What is different between this declearation tow constructor

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class Point
{
private:
		int x;
		int y;

public:
Point()
{
	x=y=0;
}
Point(int _x,int _y)
{
	x=_x;
	y=_y;
}
void  Show()
{
	cout<<"X ="<<x<<" ,Y ="<<y<<endl;
}
};
void main()
{
	Point* x=new Point();
	x->Show();
	Point X2;
	X2.Show();
		
	Point* y=new Point(1,2);
	y->Show();
	Point Y2(1,2);
	Y2.Show();
getch();
}

what is different between this two way (use new key word or without it)??

Posted 5-Apr-13 3:18am

Anderso0on

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5 solutions

Solution 5

Minor differences from other solutions, but x and y are created on the heap, X2 and Y2 are created on the stack. When this program exits, x and y will leak memory and the class destructors will not execute. X2 and Y2 destructors will execute and there will be no memory leaks with them.

Posted 7-Apr-13 16:41pm

H.Brydon

Comments

josda1000 8-Apr-13 11:06am

Getting a 5 for talking about memory leaks.

H.Brydon 8-Apr-13 11:35am

Thank you, will return the favor when I can.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-04-05T03:24:00

Solution 1

With "new", you create an object on heap, and the pointer to its object on stack. Without "new" new, you create the object itself on stack. The work with objects on stack and on heap lie deep in the fundamentals of the technology, you will need to understand it all extremely well, to do any programming at all.

—SA

Posted 5-Apr-13 3:24am

Sergey Alexandrovich Kryukov

Comments

Kenneth Haugland 5-Apr-13 9:29am

HE made both objects on the heap and stack with his little snipplet didnt he?

Sergey Alexandrovich Kryukov 5-Apr-13 9:33am

No, please look again. He has x, y (heap), X2, Y2 (stack). I also thought there were only on heap at first glance...
—SA

CPallini 5-Apr-13 9:42am

5.

Sergey Alexandrovich Kryukov 5-Apr-13 10:11am

Thank you, Carlo.
—SA

nv3 · Accepted Answer · 2013-04-05T03:24:00

With new you allocate a piece of memory from the heap you get a pointer to that memory. That implies that you are responsible for deleting that memory some time later.

The syntax

C++

Point x2;

allocates a piece of memory on the stack and refer to it with the x2 variable. When your function returns the memory will automatically be freed. (And hence you should never return the address of such a stack object!).

Allocation on the stack is faster than allocation on the heap. However the size of the stack is usually much smaller than the memory of the heap. Hence you should allocate huge chunks of memory from the heap.

CPallini · Accepted Answer · 2013-04-05T03:26:00

Solution 3

In the first case (new) you are dynamically allocating memory (on the heap) for the object and you have to explicitely delete it (I remark it since you did not delete it in your code).
In the second case (without new) you are allocating statically memory (on the stack).

Posted 5-Apr-13 3:26am

CPallini

Comments

Sergey Alexandrovich Kryukov 5-Apr-13 9:35am

Sure, a 5. I just did not find it appropriate to explain all aspects. If you start with it, you will probably need to explain many other things, how to use it; what happens if you have memory leaks, destructors, and a lot more.
—SA

josda1000 8-Apr-13 11:05am

This is getting a 5 for specifying the deletion necessity.

Richard MacCutchan · Accepted Answer · 2013-04-05T03:28:00

Solution 4

The variable x is declared as a pointer to an object, so must be initialised with the new operator, so it points to a new instance of the Point class.

The variable X2 is declared as a local instance of the object and automatically initialised, so it can be used as coded.

The same with y and Y2. If you step through your code with the debugger you will see what happens at every stage.

Posted 5-Apr-13 3:28am

Richard MacCutchan

Comments

CPallini 5-Apr-13 9:44am

My 5, however "so must be initialised with the new operator" is incorrect:
int i, * p; i = 5; p = &i;

Richard MacCutchan 5-Apr-13 9:55am

True, but I deliberately did not go into that as OP was asking for an explanation of the given code sample.

What is different between this declearation tow constructor

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