Question

我有一个类，其中一个元素是另一个类，但是是一个数组

class B
{
public:
    B()             //default
    {
        element = new A [1]; count = 0;
    }

    A add(A place)
    {
        A* newArr;
        newArr = new A [count+1];

        newArr = element;
        newArr[count+1] = place;
        delete element;

        return newArr[count+1];
    }



protected:
    int count;
    A* element;
};

我正在尝试使用动态数组，在添加元素时，我动态创建一个新数组，初始化为旧数组的大小加1，然后将旧数组的元素复制到新数组中，然后删除旧数组。但是我不确定如何修改已经在类中的数组，如果这有意义的话（基本上在我的add方法中返回什么）。

Answer 1

在C ++中，一旦声明，就没有重新调整数组大小的概念。动态数组也是如此，一旦分配就无法调整大小。但是，您可以创建一个更大的数组，将旧数组中的所有元素复制到较新的数组并删除旧数组。这是不鼓励的，也不会有效。

使用std::vector可以随意添加，也可以跟踪其大小，因此您不需要count作为课程的一部分。

class B
{
   // no need to allocate, do add when required i.e. in B::add
   B() : count(), elements() { }

   A add(A place)
   {
      // unnecessarily allocate space again
      A *new_elements = new A[count + 1];

      // do the expensive copy of all the elements
      std::copy(elements + 0, elements + count, new_elements);

      // put the new, last element in
      new_elements[count + 1] = place;

      // delete the old array and put the new one in the member pointer
      delete [] elements;
      elements = new_elements;

      // bunp the counter
      ++count;

      return place;    //redundant; since it was already passed in by the caller, there's no use in return the same back
   }

protected:
   size_t count;
   A *elements;
};

上面的代码可能会做你想要的，但是非常气馁。使用矢量;你的代码就会变成

class B
{
    // no need of a constructor since the default one given by the compiler will do, as vectors will get initialized properly by default

    void Add(A place)
    {
         elements.push_back(place);
         // if you need the count
         const size_t count = elements.size();
         // do stuff with count here
    }

    protected:
       std::vector<A> elements;
};

Answer 2

更全面的例子是更贴切地模仿std::vector：

template<typename T>
class B
{
private: // don't put data under a protected access!
    std::size_t _capacity;
    std::size_t _size;
    T* _elements;

public:
    B() : _capacity(0), _size(0), _elements(nullptr) {}

    // other methods

    void add(const T& t)
    {
        if (_size >= _capacity) // need to resize the array
        {
            _capacity++; // you can make this better by increasing by several at a time
            T* temp = new T[_capacity];
            std::copy(_elements, _elements + _size, temp);
            delete [] _elements;
            _elements = temp;
        }
        _elements[_size++] = t;
    }
};

向数组中添加内容

2 个答案: