c++ - 赋值运算符复制数组中除可用空间以外的所有内容

Question

我最近一直在研究动态分配和队列。请记住，我对数据结构还很陌生，我一直在通过观看 YouTube 教程和阅读网站上的一些教程来自学，所以如果我不知道关于这个主题的每一件事，请原谅我的无知。但是，在尝试为我的队列实现赋值运算符时，我似乎在其代码中的某个地方犯了一个错误。每当我尝试使用赋值运算符时，你都会看到

即

Queue names2;
names2 = names;

赋值运算符复制队列中的所有内容，除了数组的空元素。我是否错误地实现了我的复制构造函数或我的赋值运算符，导致它没有将空的未使用容量添加到新创建的队列中？

main.cpp

  #include "Queue.h";

int main()
{
    Queue names;
    names.enqueue("A");
    names.enqueue("B");
    names.enqueue("C");
    names.dequeue();
    names.enqueue("D");
    names.showFullQueue();
    Queue names2;
    names2 = names;
    names2.showFullQueue();
    names2.enqueue("Tom");
    names2.enqueue("Alice");
    names2.showFullQueue();

    return 0;
}

Queue.h

#ifndef Queue_H
#define Queue_H

#include <iostream>
#include <cstring>
#include <algorithm>
#include <string>

using namespace std;

class Queue {
public:
    Queue(); //Defualt costructor
    Queue(const Queue& source); //Copy constructor
    ~Queue(); //Destructor

    Queue& operator=(const Queue& source); //Assignament Operator //NOT WORKING

    void enqueue(string value); // add item to queue
    string dequeue(); // remove item from queue

    void showFullQueue() const;

    //memory allocation
    void memory(int currentCapacity);
private:
    void shift();
    string * arr;
    const static int front = 0;
    int back;
    int capacity;
    int usedCapacity;
};


#endif // !Queue_H

队列.cpp

#include "Queue.h"

Queue::Queue()
{
    back = -1;
    capacity = 1;
    usedCapacity = 0;
    arr = new string[capacity];
}

Queue::Queue(const Queue & source)
{
    cout << "Invoking copy constructor" << endl;
    this->back = source.back;
    this->capacity = source.capacity;
    this->usedCapacity = source.usedCapacity;
    arr = new string[source.capacity];
    copy(source.arr, source.arr + usedCapacity, this->arr);
}

Queue::~Queue()
{
    delete[] arr;
}

Queue & Queue::operator=(const Queue & source)
{
    if (this == &source)
        return *this;
    cout << "Invoking Assignament operator" << endl;
    Queue temp(source);
    swap(temp.back, back);
    swap(temp.capacity, capacity);
    swap(temp.usedCapacity, capacity);
    swap(temp.arr, arr);

    return *this;
}

void Queue::enqueue(string value)
{
    ++back;
    arr[back] = value;
    usedCapacity++;
    memory(usedCapacity);
}

string Queue::dequeue()
{
    string returnValue = arr[front];
    shift();
    usedCapacity--;
    memory(usedCapacity);
    return returnValue;
}

void Queue::showFullQueue() const
{
    cout << "Front: " << front << endl;
    cout << "Back: " << back << endl;
    for (int i = 0; i < capacity; i++)
    {
        cout << arr[i] << ", ";
    }
    cout << endl;
}

void Queue::memory(int currentCapacity)
{
    if (currentCapacity >= capacity)
    {
        int newCapacity = (currentCapacity * 3) / 2 + 1;
        string * arr2 = new string[newCapacity];
        copy(arr, arr + usedCapacity, arr2);
        delete[] arr;
        arr = arr2;
        capacity = newCapacity;
    }
    else
        cout << "allocation error";
}

void Queue::shift()
{
    for (int i = front; i <= back; i++)
    {
        arr[i] = arr[i + 1];
    }
    --back;
}

如有任何帮助，我们将不胜感激。先谢谢你。

Answer 1

好吧，这里有几个错误需要修复..

• 您对数据成员 back 和静态成员 front 的使用对我来说似乎是多余的。

• 我没有看到 shift 函数有任何有益的用途。

此外，在您的复制赋值运算符中...

Queue & Queue::operator=(const Queue & source)
{
    if (this == &source)
        return *this;
    cout << "Invoking Assignament operator" << endl;
    Queue temp(source);
    swap(temp.back, back);
    swap(temp.capacity, capacity);
    swap(temp.usedCapacity, capacity);    //Notice the Bug here...?
    swap(temp.arr, arr);

    return *this;
}

---

好吧，这是一个清理过的版本...(注意一些变化，比如在头文件的全局范围内不using namespace std;；并且移动不必要的#includes 到实现文件...

Queue.h

#ifndef Queue_H
#define Queue_H

#include <string>

//using namespace std; ...typically a bad idea in header files

class Queue {
public:
    Queue(); //Defualt costructor
    Queue(const Queue& source); //Copy constructor
    ~Queue(); //Destructor

    Queue& operator=(const Queue& source); //Assignment Operator // WORKING :-)

    void enqueue(std::string value); // add item to queue
    string dequeue(); // remove item from queue

    void showFullQueue() const;

private:
    std::string* arr;
    int capacity;
    int usedCapacity;

    //memory allocation
    void memory(int currentCapacity);    
};


#endif // !Queue_H

队列.cpp

#include "Queue.h"
//You can put the remaining includes here
#include <iostream>
#include <algorithm>

using namespace std; // You can us it in your cpp file... Not a bad idea.. :-)

Queue::Queue()
{
    capacity = 3;
    usedCapacity = 0;
    arr = new string[capacity]();
}

Queue::Queue(const Queue & source)
{
    cout << "Invoking copy constructor" << endl;
    capacity = source.capacity;
    usedCapacity = source.usedCapacity;
    arr = new string[source.capacity]();
    copy(source.arr, source.arr + source.capacity, arr);
}

Queue::~Queue()
{
    delete[] arr;
}

Queue & Queue::operator=(const Queue & source)
{
    if (this == &source)
        return *this;
    cout << "Invoking Assignament operator" << endl;

    using std::swap; //For ADL: ...redundant here... since you already have using namespace std;
    Queue temp(source);
    swap(temp.capacity, capacity);
    swap(temp.usedCapacity, usedCapacity);
    swap(temp.arr, arr);

    return *this;
}

void Queue::enqueue(string value)
{
    memory(usedCapacity);
    arr[usedCapacity] = value;
    ++usedCapacity;
}

string Queue::dequeue()
{
    string returnValue = arr[--usedCapacity];
    //memory(usedCapacity); //needless
    return returnValue;
}

void Queue::showFullQueue() const
{

    //This styple prevents printing the last comma on the last item
    int i = 0;
    for (; i < usedCapacity-1; i++)
    {
        cout << arr[i] << ", ";
    }
    cout << arr[i] << endl;
}

void Queue::memory(int currentCapacity)
{
    if (currentCapacity >= capacity)
    {
        int newCapacity = (currentCapacity * 3) / 2 + 1;
        string* arr2 = new string[newCapacity]();
        copy(arr, arr + capacity, arr2);
        delete[] arr;
        arr = arr2;
        capacity = newCapacity;
    }
}

并且您的 ma​​in.cpp 没有任何变化 ...:-)看到它工作 here

当然，我们还可以做一些改进

• 考虑Exception Safety
• 使用Member Initialization List
• Rule of Five