c++ - 如何使用 '=' 将一个类的对象复制到同一类的另一个对象

Question

如何仅通过使用“=”将一个类的对象复制到同一类的另一个对象。我知道我们必须使运算符重载。这是我目前所拥有的

#include<iostream>
#include<conio.h>
#include<iomanip>

using namespace std;



class arr
{
public:
    int *arr1;
    int len;

    arr& operator = (const arr& eq) //for copying two arrays. <--- my overloader
    {
        arr temp1(eq.len);
        arr *pttemp;
        int i=0;
        //temp.arr1=new int[eq.len];
        //temp.len = eq.len;
        for(i = 0 ; i < eq.len ; i++)
        {
            temp1.arr1[i] = eq.arr1[i];
        }       
        pttemp = &temp1;
        return temp1;
    };

    friend istream& operator >> (istream& ist, arr & r)
    {
        static int i = 0;
        int *arrNew;

        if (i == r.len)
        {
            r.len *=2;
            arrNew = new int[r.len]; // allocate the new array              
            for(int j = 0; j < r.len/2; j++)// copy the old array to the first half of the new array
            arrNew[j] = r.arr1[j];// delete the old array         
            delete [] r.arr1;// let arr point to the new array and continue use arr     
            r.arr1 = arrNew;
            delete arrNew;
        }

        ist>>r.arr1[i];
        i++;

        return ist;
    }

    arr() //initializing constructor
    {
        len = 5;
        arr1 = new int[len];
    };

    arr(int size) //initializing constructor with args
    {
        len = size;
        arr1 = new int[len];
    };

    arr(arr& a) : arr1(a.arr1) //copy constructor
    {
        arr1 = new int[len];
    };

    ~arr() //delete constructor
    {
        delete arr1;
    };



};




void main()
{
    int size = 5,i,temp,trig = 0;
    arr orig(size), asc(size), desc(size);  

    //generate random numbers for orig

    for (i = 0 ; i < size ; i++)
    {
        orig.arr1[i] = rand();
    }

    //copy original set to asc and desc
    asc = orig;
    desc = orig;

    //sorting ascending
    for (i = 0 ; i < size-1 ; i++)
    {
        trig = 1;
        if (asc.arr1[i] < asc.arr1[i+1])
        {
            temp = asc.arr1[i];
            asc.arr1[i] = asc.arr1[i+1];
            asc.arr1[i+1] = temp;
            trig = 0;
        }
        if (trig = 1)
            break;
        if (i == size - 1)
        {
            i = 0;
        }
    }

    //sorting descending
    for (i = 0 ; i < size-1 ; i++)
    {
        trig = 1;
        if (desc.arr1[i] > desc.arr1[i+1])
        {
            temp = desc.arr1[i];
            desc.arr1[i] = desc.arr1[i+1];
            desc.arr1[i+1] = temp;
            trig = 0;
        }
        if (trig = 1)
            break;
        if (i == size - 1)
        {
            i = 0;
        }
    }

    //printing  
    cout<<"Original Array: ";
    for (i = 0 ; i < size ; i++)
    {
        cout<<orig.arr1[i]<<" ";
    }
    cout<<endl;
    cout<<"Ascending Array: ";
    for (i = 0 ; i < size ; i++)
    {
        cout<<asc.arr1[i]<<" ";
    }
    cout<<endl;

    cout<<"Descending Array: ";
    for (i = 0 ; i < size ; i++)
    {
        cout<<desc.arr1[i]<<" ";
    }
    cout<<endl;

    getch();

}

它编译正确，但它('asc' 和 'desc')显示的数字与 'orig' 对象不同。

Answer 1

正确的解决方案是这样的:

struct Foo
{
    std::vector<int> arr;
    friend std::ifstream & operator>>(/*...*/);
};

隐式定义的赋值运算符已经完全满足您的需求，巧妙的代码重用是 C++ 编程(“库设计语言”)的核心。

如果你想手写，你必须自己复制:

struct Bar
{
    unsigned int len;
    int * arr;

    Bar & operator=(Bar const & rhs)
    {
        len = rhs.len; 
        delete[] arr;
        arr = new int[len];
        for (unsigned int i = 0; i != len; ++i) { arr[i] = rhs.arr[i]; }
        return *this;
    }

    Bar() : len(), arr() { }
    // more constructors

    Bar(Bar const &);  // exercise: write this!

    ~Bar() { delete[] arr; }
};

不过，这是一个非常糟糕的想法，因为这种模式根本无法概括:它不是异常安全的——想象一下 for 循环中的一个拷贝抛出了一个异常(exception)。现在您已经丢失了原始数据和泄漏的内存。

更好的解决方案是先分配一个临时缓冲区:

int * temp = new int[len];
for (...) temp[i] = rhs.arr[i];

delete[] arr;
arr = temp;

现在这段代码很快就会变得非常难看，想象一下你有不止一个这样的代码!

简而言之:使用std::vector。