Default Copy Constructor

[Bob]

Hi,

If a class contains only built-in types and standard library objects
such as strings and vectors, is it safe to let the compiler create the
copy constructor and assignment operator, or is it advised to always
write your own?

I know you need to write your own if there is any dynamic memory
allocation etc, but I was curious as to the "correct" way in the case
of static variables.

Thanks,
Bob.

[John Carson]

"Bob" <bo******@hotmail.com> wrote in message
news:77*************************@posting.google.co m

Hi,

If a class contains only built-in types and standard library objects
such as strings and vectors, is it safe to let the compiler create the
copy constructor and assignment operator
Yes.
or is it advised to always
write your own?

No. In fact it is safer to let the compiler do it since you might make a
mistake.

There are occasions when you might need to create your own versions (e.g.,
if you want to keep count of the number of objects created of a particular
class) but, unless you want to do something beyond mere construction or
assignment, the compiler-generated version is fine.
--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

[Rob Williscroft]

Bob wrote in news:77*************************@posting.google.co m in
comp.lang.c++:

Hi,

If a class contains only built-in types and standard library objects
such as strings and vectors, is it safe to let the compiler create the
copy constructor and assignment operator, or is it advised to always
write your own?

The rule is quite simple:

If all members have constructors, assigment operator and destructor that
"do the right thing(tm)" then the compiler generated ctor's etc
will do the right thing.

Typically pointers that point to dynamic memory don't obey this rule.
Another example would be the FILE * returned by fopen (it needs to
be closed by fclose).

Most of the types defined by the C++ part of the standard library
do obey this rule, however ther are some oddity's:

- std::auto_ptr< T > - it has strange copying behaviour (it moves).
- std::ostream et al - doesen't provide for copying (ctor or op = ).
I know you need to write your own if there is any dynamic memory
allocation etc, but I was curious as to the "correct" way in the case
of static variables.

static variables aren't part of an instance, they're globals (but globals
that are restricted to the classes scope) and don't need to be constructed
by the copy constructor or assigned to by the assignment operator.
Rob.
--
http://www.victim-prime.dsl.pipex.com/

[Victor Bazarov]

"John Carson" <do***********@datafast.net.au> wrote...

"Bob" <bo******@hotmail.com> wrote in message
news:77*************************@posting.google.co m

Hi,

If a class contains only built-in types and standard library objects
such as strings and vectors, is it safe to let the compiler create the
copy constructor and assignment operator

Yes.

Well, to be technically correct, _pointers_ _are_ in fact built-in types,
but somehow I think the OP didn't mean pointers. In any case, mentioning
"The Rule of Three" could also help.

V