Re: How to conditionally disable a copy constructor?

From:
=?UTF-8?B?RGFuaWVsIEtyw7xnbGVy?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Wed, 23 Nov 2011 14:42:40 -0800 (PST)
Message-ID:
<jajpd2$gnf$1@dont-email.me>
Am 23.11.2011 17:08, schrieb Daniel Kr??gler:

Am 22.11.2011 20:44, schrieb Andrzej Krzemie??ski:

I am trying to implement a template "wrapper" for any arbitrary type,
much like boost::optional. In fact I am trying to write my own
alternative implementation of optional. I want it to have the
following property: if the wrapped type T ptovides copy constructor,
optional<T> also provides a copy constructor; similarly, optional<T>
provides a move constructor iff T provides one. With concepts (as
defined in N2914), I would express it like this:


[..]

I just realize that it probably does not. While this approach realized
that Optional<X> has the same copy/move construction as type X, it does
still not describe how to *define* the corresponding member function
*iff* T does have an available member.

You need to extend my first approach by replacing copy_move1 and
copy_move2 by class templates, that implement the copy and move
semantics of T acting on the opaque buffer as required by the semantics
of Optional. This should still give you the advantage that you don't
need to specialize Optional completely.


I couldn't resist to implement the idea of combining CRTP and policy
base classes: Note that the following code restricts to copy/move
construction. The approach for copy/move assignment is absolutely
analogous. Enjoy!

#include <cassert>
#include <type_traits>
#include <utility>
#include <new>

struct no_move_no_copy
{
  no_move_no_copy() = default;
  no_move_no_copy(const no_move_no_copy&) = delete;
  no_move_no_copy(no_move_no_copy&&) = delete;
};

template<class T, class D>
struct copy_move
{
  copy_move() = default;

  copy_move(const copy_move& rhs)
noexcept(std::is_nothrow_copy_constructible<T>::value)
  {
    static_cast<D&>(*this).copy_construct(static_cast<const D&>(rhs));
  }

  copy_move(copy_move&& rhs)
noexcept(std::is_nothrow_move_constructible<T>::value)
  {
    static_cast<D&>(*this).move_construct(std::move(static_cast<D&>(rhs)));
  }
};

template<class T, class D>
struct copy_no_move
{
  copy_no_move() = default;

  copy_no_move(const copy_no_move& rhs)
noexcept(std::is_nothrow_copy_constructible<T>::value)
  {
    static_cast<D&>(*this).copy_construct(static_cast<const D&>(rhs));
  }
};

template<class T, class D>
struct move_no_copy
{
  move_no_copy() = default;

  move_no_copy(move_no_copy&& rhs)
noexcept(std::is_nothrow_move_constructible<T>::value)
  {
    static_cast<D&>(*this).move_construct(std::move(static_cast<D&>(rhs)));
  }
};

template<class T, class D>
struct select_optional_base :
  std::conditional<
    std::is_copy_constructible<T>::value,
    std::conditional<std::is_move_constructible<T>::value,
                     copy_move<T, D>,
                     copy_no_move<T, D>>,
    std::conditional<std::is_move_constructible<T>::value,
                     move_no_copy<T, D>,
                     no_move_no_copy>
  >::type
{
};

template<class T>
class optional_data
{
  using raw_type = typename std::aligned_storage<sizeof(T)>::type;

  bool is_init_;
  raw_type raw_data_;

public:
  optional_data() noexcept : is_init_(false) {}

  optional_data(const optional_data& rhs) noexcept : is_init_(false) {}

  optional_data(optional_data&& rhs) noexcept : is_init_(false) {}

  ~optional_data()
  {
    this->destroy();
  }

  auto is_initialized() const noexcept -> bool
  {
      return this->is_init_;
  }

  auto raw_memory() noexcept -> void*
  {
      return &this->raw_data_;
  }

  auto raw_memory() const noexcept -> const void*
  {
      return &this->raw_data_;
  }

  auto data() noexcept -> T&
  {
      return *static_cast<T*>(raw_memory());
  }

  auto data() const noexcept -> const T&
  {
      return *static_cast<const T*>(raw_memory());
  }

  template<class... U>
  void construct(U&&... u)
  {
    assert(!this->is_initialized());
    ::new (this->raw_memory()) T(std::forward<U>(u)...);
    this->is_init_ = true;
  }

  void destroy()
  {
    if (this->is_init_)
    {
      data().~T();
      this->is_init_ = false;
    }
  }
};

template<class P1, class P2>
struct and_ : std::conditional<P1::value, P1, P2>::type
{
};

template<class P>
struct not_ : std::integral_constant<bool, !P::value>
{
};

template<class, class...>
struct is_self_construct : std::false_type
{
};

template<class T, class U>
struct is_self_construct<T, U> : std::is_base_of<
  typename std::remove_reference<U>::type, T

{
};

template<typename T>
class Optional : private optional_data<T>,
                 private select_optional_base<T, Optional<T>>::type
{
  using copy_type = typename select_optional_base<T, Optional<T>>::type;
  friend copy_type;

  void copy_construct(const Optional& rhs)
  {
    if (rhs.is_initialized())
    {
      this->construct(rhs.data());
    }
  }

  void move_construct(Optional&& rhs)
  {
    if (rhs.is_initialized())
    {
      this->construct(std::move(rhs.data()));
    }
  }

public:
  Optional() = default;

  template<class... U,
    class = typename std::enable_if<
      and_<
        std::is_constructible<T, U...>,
        not_<is_self_construct<Optional<T>, U...>>
      >::value
    >::type
  >
  Optional(U&&... u)
  {
    this->construct(std::forward<U>(u)...);
  }

  auto operator*() -> T&
  {
    assert(this->is_initialized());
    return this->data();
  }

  auto operator*() const -> const T&
  {
    assert(this->is_initialized());
    return this->data();
  }

  explicit operator bool() const noexcept
  {
    return this->is_initialized();
  }
};

HTH & Greetings from Bremen,

Daniel Kr??gler

--
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