我如何定义双括号/双迭代器运算符，类似于向量

问题描述

我正在移植使用大量浮点数的代码，这可能会触发从 c 到 c++ 的 malloc 失败.我问了一个关于我应该使用向量还是双端队列的问题，Niki Yoshiuchi 慷慨地向我提供了这个安全包装的例子类型:

I'm porting code that uses a very large array of floats, which may trigger malloc failures from c to c++. I asked a question about whether I should use vectors or deques and Niki Yoshiuchi generously offered me this example of a safely wrapped type:

template<typename T>
class VectorDeque
{
private:
  enum TYPE { NONE, DEQUE, VECTOR };
  std::deque<T> m_d;
  std::vector<T> m_v;
  TYPE m_type;
  ...
public:
  void resize(size_t n)
  {
    switch(m_type)
    {
      case NONE:
      try
      {
        m_v.resize(n);
        m_type = VECTOR;
      }
      catch(std::bad_alloc &ba)
      {
        m_d.resize(n);
        m_type = DEQUE;
      }
      break;
    }
  }
};

我需要一个向量/双端队列的二维向量，因此我将其修改为以下代码:

I needed a 2D vector of vectors/deque of deques, so I modified it to the following code:

template<typename T>
class VectorDeque
{
private:
  enum STORAGE_CONTAINER { NONE, DEQUE, VECTOR };
  std::deque<std::deque<T> > x_d,y_d,z_d;
  std::vector<std::vector<T> > x_v,y_v,z_v;
  TYPE my_container;
public:
  void resize(size_t num_atoms, size_t num_frames)
  {
    switch(m_type)
    {
      case NONE:
      try
      {
        x_v.resize(num_atoms);
 for (unsigned int couter=0;couter < num_frames; counter++)
   x_v[counter].resize(num_frames);
        y_v.resize(num_atoms);
 for (unsigned int couter=0;couter < num_frames; counter++)
   y_v[counter].resize(num_frames);
        z_v.resize(num_atoms);
 for (unsigned int couter=0;couter < num_frames; counter++)
   z_v[counter].resize(num_frames);
        my_container = VECTOR;
      }
      catch(std::bad_alloc &e)
      {
        x_d.resize(num_atoms);
 for (unsigned int couter=0;couter < num_frames; counter++)
   x_d[counter].resize(num_frames);
        y_d.resize(num_atoms);
 for (unsigned int couter=0;couter < num_frames; counter++)
   y_d[counter].resize(num_frames);
        z_d.resize(num_atoms);
 for (unsigned int couter=0;couter < num_frames; counter++)
   z_d[counter].resize(num_frames);
        my_container = DEQUE;
      }
      break;
    }
  }
};

我现在希望能够定义我的括号运算符，以便我可以有一个像x[1][2] 直接访问我正在使用的真实 内存容器(由我的枚举变量的值给出.

I now want to be able to define my bracket operators so that I can have a statement like x[1][2] directly access whichever is the real memory container I'm using (given by the value of my enumerated variable.

我看过一些关于覆盖括号运算符的教程，但我完全不知道要覆盖双括号.

I've seen a couple of tutorials floating around about overriding the brackets operator, but have positively no idea to override double brackets.

如何重载双括号?

此外，您将如何重载双迭代器(以防我想使用迭代器，而不是直接索引)?

Additionally, how would you overload double iterators (in case I want to use an iterator, as opposed to direct indexing)?

编辑 1:

基于 Martin York/Matteo Italia 的解决方案，我设计了以下课程:

Based on the solution from Martin York/Matteo Italia I devised the following class:

template<typename T>
class VectorDeque2D
{
public:

  class VectorDeque2D_Inner_Set
  {
    VectorDeque2D& parent;
    int   first_index;
  public:
    // Just init the temp object
    VectorDeque2D_Inner_Set(My2D& p, int first_Index) : 
      parent(p), 
      first_Index(first_index) {} 
    // Here we get the value.
    T& operator[](int second_index)  const 
    { return parent.get(first_index,second_index);}   
  };

  // Return an object that defines its own operator[] that will access the data.
  // The temp object is very trivial and just allows access to the data via 
  // operator[]
  VectorDeque2D_Inner_Set operator[](unsigned int first_index) { 
    return (*this, x);
  }


  void resize_first_index(unsigned int first_index) {
    try {
      my_vector.resize(first_index);
      my_container = VECTOR;
    }
    catch(std::bad_alloc &e) {
      my_deque.resize(first_index);
      my_container = DEQUE;
    }
  }

  void resize_second_index(unsigned int second_index) {
    try {
      for (unsigned int couter=0;couter < my_vector.size(); counter++) {
    my_vector[counter].resize(second_index);
      }
      my_container = VECTOR;
    }
    catch(std::bad_alloc &e) {
      for (unsigned int couter=0;couter < my_deque.size(); counter++) {
    my_deque[counter].resize(second_index);
      }
      my_container = DEQUE;
    }
  }
  void resize(unsigned int first_index,
          unsigned int second_index) {
    try {
      my_vector.resize(first_index);
      for (unsigned int couter=0;couter < my_vector.size(); counter++) {
    my_vector[counter].resize(second_index);
      }
      my_container = VECTOR;
    }
    catch(std::bad_alloc &e) {
      my_deque.resize(first_index);
      for (unsigned int couter=0;couter < my_deque.size(); counter++) {
    my_deque[counter].resize(second_index);
      }
      my_container = DEQUE;
    }    
  }
private:
  enum STORAGE_CONTAINER { NONE, DEQUE, VECTOR };

  friend class VectorDeque2D_Inner_Set;

  std::vector<std::vector<T> > my_vector;
  std::deque<std::deque<T> > my_deque;
  STORAGE_CONTAINER my_container;

  T& get(int x,int y) { 
    T temp_val;
    if(my_container == VECTOR) {
      temp_val = my_vector[first_index][second_index];
    }
    else if(my_container == DEQUE) {
      temp_val = my_deque[first_index][second_index];
    }

    return temp_val;
  }

};

终于有大小安全的 2D 容器了！！谢谢各位！

Finally a size-safe 2D container!! Thanks guys!

推荐答案

主要有两种技巧:

1) 使用 operator() 而不是 operator[].
这是因为 operator() 允许多个参数.

1) Use operator() rather than operator[].
This is because the operator() allows multiple parameters.

class My2D
{
    public:
       int&   operator()(int x,int y)  { return pget(x,y);}
    private:
       int&   pget(int x,int y) { /* retrieve data from 2D storage */ }
};

2) 使用 operator[] 但返回一个中间对象.
然后，您可以将第二个 operator[] 应用于中间对象.

2) Use operator[] but return an intermediate object.
You can then apply the second operator[] to the intermediate object.

class My2D
{
    public:
       class My2DRow
       {
           My2D& parent;
           int   x;
           public:
               My2DRow(My2D& p, int theX) : parent(p), x(theX) {}     // Just init the temp object
               int& operator[](int y)  const { return parent.pget(x,y);}   // Here we get the value.
       };

       // Return an object that defines its own operator[] that will access the data.
       // The temp object is very trivial and just allows access to the data via operator[]
       My2DRow operator[](int x)        { return My2DRow(*this, x);}
    private:
       friend class My2DRow;
       int&   pget(int x,int y) { /* retrieve data from 2D storage */ }
};

int main()
{
    My2D   data;
    int&   val = data[1][2]; // works fine.

    // This is the same as
    My2D::My2DRow row  = data[1];
    int&          val2 = row[2]; 
}

我更喜欢第二种技术.
这是因为它使原始代码保持不变并且更自然地阅读(在数组上下文中).当然，您需要为实现二维数组的稍微复杂的代码支付更高级别的简单性.

I prefer the second technique.
This is because it leaves the original code untouched and more natural to read (in an array context). Of course you pay for the simplicity at the high level with slightly more complex code implementing your 2D array.

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