Difference between default copy constructor and user defined copy constructor

[Jeevang]

Hi,

We need copy constructor when we are passing the object to a function or if the function returns any object.
Suppose if we haven't defined the user defined copy constructor, it will give a call to default copy constructor. Won't it solve our purpose ?
Why we have to define the default copy constructor explicitely ??

Thanks,
Jeevan

[Meetee]

Hi,

We need copy constructor when we are passing the object to a function or if the function returns any object.
Suppose if we haven't defined the user defined copy constructor, it will give a call to default copy constructor. Won't it solve our purpose ?
Why we have to define the default copy constructor explicitely ??

Thanks,
Jeevan

Hi Jeevan,

C++ calls a copy constructor to make a copy of an object in each of the above cases. If there is no copy constructor defined for the class, C++ uses the default copy constructor which copies each field. So there is no need to define it explicitly. Default is default and not made defined by user.

Regards

[JosAH]

Hi Jeevan,

C++ calls a copy constructor to make a copy of an object in each of the above cases. If there is no copy constructor defined for the class, C++ uses the default copy constructor which copies each field. So there is no need to define it explicitly. Default is default and not made defined by user.

Regards

Whether or not there's a need for a user supplied copy ctor is entirely up on the
application. The default copy ctor builds a shallow copy of the original which may
not be what you want, i.e. maybe you need a deep copy instead in which case
you have to define a copy ctor yourself.

kind regards,

Jos

[Banfa]

The default copy constructor does a 'shallow' copy, that is it copies the values of the members from 1 object to the other.

However if one of those members happens to be a pointer to allocated memory then both objects end up with pointers to the same piece of memory.

This is a problem, imagine that that memory is deleted in the destructor. When one of the objects is deleted the memory will be deleted but the other object still has a pointer to it so it now has an invalid pointer.

In this case a 'deep' copy is required, this is a copy that takes account of the types of the member variables and where required allocates and copies the allocated memory.

I deep copy has to be hand coded specifically for a class and that is when you would define your own opy constructor an discard the compilers auto generated deafult one.

[Premier]

hey Banfa, you should write a book. your ans was very clear. i now understand when i should define my own copy constructor. if you do write a book email me [e-mail]