c++ - 如何从 C++ 程序中的容器类中删除第一个元素？

Question

每次我运行一个名为 Loan &removeFirst () {} 的方法时，我的程序都会崩溃。每当我试图删除我创建的临时贷款结构时，程序就会崩溃。我可以展示容器类的一些背景知识和我的函数的代码:

class ListOfLoans {

public:

    //default constructor allocate appropriate heap storage 
    //store elements on heap array declared like this: 
    //new Loan*[initial_size];
    ListOfLoans(int initial_size = 4) {
        elements = new Loan*[initial_size];
        numberOfElements = 0;
        capacity = initial_size;
        index = 0;
    }

    ~ListOfLoans(void) {
        cout << "Deleting \n";
        delete [] elements;
    }


    // answer the first element from the list but don't remove it
    Loan & first() const {
        if (capacity == 0) {
            cout << "Attempted on empty list. Exitting!" << endl;
            exit;
        } else {
            return *elements[0];
        }
    }

    // answer the first element from the list and remove it from the list
    // if the resulting list is more than three quarters empty release some memory
Loan & removeFirst() {
    index--;

    if (capacity == 0) {
           cout << "Attempted on empty list. Exitting!" << endl;
           exit;
    }

    if(size() < capacity/4) {  //shrink the container when 1/4 full 
      cout << "shrinking\n"; 
      Loan **temp = elements; 
      elements = new Loan*[capacity/2]; 
      for(index = (numberOfElements - 1); index >= 0; index--) {elements[index] = temp[index];}
      capacity /= 2; 
      delete [] temp; // my program crashes at this line, I want to delete the temp structure
    } 
      return first();
    }


private: 

    Loan ** elements; 
    int     numberOfElements; //number of elements in the list 
    int     capacity; //size of the available array memory 
    mutable int index; //used to help with the iteration

};

Answer 1

删除指针数组(指向指针的指针)时，您通常会执行以下操作:

for(int i = 0; i < capacity; ++i)
{
    delete temp[i];
}
delete [] temp;

如果没有 for 循环，您将泄漏内部指针中的内存。

size() 是否返回 numberOfElements？我在这里担心的是，您的 for 循环将数据从 temp 复制到元素中，并且可能在 temp 范围之外开始。如果是这种情况，您可能正在覆盖内存，这可能是崩溃的根源。为什么不直接从 0 循环到 size()？如果要删除第一个元素，只需使用以下内容复制它:

elements[index] = temp[index+1];

最后，如果您要删除列表中的第一个元素，那么 first() 在内部做了什么？从我上面看到的情况来看，您似乎已经删除了第一个元素，或者本来打算这样做。如果它被删除，它可能会在您返回它时被删除，因此您需要在本地复制该指针并让您的 for 循环删除所有元素并跳过第一个元素，这样您仍然可以返回一些有效的东西!