Question

我一直在教自己C ++，并开始创建一个列表组织器来处理指针的概念。

我已经定义了一个名为List的类，它有三个属性：

int itemTotal;
Item* firstItem;
Item* lastItem;

构造函数将其值设置为：

itemTotal = 0;
firstItem = NULL;
lastItem = NULL;

我已经构建了一个函数来返回itemTotal的值：

int List::getItemTotal()
{
    return itemTotal;
}

在我的驱动程序中构建对象之后，itemTotal立即开始表现得很有趣并返回非常大的数字（每次都是-858993460），即使在List上没有完成任何工作，并且在程序中没有发生任何事情。我在构造函数中添加了一个cout，只是为了看看那里发生了什么，并且它回显了0值，但是一旦构造函数完成，值就会立即改变。

我一直试图用我的书来解决这个问题，但我似乎无法解决这个问题，所以我想我会转向有经验的人。主要在下面：

int main()
{
    List grocery;
    cout << "itemTotal is now: " << grocery.getItemTotal() << endl; // Returns wrong value...


    system("Pause");
    return 0;
}

输出如下：

grocery List is built!
itemTotal inside of the constructor is 0!
itemTotal is now: -858993460

有什么想法吗？ = /

编辑：每个请求，整个类（抱歉格式是丑陋的，我不想重新做这一切）：

class List
{
public:
// Constructor
// Purpose: Builds object.
// Returns: Nothing.
// Pre-Conditions: None.
// Post-Conditions: Initializes null.
List();

// push_back function
// Purpose: Adds Item to end of List.
// Returns: None.
// Pre-Conditions: Must pass a declared Item object.
// Post-Conditions: None.
void push_back(Node*);

// push_front function
// Purpose: Adds Item to beginning of List.
// Returns: None.
// Pre-Conditions: Must pass a declared Item object.
// Post-Conditions: None.
void push_front(Node*);

// pop_back function
// Purpose: Removes last Item from List. Item is NOT deleted.
// Returns: Pointer to removed Item.
// Pre-Conditions: None.
// Post-Conditions: None.
Node* pop_back();

// pop_front function
// Purpose: Removes first Item from List. Item is NOT deleted.
// Returns: Pointer to removed Item.
// Pre-Conditions: None.
// Post-Conditions: None.
Node* pop_front();

// getFirst function
// Purpose: Returns pointer to first Item in List.
// Returns: Pointer.
// Pre-Conditions: List must have a Item object.
// Post-Conditions: None.
Node* getFirst();

// getItemTotal function
// Purpose: Returns the itemTotal
// Returns: Int
// Pre-Conditions: None.
// Post-Conditions: None.
int getItemTotal();
private:
Item* firstitem;
Item* lastitem;
int itemTotal;

}

和构造函数：

List::List()
{
Item* firstNode = NULL;
Item* lastNode = NULL;
int itemTotal = 0;
cout << "item total should start at 0, it is " << nodeTotal << " inside of the constructor." << endl;
}

Answer 1

HAAA！您正在构造函数中初始化局部变量而不是成员！而不是int itemTotal = 0;写this->itemTotal = 0;或只是itemTotal = 0甚至使用像这样的构造函数初始化列表

List::list()
   :itemTotal(0),
    firstNode(NULL),
    lastNode(NULL)   
{
   cout << "List ctor Called"
}

Answer 2

您在构造函数中声明了与成员同名的本地值。它们隐藏了成员的值，因此当您设置itemTotal = 0;时，实际上只是设置局部变量的值。你的构造函数应该如下所示：

List::List()
    :itemTotal(0), firstNode(NULL), lastNode(NULL)
{
    cout << "item total should start at 0, it is " << itemTotal << " inside of the constructor." << endl;

}

Answer 3

您正在初始化局部变量而不是类成员。

替换：

List::List()
{
Item* firstNode = NULL;
Item* lastNode = NULL;
int itemTotal = 0;
cout << "item total should start at 0, it is " << nodeTotal << " inside of the constructor." << endl;
}

人：

List::List()
{
  firstNode = NULL;
  lastNode = NULL;
  itemTotal = 0;
  cout << "item total should start at 0, it is " << nodeTotal << " inside of the constructor." << endl;
}

Answer 4

您的问题是您在构造函数中声明隐藏成员变量的局部变量：

List::List()
{
    Item* firstNode = NULL;  // declares a new variable.
    Item* lastNode = NULL;
    int itemTotal = 0;
    cout << "item total should start at 0, it is " << nodeTotal << " inside of the constructor." << endl;
}

您的代码应如下所示：

List::List()
{
    firstNode = NULL;
    lastNode = NULL;
    itemTotal = 0;

    // fix this line: cout << "item total should start at 0, it is " << nodeTotal << " inside of the constructor." << endl;
}

如果你使用初始化列表会更好：

List::List()
    : firstNode(NULL)
    , lastNode(NULL)
    , itemTotal(0)
{
    // Fix this line cout << "item total should start at 0, it is " << nodeTotal << " inside of the constructor." << endl;
}

C ++对象 - 私有int返回奇怪的值

4 个答案: