Avoid automatic copy constructor generation

[=?ISO-8859-1?Q?Marcel_M=FCller?=]

Hi,

is there a way to avoid the automatic copy constructor generation. I do
not want the object to be non-copyable. I simply do not want that
copying is done by the default copy constructor. But there is a
constructor that accepts the base class. This one should be used for
copying.

In fact i have a set of classes with a common abstract base. The
implementations do not have own data members. They only implement
different functionality. But is makes sense to do assignments and copy
construction between these set of types. This effectively changes the
functionality depending on the current type.
class ContainerBase
{ // ...
};

class Container1
: public ContainerBase
{ // no own data members!
public:
Container1()
{}
Container1(cons t ContainerBase& r)
: ContainerBase(r )
{}
Container1& operator=(const ContainerBase& r)
{ ContainerBase:: operator=(r); return *this; }
};

class Container2
: public ContainerBase
{ // no own data members!
public:
Container2()
{}
Container2(cons t ContainerBase& r)
: ContainerBase(r )
{}
Container2& operator=(const ContainerBase& r)
{ ContainerBase:: operator=(r); return *this; }
};
int main()
{ Container1 c1;
Container2 c2 = c1; // OK
Container1 c3 = c1; // Wrong! Invokes Container1(cons t Contarine1& r)
return 0;
}
Of course, I could implement the copy constructors to do the same thing
as the constructor from const ContainerBase&. But this is really
unneeded redundancy. And the copy constructor is much more complicated
in the real application than in this small example. Furthermore the auto
generated copy constructor is unusable. It has undefined behavior
because of some special internal storage dependencies. It also has to do
with multiple inheritance, so I could not move the code to a member
function either.

Is there a way to avoid the required redefinition of the copy
constructor for each class?
Marcel

[Ian Collins]

Erik Wikström wrote:

On 2008-10-27 19:29, Jeff Schwab wrote:

>Juha Nieminen wrote:

>>Ian Collins wrote:
I can see the benefit of "= default", but what benefits does "= delete"
offer over private constructors?
Declaring a constructor private is more a hack than a clean idiom. If
you accidentally call the copy constructor from the class itself, you
will not get a clean compiler error, but an obscure linker error, which
is not really the best possible solution.

That's true if the private constructors are declared directly in the
non-copyable class. I wonder whether =delete offers anything over
EBO/inheritance-based utilities like boost::noncopya ble.

Elegance and simplicity. A private copy-constructor is a hack but
boost::noncopya ble is much worse, a complex solution to a simple
problem. Why should I have to inherit from some other class just to make
my class non-copyable?

It may be a hack, but it's an idiomatic one that's been used since the
epoch of C++.

I don't consider it to be a hack, but an aid to documentation. The
reader can tell at a glance that the class isn't copyable. It also
saves typing!

--
Ian Collins

[Juha Nieminen]

Pete Becker wrote:

>If the member
variable is just eg. an int, then it the default copying would be enough
for it, so calling the compiler-generated copy constructor would
automaticall y copy it without having to do anything special about it.
(The user-written copy constructor could then simply concentrate on the
members which do need special copying.)

So the default copy constructor would only copy members that were not
copied by the user-written one? How could it know?

No, you first call the compiler-generated copy constructor, and then
you copy the special members appropriately.

For example, if you had, let's say, a pointer to a dynamically
allocated object, the compiler-generated copy constructor would simply
copy the value of the pointer, so the copy would point to the same
dynamically allocated object. However, that doesn't matter: Once it has
done that, you simply make a deep copy of the object (or whatever) and
make the pointer point to that new object.

The idea is that you only need to write this special copying code for
that one pointer, and you don't need to write anything for any of the
other member variables (because those are handled by the
compiler-generated copy constructor).

[Jeff Schwab]

Ian Collins wrote:

Erik Wikström wrote:

>On 2008-10-27 19:29, Jeff Schwab wrote:

>>Juha Nieminen wrote:
Ian Collins wrote:
I can see the benefit of "= default", but what benefits does "= delete"
offer over private constructors?
Declaring a constructor private is more a hack than a clean idiom. If
you accidentally call the copy constructor from the class itself, you
will not get a clean compiler error, but an obscure linker error, which
is not really the best possible solution.
That's true if the private constructors are declared directly in the
non-copyable class. I wonder whether =delete offers anything over
EBO/inheritance-based utilities like boost::noncopya ble.

Elegance and simplicity. A private copy-constructor is a hack but
boost::noncopy able is much worse, a complex solution to a simple
problem. Why should I have to inherit from some other class just to make
my class non-copyable?

It may be a hack, but it's an idiomatic one that's been used since the
epoch of C++.

I don't consider it to be a hack, but an aid to documentation. The
reader can tell at a glance that the class isn't copyable. It also
saves typing!

Agreed. It's self-documenting, and doesn't require the reader to be
familiar with yet another language feature.

[Pete Becker]

On 2008-10-27 18:03:07 -0400, Juha Nieminen <no****@thanks. invalidsaid:

Pete Becker wrote:

>>If the member
variable is just eg. an int, then it the default copying would be enough
for it, so calling the compiler-generated copy constructor would
automatical ly copy it without having to do anything special about it.
(The user-written copy constructor could then simply concentrate on the
members which do need special copying.)

So the default copy constructor would only copy members that were not
copied by the user-written one? How could it know?

No, you first call the compiler-generated copy constructor, and then
you copy the special members appropriately.

Okay, you copy the special members twice.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[Juha Nieminen]

Jeff Schwab wrote:

> One problem with copy constructors (if you have implemented one
explicitly) is that each time you add a new member variable to your
class, you have to remember to add it to the copy constructor (and
assignment operator). This is extremely easy to forget.

You have a reasonable point, but IMO such a class is already responsible
for too many things. If a member requires special handling during
copying, then it can be wrapped in a class whose copy constructor does
the right thing. That special handling should be the wrapper's only
job.

That's a reasonable design. On one hand it simplifies the outer class,
because you don't have to write a copy constructor for the entire class.
On the other hand it adds a small layer of complexity to that special
member. For example assume you have something like this:

class MyClass
{
std::list<Objec tobjectList;
std::list<Objec t>::iterator currentObject;
...
};

Let's assume the specification of this code is that 'currentObject'
always points to either an object inside 'objectList', or to
objectList.end( ).

This class requires a copy constructor and assignment operator if
instances of this class must be copyable. The copy constructor and
assignment operator need to make 'currentObject' to point to the copied
list (rather than the original).

By your suggested design both 'objectList' and 'currentObject' need to
be wrapped inside an inner class if we want to avoid having to write a
copy constructor and assignment operator on the outer class, so it would
become something like:
class MyClass
{
struct ObjectList
{
std::list<Objec tobjectList;
std::list<Objec t>::iterator currentObject;

ObjectList(cons t ObjectList&); // Implement these appropriately
ObjectList& operator=(const ObjectList&);
};

ObjectList objectList;
...
};

Now accessing the 'objectList' member has to always be done with a
"objectList.obj ectList" (or whatever we want to call the struct
instantiation). Well, I suppose that's a small price to pay for not
having to write a copy constructor and assignment operator for the outer
class...

[Juha Nieminen]

Pete Becker wrote:

Okay, you copy the special members twice.

Basically yes. The alternative is the current one: You have to copy
everything explicitly.

[James Kanze]

On Oct 27, 2:05 am, Pete Becker <p...@versatile coding.comwrote :

On 2008-10-26 17:37:53 -0400, Ian Collins <[e-mail] msaid:

Pete Becker wrote:

On 2008-10-25 21:55:54 -0400, Victor Bazarov
<v.Abaza...@com Acast.netsaid:

>Juha Nieminen wrote:
Marcel Müller wrote:
is there a way to avoid the automatic copy constructor
generation .

>>Yes: Implement your own copy constructor. That will make
the compiler to not to create a default one.

>You don't have to implement it. Just declare one. If
your program does not use it, you don't have to define it.
And to make sure the program does not use it, declare it
'private'.

And a note for the future: with C++0X you don't need to
declare it private.

class C
{
public:
C(const C&) = delete;
};

The "= delete" says that it doesn't exist. There's also "=
default", to tell the compiler to generate the default
version even if it wouldn't otherwise. These also apply to
default constructors, generated assignment operators, etc.

I can see the benefit of "= default", but what benefits does
"= delete" offer over private constructors?

A bit of clarity. A private constructor can be implemented,
and used by members and friends. "= delete" says it doesn't
exist.

And can't be used even by members and friends.

Not a big thing, but the fact that you cannot copy an object is
part of the public interface. So the fact that you explicitly
say so should logically appear in the public part of the class
definition.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Richard Herring]

In message <Ga************ ***@read4.inet. fi>, Juha Nieminen
<no****@thanks. invalidwrites

>Pete Becker wrote:

>Okay, you copy the special members twice.

Basically yes. The alternative is the current one: You have to copy
everything explicitly.

How do you handle reference members?

--
Richard Herring

[James Kanze]

On Oct 27, 10:20 pm, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2008-10-27 19:29, Jeff Schwab wrote:

Juha Nieminen wrote:

Ian Collins wrote:
I can see the benefit of "= default", but what benefits
does "= delete" offer over private constructors?

Declaring a constructor private is more a hack than a clean
idiom. If you accidentally call the copy constructor from
the class itself, you will not get a clean compiler error,
but an obscure linker error, which is not really the best
possible solution.

That's true if the private constructors are declared
directly in the non-copyable class. I wonder whether
=delete offers anything over EBO/inheritance-based utilities
like boost::noncopya ble.

Elegance and simplicity. A private copy-constructor is a hack
but boost::noncopya ble is much worse, a complex solution to a
simple problem. Why should I have to inherit from some other
class just to make my class non-copyable?

Documentation. Inheritance defines an isA relationship. Your
class isA UncopiableObjec t.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[James Kanze]

On Oct 27, 11:03 pm, Jeff Schwab <j...@schwabcen ter.comwrote:

Ian Collins wrote:

Erik Wikström wrote:

On 2008-10-27 19:29, Jeff Schwab wrote:
Juha Nieminen wrote:
Ian Collins wrote:
I can see the benefit of "= default", but what benefits does "=delete"
offer over private constructors?
Declaring a constructor private is more a hack than a clean idiom.If
you accidentally call the copy constructor from the class itself, you
will not get a clean compiler error, but an obscure linker error, which
is not really the best possible solution.
That's true if the private constructors are declared directly in the
non-copyable class. I wonder whether =delete offers anything over
EBO/inheritance-based utilities like boost::noncopya ble.
Elegance and simplicity. A private copy-constructor is a hack but
boost::noncopya ble is much worse, a complex solution to a simple
problem. Why should I have to inherit from some other class just to make
my class non-copyable?

It may be a hack, but it's an idiomatic one that's been used
since the epoch of C++.

I don't consider it to be a hack, but an aid to
documentation. The reader can tell at a glance that the
class isn't copyable. It also saves typing!

Agreed. It's self-documenting, and doesn't require the reader
to be familiar with yet another language feature.

Yes and no. I used the techique back in the early 1990's. I
dropped it for awhile because readers weren't familiar with it.
The private copy constructor was the standard idiom, which they
immediately recognized, where as they didn't know what
UncopiableObjec t did; at the time, at least, it didn't occur to
them that a base class might have not direct interface, and they
expected something more. Thanks to things like std::iterator<> ,
std::unary_func tion<>, etc., modern C++ programmers are far more
open about this, and when they see a class called
UncopiableObjec t, they don't expect anything more than that it
can't be copied. Some I've reverted to doing things the way I
did 25 years ago.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]