C++ Int 似乎没有初始化并抛出异常：读取访问冲突

Question

我对 C++ 很陌生，所以请原谅草率的代码。 这是有问题的代码：

包类

class Bag {
protected:
    Item* _myItems;
    int _numItems;
    int _size;
public:
    Bag();
    Bag(int size);
    ~Bag();
    Bag(Bag& original);
    void add(Item a);
    void remove(int itemnum);
    int size();
    int numItems();
    void operator=(Bag& bag);
    Item& operator[] (int i);
};

//Empty constructor
Bag::Bag() {
    _numItems = 0;
}

//overloaded constructor
Bag::Bag(int size) {
    _numItems = 0;
    _myItems = new Item[size];
}

//copy constructor
Bag::Bag(Bag& original) {
    //Copies the numItems
    _numItems = original._numItems;
    //Makes a new copy of the original array
    _myItems = new Item[_numItems];
    //Copies each element of the original into the new
    for (int i = 0; i < _numItems; ++i) {
        _myItems[i] = original[i];
    }
}

//Destructor
Bag::~Bag(){
    delete[] _myItems;
}

//Returns the size of the bag
int Bag::size()
{
    return _size;
}

//Returns the number of items in the bag
int Bag::numItems() {
    return _numItems;
}

//Add a new item to the bag
void Bag::add(Item a) {
    int s = _numItems;
    //Create a Item pointer and assign it to the array of the bag
    Item* temp = _myItems;
    //Assign _myItems to a new, larger array
    _myItems = new Item[_numItems++];
    //Copy the old array into the new one and nullify all the old array's items
    for (int i = 0; i < _numItems - 1; i++) { 
        _myItems[i] = temp[i];  
    }
    //Destroy the old array
    delete[] temp;
    //Add the item to the last position
    _myItems[_numItems] = a;
}

我正在逐行读取文本文件。阅读似乎进行得很好。当我阅读时，我执行这部分代码：

//The main program
int main() {

    Pens * onePen = new Pens(1, 2);
    Pens * twoPen = new Pens(2, 3);

    Bag* bag = new Bag(5);

    (*bag).add(onePen);
    (*bag).add(twoPen);

    bag[0];
    bag[1];

    int d = 0;

    return 0;
}

当我进入 add 方法时，我不断收到读取访问冲突（这是 0xc）。我还注意到，当我设置断点检查代码时，_numItems 不是 0 而是 211。我是否以某种方式破坏了我的内存？

Here is a sample text file that we are using

Bag 和 Pen 类的简化版本（由 PaulMcKenzie 提供）：

class Item {
protected:
    int code_;

    //Sets the method definition for the get/set methods and constructors
public:
    Item(int code = -1);
    virtual ~Item() {}
    int getcode() const;
    void setcode(int code);
    std::ostream& operator<< (std::ostream& s);
    bool operator== (const Item& a) const;
};

Item::Item(int code) : code_(code) {}
int Item::getcode() const { return code_; }
void Item::setcode(int code) { code_ = code; }

std::ostream & Item::operator<<(std::ostream& s)
{
    s << " Code - " << code_ << "\n";
    return s;
}

bool Item::operator==(const Item & a) const
{
    return (code_ == a.getcode());
}

class Pens : public Item
{
private: int packetsize_;
public:
    Pens();
    Pens(int code, int packetsize);
    int getpacketsize() const;
    void setpacketsize(int packetsize);
    bool operator== (const Pens& a) const;
};

Pens::Pens() :Item() { }
Pens::Pens(int code, int packetsize) : Item(code), packetsize_(packetsize) {}
int Pens::getpacketsize() const { return packetsize_; }
void Pens::setpacketsize(int packetsize) { packetsize_ = packetsize; }

std::ostream& operator<<(std::ostream& s, const Pens& pen)
{
    s << " Packet size: " << pen.getpacketsize() << "\n";
    return s;
}

bool Pens::operator==(const Pens & a) const
{
    return code_ == a.getcode() && packetsize_ == a.getpacketsize();
}

Answer 1

我没有深入研究，但这段片段引起了我的注意：

//Add a new item to the bag
void Bag::add(Item a) {
int s = _numItems;
//Create a Item pointer and assign it to the array of the bag
Item* temp = _myItems;
//Assign _myItems to a new, larger array
_myItems = new Item[_numItems++];
//Copy the old array into the new one and nullify all the old array's items
for (int i = 0; i < _numItems - 1; i++) { 
    _myItems[i] = temp[i];  
}
//Destroy the old array
delete[] temp;
//Add the item to the last position
_myItems[_numItems] = a;
}

请看这一行：

_myItems = new Item[_numItems++];

您创建大小为 _numItems 的新数组，然后将 _numItems 增加 1。

以我的拙见，这会让您得到大小为 _numItems-1 的数组。 然后你尝试使用元素 _myItems[_numItems] 所以这可能是内存损坏的原因。