使用奇怪的循环模板模式 (CRTP) 在抽象基础 class 中实现赋值运算符

Question

我正在为 static/dynamic 数组编写基于 CRTP 的抽象基础 class。我打算在基础 class 中放置尽可能多的方法，以便在派生的 class 中没有代码重复。我已经让索引运算符工作，但我正在努力处理赋值 (=) 运算符。

/** Array base class. */
template <class t_derived_class, class data_type>
class ArrayBase
{
  private:
  t_derived_class& Derived;

  public:
  /** Default constructor. */
  ArrayBase() : Derived(static_cast<t_derived_class&>(*this)) {}

  /** Pure virtual desctructor. */
  virtual ~ArrayBase() {};

  /** Index operator overloads. */
  data_type& operator[](const std::size_t _index)
  {
    return *(Derived.begin() + _index);
  }

  const data_type& operator[](const std::size_t _index) const
  {
    return *(Derived.begin() + _index);
  }

  /** Assignment operator overloads. */
  t_derived_class& operator=(const t_derived_class& _other_derived)
  {
    for(std::size_t i = 0; i < Derived.size(); ++i) Derived[i] = _other_derived[i];
    return Derived;
  }
};

/** Static array class. */
template <class data_type, int array_size>
class StaticArray : public std::array<data_type, array_size>, public ArrayBase<StaticArray<data_type, array_size>, data_type>
{
  using Base = ArrayBase<StaticArray<data_type, array_size>, data_type>;
  friend Base;

  public:
  /** Default constructor. */
  StaticArray() : std::array<data_type, array_size>() {}

  /** Default destructor. */
  ~StaticArray() = default;

  using Base::operator[];
  using Base::operator=;
};

/** Dynamic array class. */
template <class data_type>
class DynamicArray : public std::vector<data_type>, public ArrayBase<DynamicArray<data_type>, data_type>
{
  // ...
};

int main()
{
  StaticArray<double, 3> array1;
  array1[0] = 1.0;
  array1[1] = 2.0;
  array1[2] = 3.0;

  // This code compiles
  StaticArray<double, 3> array2 = array1;

  // This code does not compile
  array2 = array1; 

  return 0;
}

我的 IDE (CLion) 在我使用上述赋值运算符时出现以下错误：

Object of type 'StaticArray<Apeiron::Float, 3>' (aka 'StaticArray<double, 3>') cannot be assigned because its copy assignment operator is implicitly deleted

编译器错误是：

error: ‘Apeiron::StaticArray<double, 3>& Apeiron::StaticArray<double, 3>::operator=(const Apeiron::StaticArray<double, 3>&)’ cannot be overloaded with ‘t_derived_class& Apeiron::ArrayBase<t_derived_class, data_type>::operator=(const t_derived_class&) [with t_derived_class = Apeiron::StaticArray<double, 3>; data_type = double]’
   90 | class StaticArray : public std::array<data_type, array_size>, public ArrayBase<StaticArray<data_type, array_size>, data_type>
      |       ^~~~~~~~~~~
Array.h:62:20: note: previous declaration ‘t_derived_class& Apeiron::ArrayBase<t_derived_class, data_type>::operator=(const t_derived_class&) [with t_derived_class = Apeiron::StaticArray<double, 3>; data_type = double]’
   62 |   t_derived_class& operator=(const t_derived_class& _other_derived)

任何人都可以告诉我如何让它工作吗？

Answer 1

由于您的 ArrayBase 的成员变量是 reference，隐式声明的 ArrayBase::operator= 将是 automatically deleted.

另一种方法是去掉成员变量，直接使用帮助函数得到派生的reference：class:

template <class t_derived_class, class data_type>
class ArrayBase
{
  private:
  t_derived_class& Derived() noexcept {
    return static_cast<t_derived_class&>(*this);
  };

  const t_derived_class& Derived() const noexcept {
    return static_cast<const t_derived_class&>(*this);
  };

  public:
  /** Pure virtual desctructor. */
  virtual ~ArrayBase() {};

  /** Index operator overloads. */
  data_type& operator[](const std::size_t _index)
  {
    return *(Derived().begin() + _index);
  }

  // ...
};

使用奇怪的循环模板模式 (CRTP) 在抽象基础 class 中实现赋值运算符

Implementing the assignment operator in an abstract base class using the curiously recurring template pattern (CRTP)

c++

c++17

operator-overloading

assignment-operator

crtp