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tpochep@mail.ru
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 Posted: Fri Jun 17, 2005 10:23 am Post subject: implicitly-defined copy ctor |
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Hello.
struct A
{
A(int & r) : ri_(r){}
int & ri_;
};
int main()
{
int i = 0;
A a(i);
A b(a);
}
Clause 12 (12.8/ says:
The implicitly-defined copy constructor for class X performs a
memberwise copy of its subobjects. The
order of copying is the same as the order of initialization of bases
and members in a user-defined constructor
(see 12.6.2). Each subobject is copied in the manner appropriate to its
type:
- if the subobject is of class type, the copy constructor for the
class is used;
- if the subobject is an array, each element is copied, in the manner
appropriate to the element type;
- if the subobject is of scalar type, the built-in assignment
operator is used.
Virtual base class subobjects shall be copied only once by the
implicitly-defined copy constructor (see
12.6.2).
What about references?
AFAIK references are not scalar types. Should I explicitly declare copy
ctor? Or compiler-generated is everywhere (everywhere == all
implementations) good?
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Ben Hutchings
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| Posted: Sat Jun 18, 2005 9:30 am Post subject: Re: implicitly-defined copy ctor |
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<tpochep (AT) mail (DOT) ru> wrote:
<snip>
| Quote: | Clause 12 (12.8/ says:
The implicitly-defined copy constructor for class X performs a
memberwise copy of its subobjects. The
order of copying is the same as the order of initialization of bases
and members in a user-defined constructor
(see 12.6.2). Each subobject is copied in the manner appropriate to its
type:
- if the subobject is of class type, the copy constructor for the
class is used;
- if the subobject is an array, each element is copied, in the manner
appropriate to the element type;
- if the subobject is of scalar type, the built-in assignment
operator is used.
Virtual base class subobjects shall be copied only once by the
implicitly-defined copy constructor (see
12.6.2).
What about references?
AFAIK references are not scalar types.
|
Nor are they objects!
| Quote: | Should I explicitly declare copy ctor? Or compiler-generated is
everywhere (everywhere == all implementations) good?
|
The implicitly-defined copy constructor will copy members of reference
type along with the other members. I'm not aware of any
implementations that fail to do this.
--
Ben Hutchings
Having problems with C++ templates? Your questions may be answered by
<http://womble.decadentplace.org.uk/c++/template-faq.html>.
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tpochep@mail.ru
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| Posted: Mon Jun 20, 2005 9:16 am Post subject: Re: implicitly-defined copy ctor |
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Hello.
Ben Hutchings пиÑ?аЛ(а):
| Quote: | Nor are they objects!
|
Yes, I know.
| Quote: | The implicitly-defined copy constructor will copy members of reference
type along with the other members. I'm not aware of any
implementations that fail to do this.
|
Yes, all my compilers do the same, but is this behaviour defined by
standard?
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kanze@gabi-soft.fr
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| Posted: Mon Jun 20, 2005 3:34 pm Post subject: Defect report: handling of reference members not specified i |
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Ben Hutchings wrote:
| Quote: | tpochep (AT) mail (DOT) ru> wrote:
snip
Clause 12 (12.8/ says:
The implicitly-defined copy constructor for class X performs a
memberwise copy of its subobjects. The
order of copying is the same as the order of initialization of bases
and members in a user-defined constructor
(see 12.6.2). Each subobject is copied in the manner appropriate to its
type:
- if the subobject is of class type, the copy constructor for the
class is used;
- if the subobject is an array, each element is copied, in the manner
appropriate to the element type;
- if the subobject is of scalar type, the built-in assignment
operator is used.
Virtual base class subobjects shall be copied only once by the
implicitly-defined copy constructor (see
12.6.2).
What about references?
AFAIK references are not scalar types.
Nor are they objects!
|
Nor are they mentionned in the above paragraph.
| Quote: | Should I explicitly declare copy ctor? Or compiler-generated
is everywhere (everywhere == all implementations) good?
The implicitly-defined copy constructor will copy members of
reference type along with the other members. I'm not aware of
any implementations that fail to do this.
|
I think the original poster has a point. All of the
implementations I know do copy initialize member references, and
I can't imagine that anything else was intended. But what the
standard actually says is that "the implicitly-defined copy
constructor for class X performs a memberwise copy of its
subobjects." References aren't objects; can they be subobjects?
If not, then the standard doesn't say what an implementation
should do with them, and when the standard doesn't specify
something, it's undefined behavior.
I suspect that this is a defect in the standard. Minor, since
we all know what is wanted (and all implementations seem to
agree with us), but technically a defect. The first sentence
should probably read "[...] a memberwise copy of its subobjects
and members of reference type," and there should be an
additional point for references.
I've added comp.std.c++ to the groups.
--
James Kanze GABI Software
Conseils en informatique oriente objet/
Beratung in objektorientierter Datenverarbeitung
9 place Smard, 78210 St.-Cyr-l'cole, France, +33 (0)1 30 23 00 34
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tony_in_da_uk@yahoo.co.uk
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| Posted: Tue Jun 21, 2005 4:12 pm Post subject: Re: implicitly-defined copy ctor |
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References are little different to pointers in terms of storage and
copying issues - basically a plain-old-data (POD) type storing a number
that happens to be a memory address. In the list above I'm sure
they're both intended to be covered by scalar types, and IMHO the
standard's wording is fine. Subobjects is obviously used in a general
english sense, and defined by the list provided in the quoted section
of the standard.
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