problem with exception

[vivmen]

hi all
i am trying following code and have some query.

class myexn
{
	
public:
	int* ptr;
	myexn()
	{
		ptr = new int;
		cout<<"ctor myexn \n";
	}
	
	~myexn()
	{
		delete ptr;
		cout<<"dtor myexn \n";
	}
	
	myexn(myexn& i)
	{
		cout<<"copy ctor myexn \n";
	}
};
int main()
{
	try
	{
		throw myexn();	//object  created
	
	} // goes out of scope destructor called
	
	catch(myexn& i) // same object refer (destroyed one ? or copy of oreginal ?)
	{
		*(i.ptr) = 24; // fail , but work when copy constructor provided why ?
	}

	return 0;
}

above code works when i use copy constructor, why?
it fails if i do not provide copy constructor (why ? well compiler will any way
provide one if i don't)
surprisingly copy constructor did not get called nothing printed.

can any one help my to find out whats going on ?

using windows visual studio 2010.

thanks in advance.

[zb0S216C]

The copy constructor should take a constant reference to another instance.

_Wazzak

[TheDestroyer]

Framework is right, just add a const to your copy constructor. What you defined is not a copy contructor.

	myexn(const myexn& i)
	{
		cout<<"copy ctor myexn \n";
	}

[kbw]

You're not copying the object, so I wouldn't expect the copy constructor to be called. And gcc behaves as I'd expected.

However, Visual Studio (all versions) catch a copy. You can verify this by printing the address of the object. I would not have expected this behaviour.

[vivmen]

i tried all your suggestion well one thing sure that copy of object got send even if i am using reference. i think that is correct as after throw, control leaves block , did not return and all object up to that point got destroyed.
but still many confusing things are there here is new code.

class myexn
{
	
public:
	int* ptr;
	myexn()
	{
		ptr = new int;
		cout<<"ctor myexn \n";
	}
	
	~myexn()
	{
		delete ptr;
		cout<<"dtor myexn \n";
	}
	
	myexn(const myexn& i)
	{
		cout<<"copy ctor myexn \n";
	}
};
int main()
{
	try
	{
		myexn i ;	//object  created
		cout<<&i<<endl;
		throw i; // calls copy constructor

		//throw myexn(); // no copy constructor called code works
	
	} // goes out of scope destructor called
	
	catch(myexn& i) // ok now its copy only
	{
		cout<<&i<<endl;
		*(i.ptr) = 24; // now fails with throw i , but suceed with throw() why ?
	}

	//both address are different so its not oreginal object either.

	return 0;
}

the only surprising part now is throw myexn() , no copy constructor called and code works.