Question

#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.

Answer 1

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.