auto_ptr 令人困惑的行为

发布于 2024-08-17 07:45:44 字数 1047 浏览 5 评论 0原文

#include<iostream>
#include<memory>
#include<stdio>

using namespace std;

class YourClass
{
   int y;
public:
   YourClass(int x) {
      y= x;
   }
};
class MyClass
{
   auto_ptr<YourClass> p;
public:
   MyClass() //:p(new YourClass(10)) 
   {
      p= (auto_ptr<YourClass>)new YourClass(10);
   }
   MyClass( const MyClass &) : p(new YourClass(10)) {}
   void show() {
      //cout<<'\n'<<p; //Was not working hence commented
      printf("%p\n",p);
   }
};

int main() {
   MyClass a;
   a.show();
   MyClass b=a;
   cout<<'\n'<<"After copying";
   a.show();//If I remove copy constructor from class this becomes NULL(the value of auto_ptr becomes NULL but if class has copy constructor it remains same(unchanged)
   b.show();//expected bahavior with copy construcotr and withought copy constructor
}

让问题更具体：目前该类具有复制构造函数，因此 a.show() 打印的 auto_ptr 值没有问题（当第二次调用时）。它保持与启动时相同的状态）。它保持不变。如果我从类 MyClass 中删除复制构造函数，则 a.show() （第二次调用时）打印的 auto_ptr 的值为 NULL。

原文

#include<iostream>
#include<memory>
#include<stdio>

using namespace std;

class YourClass
{
   int y;
public:
   YourClass(int x) {
      y= x;
   }
};
class MyClass
{
   auto_ptr<YourClass> p;
public:
   MyClass() //:p(new YourClass(10)) 
   {
      p= (auto_ptr<YourClass>)new YourClass(10);
   }
   MyClass( const MyClass &) : p(new YourClass(10)) {}
   void show() {
      //cout<<'\n'<<p; //Was not working hence commented
      printf("%p\n",p);
   }
};

int main() {
   MyClass a;
   a.show();
   MyClass b=a;
   cout<<'\n'<<"After copying";
   a.show();//If I remove copy constructor from class this becomes NULL(the value of auto_ptr becomes NULL but if class has copy constructor it remains same(unchanged)
   b.show();//expected bahavior with copy construcotr and withought copy constructor
}

Making the problem more specific:
Currently the class has copy constructor so there is no problem with the value of auto_ptr printed by a.show()(when it is called second time). It remians the same as it was when it was initiazed). It remians unchanged.
If I remove the copy contructor from the class MyClass , the value of auto_ptr printed by a.show()(when it is called second time) is NULL.

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

靖瑶 2024-08-24 07:45:44

发生的事情是由于分配或复制 auto_ptr 的奇怪（但如果您考虑一下才合理）语义，例如

auto_ptr<T> a;
auto_ptr<T> b(new T());
a = b;

...或...

auto_ptr<T> b(new T());
auto_ptr<T> a(b);

这些将按预期将 a 设置为 b，但它们也会将 b 设置为 NULL （参见http://www.cplusplus.com/reference/std/memory /auto_ptr/auto_ptr/）。

如果您没有为 MyClass 定义复制构造函数，那么编译器将为您生成一个复制构造函数，并在复制 auto_ptr 成员时执行与上述类似的操作。因此，在调用复制构造函数后，从类复制的将具有 NULL 成员。

What's happening is due to the strange (but only justifiable if you think about it) semantics of assigning or copying an auto_ptr, e.g.

auto_ptr<T> a;
auto_ptr<T> b(new T());
a = b;

... or ...

auto_ptr<T> b(new T());
auto_ptr<T> a(b);

These will set a to b as expected, but they will also set b to NULL (see http://www.cplusplus.com/reference/std/memory/auto_ptr/auto_ptr/).

If you don't define a copy constructor for MyClass, then the compiler will generate one for you and will do just something similar to the above when it copies the auto_ptr member. Hence the copied from class will have a NULL member after the copy constructor has been called.

回复收藏 0 原文