copy costructor

[Geevi]

Hi all,

Differentiate between copy constructor and assignment operator?

Explain each with an example.

Thanks in advance....

[willakawill]

Hi all,

Differentiate between copy constructor and assignment operator?

Explain each with an example.

Thanks in advance....

A constructor is used when a class is declared and parameters in brackets are used to initialize variables in the constructor

MyClass obj1("hello");

A copy constructor uses the data already in another instance of this class to initialize the variables in the new object

MyClass obj1("hello");
Myclass ojb2(obj1);

An overloaded operator is used to copy the values in the member variables of one instance into the member variables of a second instance after both instances have been initialized

MyClass obj1("hello");
MyClass obj2("banana");
 
obj2 = obj1;

the '=' operator must be overloaded to accomodate this assignment.

[Banfa]

the '=' operator must be overloaded to accomodate this assignment.

Strictly speaking not quite true, the compiler will produce a default assignment operator (and copy constructor) if you don't explicitly write one. However it may not do the right thing as it just does a straight member to member copy and if some of the members are pointers with allocated memory you will end up with 2 classes both pointing to the same allocated memory. This will probably result in an exception at some point after 1 of the objects has been release (and as presumable freed the memory) when the other object tries to access it.

[willakawill]

Strictly speaking not quite true, the compiler will produce a default assignment operator (and copy constructor) if you don't explicitly write one. However it may not do the right thing as it just does a straight member to member copy and if some of the members are pointers with allocated memory you will end up with 2 classes both pointing to the same allocated memory. This will probably result in an exception at some point after 1 of the objects has been release (and as presumable freed the memory) when the other object tries to access it.

Correct. So, strictly speaking, never use default constructors, copy constructors, destructors or operators. Unless, of course, you are fond of chasing random memory errors :)

[Banfa]

Correct. So, strictly speaking, never use default constructors, copy constructors, destructors or operators. Unless, of course, you are fond of chasing random memory errors :)

Probably very wise advice, however people generally do not write copy constructors and assignment operators for all there structures which they would have to in order to follow it.

[Ganon11]

In general, a copy constructor, assignment operator, and destructor must be written for classes involving pointers to avoid shallow copy of pointers (as Banfa described above). I'm not sure they're 100% necessary for pointer-less classes, but if your problem specification requires special assignment, copy, etc, then it's not a bad thing to overload these operations/constructors.

[willakawill]

Probably very wise advice, however people generally do not write copy constructors and assignment operators for all there structures which they would have to in order to follow it.

And who are these 'people' for whom you speak? :)

Of course I would only write a constructor, copy constructor, destructor, overloaded operator in circumstances that demand their use. Never just for the sake of it.

[r035198x]

Hi all,

Differentiate between copy constructor and assignment operator?

Explain each with an example.

Thanks in advance....

Has someone just done someone's homework?

[willakawill]

Has someone just done someone's homework?

Dammit you're right!!
I usually avoid those and didn't see it.
Glad somebody is awake :)

[Geevi]

Thanks to all,