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= operator overloading

General and Gameplay Programming Programming

Started by Floating July 03, 2003 09:38 PM

7 comments, last by Floating 20 years, 9 months ago

Floating

122

Author

July 03, 2003 09:38 PM

Hi, Can you please tell me what is wrong with the following class? Class CMyObject header:


void operator= (const CMyObject& obj);

Class CMyObject source:


void CMyObject::operator= (const CMyObject& obj)
{
   ... copy all values from obj to this
}

and somewhere else I have:


CMyObject objectA;
objectA.initialise(values...);
CMyObject objectB=objectA;

I always get my computer to crash. What is wrong? Is it possible that the above call doesn''t call the overloaded = operator? And what happens if = is not overloaded? What is the default behaviour? What would my objectB contain? Thanks a lot

daerid

354

July 03, 2003 09:42 PM

Yes it is. In this case:

CMyObject objectB=objectA;

you''re calling the copy constructor, because you''re initializing objectB with objectA.

To call the operator=, something like this is required:

CMyObject objectA;CMyObject objectB;objectB = objectA;

daerid@gmail.com

DemiGod25

122

July 03, 2003 09:44 PM

The problem is that you are returning void. I am pretty sure you have to return an object of that type for it to work.

Enigma1625

122

July 03, 2003 10:20 PM

yeah with assignment operators you need to return *this as a reference to a constant class.

unkn.Enigma1625

unkn.Enigma1625

Floating

122

Author

July 03, 2003 10:59 PM

quote:Original post by daerid
...
you''re calling the copy constructor, because you''re initializing objectB with objectA.
...

Will the copy constructor make objectB exactly identical to objectA?
Then if I point to an objectC in objectA, objectB will point to the same objectC (not a copy of it) right?

Anonymous

July 03, 2003 11:10 PM

you need to provide a copy constructor like this.

in CMyObject add

CMyObject( CMyObject &rhs)
{
*this = rhs;

}

Zipster

2,420

July 04, 2003 12:01 AM

quote:Original post by Floating
Will the copy constructor make objectB exactly identical to objectA?
Then if I point to an objectC in objectA, objectB will point to the same objectC (not a copy of it) right?

In the case of pointers inside your class, usually you will need to perform a deep copy, which actually makes a copy of what any pointers are pointing to for the object being assigned to. If you just copy the pointers, then you run into trouble at destruction time when both objects try to free the same pointer.

And "exactly identical" is a bit misleading. As I mentioned above, you will want to make copies of data that''s being pointed to. Obviously then the pointer values themselves will be different for each class, and nothing will be truly identical. However, rather than thinking of things in terms of identical, think in terms of operators. After the copy constructor is done, B == A should be true. Then you can implement the equality operator any way you wish and define "identical" in your own terms.

Floating

122

Author

July 04, 2003 12:19 AM

I understand now. Thanks a lot for your help

Lektrix

106

July 04, 2003 07:36 AM

Your class's assignment operator should check that the user is not assigning an object to the same object. Have a think about why, or have a search on Google for an example. You would generally do something like so:

class CMyObject{public:    ...    CMyObject& operator=( const CMyObject& rhs );    ...private:    ...}; CMyObject& CMyObject::operator=( const CMyObject& rhs ){    if ( this == &rhs )     // or if ( this != &rhs ) {do stuff}, return *this; ... or what ever your style is. You get the idea.        return *this;     // do stuff     return *this;}

[ Google || Start Here || ACCU || STL || Boost || MSDN || GotW || MSVC++ Library Fixes || BarrysWorld || E-Mail Me ]

[edited by - Lektrix on July 4, 2003 8:38:16 AM]

[ Google || Start Here || ACCU || STL || Boost || MSDN || GotW || CUJ || MSVC++ Library Fixes || BarrysWorld || [email=lektrix@barrysworld.com]E-Mail Me[/email] ]

= operator overloading

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