C++ what does & do in overloaded assignment operator declaration?

Question

Possible Duplicate:
Why does the use of ‘new’ cause memory leaks?

What is the difference between (if there is one):

   Player player=*(new Player());

and:

   Player &player=*(new Player());

Both (seem to) behave the same way, but I surely miss something?!?

This must have been asked an answered many many times before? — The Unfun Cat, Nov 12 '12 at 21:01
@Joe: The second one isn't _inherently_ a memory leak, it can be freed properly, if oddly. — Mooing Duck, Nov 12 '12 at 21:02
....and what's wrong with `Player player;`? :P Looks like someone's stuck in a Java mindset, unaware that an object can exist without you saying `new`. — cHao, Nov 12 '12 at 21:02
@Joe As [Armen's answer says](http://stackoverflow.com/a/13351496/845092) `delete &player;` — Mooing Duck, Nov 12 '12 at 21:06
@cHao Not able to vote your answer up, but your answer was most helpful actually. Thx. C#, BTW. :) — mipesom, Nov 12 '12 at 21:16
@MooingDuck Thanks, I don't go around taking the address of what a reference is pointing to and deleting them very often (read never). — Joe, Nov 12 '12 at 21:16
@Joe: Oh, I agree. Such a thing is absolutely foolish and should never be done. But that's different from impossible. — Mooing Duck, Nov 12 '12 at 21:38

score 4 · Accepted Answer · answered Nov 12 '12 at 21:00

4

The difference is that the first makes a copy, whereas the second creates a reference to the object pointed to by the pointer returned by new Player().

Player player=*(new Player());

copy-initializes player using the copy-constructor.

Player &player=*(new Player());

just creates an alias for *(new Player()), which is valid because new Player() isn't a temporary. Player& player = Player() would be illegal because of that.

They're the same in that they both suck.

answered Nov 12 '12 at 21:00

Luchian Grigore

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Thanks for the quick reply. Why does the second suck? – mipesom Nov 12 '12 at 21:07
@mipesom because of the memory leak (and fixing the memory leak is ugly) and because it's not needed. You can simply write `Player player = Player()` to the same effect - no need for `new`. – Luchian Grigore Nov 12 '12 at 21:09
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@LuchianGrigore: Why on Earth would you write `Player player = Player();` instead of simply writing `Player player;`? – Armen Tsirunyan Nov 12 '12 at 21:09
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@ArmenTsirunyan to guarantee the same effect. If `Player` is a POD type, the latter doesn't initialize the members, while the former does. – Luchian Grigore Nov 12 '12 at 21:10
@LuchianGrigore: I see. It would be strange for a class named `Player` to be a POD type though... – Armen Tsirunyan Nov 12 '12 at 21:12
@ArmenTsirunyan it would, but I prefer being thorough. :) – Luchian Grigore Nov 12 '12 at 21:13
@LuchianGrigore Or `Player player{};`, right? – Joseph Mansfield Nov 12 '12 at 21:14
@sftrabbit C++11 is mostly a mystery to me. :D – Luchian Grigore Nov 12 '12 at 21:15

score 2 · Answer 2 · answered Nov 12 '12 at 21:01

2

new Player() is an expression that creates an object (unnamed) on the so-called heap. The result of the expression is a pointer to the newly created object. Now when you do

Player player = *(new Player())

you define a variable named player that is a copy of the newly created object. Moreover, you've lost all handles (access) to the heap object, and you can never free the memory it occupies.

On the other hand,

Player &player=*(new Player());

creates a reference named player to the newly created object. Thus, you have access to that object. In particular, you can free the memory and destroy that object by

delete &player;

answered Nov 12 '12 at 21:01

Armen Tsirunyan

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I was about to write the same thing, but you can actually free the memory in the first case (granted, in an ugly, UGLY way, but still possible). – Luchian Grigore Nov 12 '12 at 21:03
@LuchianGrigore: How? You intrigued me :) – Armen Tsirunyan Nov 12 '12 at 21:05
Well, if you have the copy constructor of `Player` hold a reference to the original object, you can delete it in the same way you `delete &player;`. – Luchian Grigore Nov 12 '12 at 21:06
@LuchianGrigore: Ah... I thought the trick could be done without touching the Player type... – Armen Tsirunyan Nov 12 '12 at 21:07

C++ what does & do in overloaded assignment operator declaration?

2 Answers2