cpp-descriptive-pointers/include/pointers/value_ptr.hpp

//
//  copy_ptr.hpp
//  pointers
//
//  Created by Sam Jaffe on 12/6/16.
//

#pragma once

#include <memory>

#include "detail/compare.hpp"
#include "pointer_fwd.hpp"

namespace pointers {
  namespace detail {
    template <typename T> using clone_func = T * (T::*)() const;

    template <typename T, typename = void> class value_ptr_copy;

    template <typename T>
    struct value_ptr_copy<T, std::enable_if_t<!std::is_polymorphic<T>::value>> {
      T * Copy(T * ptr) const { return ptr ? new T(*ptr) : nullptr; }
    };

#define POLYMORPHIC_VALUE_PTR_GROUP_FROM_CLONE_FUNCTION(fclone)                \
  template <typename T>                                                        \
  struct value_ptr_copy<                                                       \
      T, std::enable_if_t<                                                     \
             std::is_polymorphic<T>::value &&                                  \
             std::is_same<clone_func<T>, decltype(&T::fclone)>::value>> {      \
    T * Copy(T * ptr) const { return ptr ? ptr->fclone() : nullptr; }          \
  }

    POLYMORPHIC_VALUE_PTR_GROUP_FROM_CLONE_FUNCTION(Clone);
    POLYMORPHIC_VALUE_PTR_GROUP_FROM_CLONE_FUNCTION(clone);
    POLYMORPHIC_VALUE_PTR_GROUP_FROM_CLONE_FUNCTION(Copy);
    POLYMORPHIC_VALUE_PTR_GROUP_FROM_CLONE_FUNCTION(copy);
  }

  template <typename T>
  class value_ptr : private detail::value_ptr_copy<T>,
                    public detail::pointer_compare<value_ptr<T>> {
  public:
    using element_type = T;
    using pointer = element_type *;
    using reference = element_type &;

    value_ptr() noexcept : _ptr(nullptr) {}
    value_ptr(T * const & p) = delete;
    value_ptr(T *&& p) noexcept(std::is_nothrow_move_constructible<T>::value)
        : _ptr(std::move(p)) {}
    value_ptr(value_ptr const & other)
        : _ptr(detail::value_ptr_copy<T>::Copy(other._ptr)) {}
    value_ptr(value_ptr && other) noexcept(noexcept(std::swap(_ptr,
                                                              other._ptr)))
        : _ptr(nullptr) {
      std::swap(_ptr, other._ptr);
    }
    static value_ptr copy_of(T * const & p) {
      return detail::value_ptr_copy<T>().Copy(p);
    }

    template <typename Y> explicit operator value_ptr<Y>() const {
      return Copy(_ptr);
    }

    ~value_ptr() { delete _ptr; }

    value_ptr & operator=(value_ptr const & other) {
      swap(_ptr, value_ptr{other}._ptr);
      return *this;
    }
    value_ptr & operator=(value_ptr && other) noexcept {
      swap(_ptr, other._ptr);
      return *this;
    }

    operator bool() const noexcept { return static_cast<bool>(_ptr); }

    pointer get() const noexcept { return _ptr; }
    pointer operator->() const noexcept { return get(); }
    reference operator*() const noexcept { return *get(); }

  private:
    static void swap(T * a, T * b) {
      T * tmp = a;
      a = b;
      b = tmp;
    }
    T * _ptr;
  };

  template <typename T, typename... Args>
  value_ptr<T> make_value(Args &&... args) {
    return value_ptr<T>(new T(std::forward<Args>(args)...));
  }

}