
 
#endif // LIBSBX_MEMORY_OBSERVER_PTR_HPP_