Copy constructor and assignment operator

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Hi, I thought I had got it right but am confused now. Say we have following code:

C++

MyClass object1;
MyClass object2 = object1; // I thought here Copy Constructor is called instead of = operator right??

// But here, if I have:
MyClass object3;
object3 = object1; // In this case = operator would be called right??

I am a bit confused because in my code I have mostly everywhere only MyClass object2 = object1; type of calls and still my assignment (=) operator gets called.

Any help? Thank you.

Posted 1-Apr-13 1:09am

Dan page

Updated 1-Apr-13 1:24am

v2

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3 solutions

Solution 1

If you want the copy constructor to be called, use

C++

MyClass object2 (object1);

Other than that, the copy constructor is called implicitly when transfering arguments by value or returning by value. Hence, a function

C++

MyClass myFunction (MyClass obj)
{
    ...
    return x;
}

would implicitly call the copy constructor for argument obj and for returning value x. Notice, however, that by modifying the function to

C++

MyClass& myFunction (MyClass& obj)
{
    ...
    return x;
}

the copy constructor will not be called. In these cases a reference is passed, which means internally that just a pointer is being transferred in and out.

Posted 1-Apr-13 2:01am

nv3

Comments

Dan page 1-Apr-13 8:04am

Hi, thanks for your reply. What gets called in such a case? MyClass object2 = object1; I guess the copy constructor also?

nv3 1-Apr-13 8:30am

In that case the object is first created by the default constructor and then the assignment operator is called.

Dan page 1-Apr-13 8:32am

No, I think that's not true :)

nv3 1-Apr-13 9:58am

You are right. If the assignment happens in an initialization part of a definition, the copy constructor will be called.

Solution 2

Hi,

Your copy constructor will be called, if the object does not exist. Otherwise your assignment operator is used to copy the values from one object to another already existing object.

In your question in both cases your assignment operator will be invoked because your object already exist.

Posted 1-Apr-13 2:14am

ramrooney

Comments

Dan page 1-Apr-13 8:24am

Hi, no not true in the first case object 2 does not exist so copy constructor should called.

Dan page 1-Apr-13 10:04am

no :) check last comment of nv3 also.
ps. THis line: MyClass object2 = object1; will call the Copy Constructor.

ramrooney 1-Apr-13 13:37pm

Yup. Dan page - you are correct, in the first case the object is not present, so the copy constructor will be invoked. Hope you are clear now.

SoMad 1-Apr-13 15:37pm

Sorry, you are right, I was wrong. Comment retracted.

Soren Madsen

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Dan page · Accepted Answer · 2013-04-01T02:25:00

Solution 3

ps. I've checked it seems I had leaked somewhere usage of a normal = operator
that is why the program was calling my = operator.
Otherwise, what I mentioned in my initial reply seem to be true.

Posted 1-Apr-13 2:25am

Dan page

Comments

SoMad 1-Apr-13 8:31am

Now I am confused.

Soren Madsen

Dan page 1-Apr-13 8:33am

Hi, meaning all the assumptions I had made in my comments (in the "code" section) in my initial post seem to be true.

SoMad 1-Apr-13 8:49am

I see. You wrote "...in my initial reply..." so I thought you were referring to the first reply you made to Solution 1.

Soren Madsen