深度複製和移動支援

如果型別希望具有值語義，並且它需要儲存動態分配的物件，那麼在複製操作時，型別將需要分配這些物件的新副本。它也必須為複製分配執行此操作。

這種複製稱為深層複製。它有效地採用了本來就是引用語義的東西並將其轉換為值語義：

struct Inner {int i;};

const int NUM_INNER = 5;
class Value
{
private:
  Inner *array_; //Normally has reference semantics.

public:
  Value() : array_(new Inner[NUM_INNER]){}

  ~Value() {delete[] array_;}

  Value(const Value &val) : array_(new Inner[NUM_INNER])
  {
    for(int i = 0; i < NUM_INNER; ++i)
      array_[i] = val.array_[i];
  }

  Value &operator=(const Value &val)
  {
    for(int i = 0; i < NUM_INNER; ++i)
      array_[i] = val.array_[i];
    return *this;
  }
};

Version >= C++ 11

移動語義允許像 Value 這樣的型別避免真正複製其引用的資料。如果使用者以引發移動的方式使用該值，則物件的複製可以留空其引用的資料：

struct Inner {int i;};

constexpr auto NUM_INNER = 5;
class Value
{
private:
  Inner *array_; //Normally has reference semantics.

public:
  Value() : array_(new Inner[NUM_INNER]){}

  //OK to delete even if nullptr
  ~Value() {delete[] array_;}

  Value(const Value &val) : array_(new Inner[NUM_INNER])
  {
    for(int i = 0; i < NUM_INNER; ++i)
      array_[i] = val.array_[i];
  }

  Value &operator=(const Value &val)
  {
    for(int i = 0; i < NUM_INNER; ++i)
      array_[i] = val.array_[i];
    return *this;
  }

  //Movement means no memory allocation.
  //Cannot throw exceptions.
  Value(Value &&val) noexcept : array_(val.array_)
  {
    //We've stolen the old value.
    val.array_ = nullptr;
  }

  //Cannot throw exceptions.
  Value &operator=(Value &&val) noexcept
  {
    //Clever trick. Since `val` is going to be destroyed soon anyway,
    //we swap his data with ours. His destructor will destroy our data.
    std::swap(array_, val.array_);
  }
};

實際上，如果我們想要禁止深拷貝同時仍然允許移動物件，我們甚至可以使這種型別不可複製。

struct Inner {int i;};

constexpr auto NUM_INNER = 5;
class Value
{
private:
  Inner *array_; //Normally has reference semantics.

public:
  Value() : array_(new Inner[NUM_INNER]){}

  //OK to delete even if nullptr
  ~Value() {delete[] array_;}

  Value(const Value &val) = delete;
  Value &operator=(const Value &val) = delete;

  //Movement means no memory allocation.
  //Cannot throw exceptions.
  Value(Value &&val) noexcept : array_(val.array_)
  {
    //We've stolen the old value.
    val.array_ = nullptr;
  }

  //Cannot throw exceptions.
  Value &operator=(Value &&val) noexcept
  {
    //Clever trick. Since `val` is going to be destroyed soon anyway,
    //we swap his data with ours. His destructor will destroy our data.
    std::swap(array_, val.array_);
  }
};

我們甚至可以通過使用 unique_ptr 來應用零規則：

struct Inner {int i;};

constexpr auto NUM_INNER = 5;
class Value
{
private:
  unique_ptr<Inner []>array_; //Move-only type.

public:
  Value() : array_(new Inner[NUM_INNER]){}

  //No need to explicitly delete. Or even declare.
  ~Value() = default; {delete[] array_;}

  //No need to explicitly delete. Or even declare.
  Value(const Value &val) = default;
  Value &operator=(const Value &val) = default;

  //Will perform an element-wise move.
  Value(Value &&val) noexcept = default;

  //Will perform an element-wise move.
  Value &operator=(Value &&val) noexcept = default;
};