c ++复制控制问题

Question

#include <iostream>
#include <string>
#include <vector>
using namespace std;

class HasPtr{
 public:
    //constructor accepts a string
    HasPtr(const string &s = string()) : ps(new string(s)), i(0), use(new size_t(1)) {}  
    //copy constructor
    HasPtr(const HasPtr &h) : ps(h.ps), i(h.i), use(h.use) { ++*use; }
    //copy assignment operator 
    HasPtr &operator=(const HasPtr &h)
    {
        ++*h.use;
        if(--*use == 0)
        {
            delete ps;
            delete use;
        }
        ps = h.ps;
        i = h.i;
        use = h.use;

        return *this;
    }
    ~HasPtr()
    {
        if(--*use == 0)
        {
            delete ps;
            delete use;
        }
    }

//private:
    string *ps;
    int i;

    size_t *use;
};


int main()
{
    HasPtr h("hi mom");
    HasPtr h2 = h;
    HasPtr h3("hi dad");
    h2 = h3;

    cout << "h: " << *h.ps << endl;
    cout << "h2: " << *h2.ps << endl;
    cout << "h3: " << *h3.ps << endl;
}

输出为：

h: hi mom
h2: hi dad
h3: hi dad

我认为输出应该是：

h: hi dad
h2: hi dad
h3: hi dad

我认为输出应该如上的原因：

h 是“嗨妈妈”，h2 共享 h1，所以 h2 是“嗨妈妈”，h3 是“嗨爸爸”，我认为 h2 = h3 更改了 h1 或者因为 h2 共享 h1，但事实并非如此。

我做错了什么？

Answer 1

这是怎么回事：

HasPtr h("hi mom");
HasPtr h2 = h;

h.ps ---> "hi mom"  <--- h2.ps

然后在下一条语句中，您更改h2，但这不会影响h：

HasPtr h3("hi dad");
h2 = h3;

h2.ps ----> "hi dad" <--- h3.ps
 |
XXX changed
 |
"hi mom" <--- h.ps

指针是指向同一地址的不同实例。示例：

int a = 5, c = 1; 
int *p1 = &a, *p2 = &a;

更改p1（即p1 = &c）不会影响p2。

Answer 2

让我们考虑一个简化的例子会更清楚

int x = 10;
int y = 20;

int *h = &x;
int *h2 = h;

现在h 和h2 指向x。您可以检查此输出指针指向的对象。例如

std::cout << "*h = " << *h << ", *h2 = " << *h2 << std::endl;

那么你使用的是第三个指针

int *h3 = &y;
h2 = h3;

现在h2 和h3 指向y。 h 没有改变。你也可以看看这个

std::cout << "*h2 = " << *h2 << ", *h3 = " << *h3 << std::endl;
std::cout << "*h = " << *h << std::endl;

语句中的对象也是如此

HasPtr h("hi mom");
HasPtr h2 = h;
HasPtr h3("hi dad");
h2 = h3;

只有对应的指针（ps）被包裹在类的对象中。

这是一个演示解释的程序

#include <iostream>
#include <string>


int main()
{
    std::string hi_mom( "hi mom" );
    std::string hi_dad( "hi dad" );

    std::string *h  = &hi_mom;
    std::string *h2 = h;

    std::cout << "*h = " << *h << ", *h2 = " << *h2 << std::endl;

    std::string *h3 = &hi_dad;
    h2 = h3;

    std::cout << "*h2 = " << *h2 << ", *h3 = " << *h3 << std::endl;
    std::cout << "*h = " << *h << std::endl;
}        

它的输出是

*h = hi mom, *h2 = hi mom
*h2 = hi dad, *h3 = hi dad
*h = hi mom