C++ copy constructor, with pointers

Question

When we define the copy constructor, is it required that we clear the contents of the object we are writing over? I am implementing a binary search tree, and wondering if I wouldn't have a memory leak unless I clean it up when implementing the copy constructor and the assignment operator.

Clear what? It hasn't been constructed yet, there isn't anything to clear. — Cory Kramer, Jan 04 '16 at 19:29
Unclear what you asking. Just ensure any allocation is paired with exactly one deallocation (new/delete, new[]/delete[], malloc/free, ...) — , Jan 04 '16 at 19:29
If you are defining a copy constructor then there is nothing to clean as you are building from scratch. — Jean-Baptiste Yunès, Jan 04 '16 at 19:29
What object you are writing over? Constructors are called on object creation. — Algirdas Preidžius, Jan 04 '16 at 19:30
Are you confusing copy constructor with assingment operator? — SergeyA, Jan 04 '16 at 19:36

score 6 · Accepted Answer · edited May 23 '17 at 11:59

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When we define the copy constructor, is it required that we clear the contents of the object we are writing over?

At time of construction there's no content that needs to be overwritten. Just make sure you're doing all of the copies of member variables in the constructor member initializer list, rather than doing in the constructor's body.

Otherwise the compiler would generate default initialization for members, and then there could be something allocated, that actually needs to be cleared and overwritten (or deep copied).

Especially if you're dealing with pointers as you claim to do.

Best you should follow The Rule of Three (Five) to ensure everything is done consistently.

edited May 23 '17 at 11:59

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answered Jan 04 '16 at 19:38

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Upvoting for actually making sense of the seemingly senseless question by bringing members into the picture. – SergeyA Jan 04 '16 at 19:39
@SergeyA: I shall try not to take that personally. – Lightness Races in Orbit Jan 04 '16 at 19:40
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@LightnessRacesinOrbit, how did you come into picture? And what happened to the red herring comment I was going to address? – SergeyA Jan 04 '16 at 19:41
@SergeyA: I came into the picture because you didn't want to also upvote my answer :( – Lightness Races in Orbit Jan 04 '16 at 19:42
That comment was a red herring so I deleted it ;) – Lightness Races in Orbit Jan 04 '16 at 19:42
@LightnessRacesinOrbit There are differences for implementations in the member initializer list vs copying in the constructors body? Don't see you mentioned that, neither the _red herrings_ so much? But have an upvote, so what :-P ... – πάντα ῥεῖ Jan 04 '16 at 19:45
@LightnessRacesinOrbit, I didn't expect someone with almost 200K rep to even think about reputation :) But I see no reason not to upvote it. – SergeyA Jan 04 '16 at 19:48
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@SergeyA: I didn't say anything about reputation! – Lightness Races in Orbit Jan 04 '16 at 19:49
@SergeyA It's more about accepting _correctness_ ... I don't care about the rep so much. – πάντα ῥεῖ Jan 04 '16 at 19:50
@LightnessRacesinOrbit, πάντα ῥεῖ - OK. I am still not clear what is considered a good answer here (for example, I was often downvoted for answers which I believe to be similar to LRiOs with comments - *useless* or *this should be a comment*. – SergeyA Jan 04 '16 at 19:53
@SergeyA: People are silly – Lightness Races in Orbit Jan 04 '16 at 20:05
@LightnessRacesinOrbit _"People are silly"_ I've been prooving this soo many times ;-) ... – πάντα ῥεῖ Jan 04 '16 at 20:09

score 4 · Answer 2 · answered Jan 04 '16 at 19:37

If a copy constructor is running, there is no "object we are writing over". You are creating ("constructing") a new object, from scratch, but copying data from some other object to populate its initial state.

For an assignment operator, yes this is definitely a consideration and you need to be sure to avoid leaks. Though, if you employ RAII so you don't have any manual memory management, then you have nothing to worry about. (Beyond that, I don't understand how pointers come into it.)

score 1 · Answer 3 · answered Jan 04 '16 at 19:53

As the others have stated, using the copy constructor will create a new object.
What you have to consider, tho:
If your original object is the owner of some other data e.g. foo* allocated with new or a shared_ptr, copying the object will simply copy the pointer and its adress.
You will not actually copy the owned data.
In case of a raw foo* and a delete foo in your destructor, the second object being destroyed will try to delete removed memory.

C++ copy constructor, with pointers

3 Answers3