如何在 C++ 中实现 resize() 来改变动态成员数据的容量

Question

我需要实现函数resize()：

 void IntSet::resize(int new_capacity)
 {
     if (new_capacity < used)
         new_capacity = used;
     if (used == 0)
         new_capacity = 1;

     capacity = new_capacity;

     int * newData = new int[capacity];

     for (int i = 0; i < used; ++i)
         newData[i] = data[i];

     delete [] data;
     data = newData;
}

函数内部：

 IntSet IntSet::unionWith(const IntSet& otherIntSet) const
 {
       IntSet unionSet = otherIntSet;

       for (int i = 0; i < used; i++)
       {
           if (unionSet.contains(data[i]))
              unionSet.add(data[i]);
       }
       return unionSet;
 }

还有这个：（注意：我已经在 add() 函数中找到了它，但我认为它不正确）

 bool IntSet::add(int anInt)
 {
     if (contains(anInt) == false)
     {
        if (used >= capacity)
           resize(used++);

        data[used++] = anInt;

        return true;
     }
     return false;
 }

程序正确编译没有错误，但它确实给了我一个分段错误

的错误

注意：主要是我需要帮助来学习如何使用 resize 函数来调整动态成员数据的 容量。另外，我知道向量在这种情况下会有所帮助，但我们还不能使用向量

这是教授的特殊要求：

>Special Requirement (You will lose points if you don't observe this.) <br/>

>When calling resize *(while implementing some of the member functions)* to 
>increase the capacity of the dynamic arrays, use the following resizing 
>rule (unless the new capacity has to be something else higher as dictated 
>by other >overriding factors): <br/>
>
>*"new capacity" is "roughly 1.5 x old capacity" and at least "old capacity 
> + 1".* <br/>
>
>The latter *(at least " old capacity + 1 ")* is a simple way to take care 
>of the subtle case where " 1.5 x old capacity " evaluates (with truncation) 
>to the >same as "old capacity". <br/>

Answer 1

当您因为add 而resize 时，您将used 增加两次。

bool IntSet::add(int anInt)
{
    if (contains(anInt) == false)
    {
       if (used >= capacity)
          resize(used++);  // Here And this is a post increment. 
                           // resize will be called with used before the increment
                           // so you will wind up asking for a buffer the same size.

       data[used++] = anInt; // and here. 

       return true;
    }
    return false;
}  

所以没有任何东西被使用。您跳过一个空格并在之后写入空格。加上resize(used++); 没有要求更多空间，所以你实际上在分配的存储之外写了两个位置，这可能会触发段错误。

解决方案

您不想在resize(used++); 处增加任何内容。您想在capacity 中添加一个，但不增加它，所以

resize(capacity +1);

看起来不错。然而，指令要求的更像是：

int newcap = capacity * 1.5;
if (newcap == capacity) // newcap didn't change. eg: 1*1.5 = 1
{
    newcap ++;
}
resize(newcap);

虽然这是蛮力。有更聪明的方法可以做到这一点。