VS2005 - question about copy-ctor

[Patrick Kowalzick]

Hello all,

find below an example which IMO should refuse to compile. It compiles fine
with VS2005.

What happens in the call?
What is your opinion?

Kind regards,
Patrick

class noncopyable
{
protected:
noncopyable() {}
~noncopyable() {}
private:
// hide the copy-ctor and assignment
noncopyable( noncopyable const & );
const noncopyable& operator=( noncopyable const & );
};
class A : public noncopyable // inherit to avoid copying
{
noncopyable copy_protection ; // make member to avoid copying
};
int main()
{
//A a;
//A b = a; // this fails to compile

A c = A(); // this compiles fine

return 0;
}

[Tom Widmer [VC++ MVP]]

Patrick Kowalzick wrote:

Hello all,

find below an example which IMO should refuse to compile. It compiles fine
with VS2005.

What happens in the call?
What is your opinion?
I agree that it shouldn't compile, and for 2 separate reasons:

1: A's default constructor cannot access the default constructor for the
member copy_protection - remember that protected access only gives you
access to protected members of bases through pointers, references and
objects of the derived class, not the base class (this is to prevent one
derived class from breaking objects of other derived classes derived
from the same base). As a result, A doesn't have a well-formed default
constructor, so even "A a;" shouldn't compile. This is covered in 11.5/1
of the standard.

Delete the member copy_protection to get around this, and try a
different compiler to get the error in the first place.

2: In the call, "c" is copy initialized from a default constructed A.
The compiler is allowed to optimize out the copy, but according to the
standard it must check that the copy could be made (e.g. that there is
an accessible copy constructor). This is covered in 12.2/1 of the standard.

Again, use a different compiler to get the error.
class A : public noncopyable // inherit to avoid copying
{
noncopyable copy_protection ; // make member to avoid copying
};
Should be:

class A : noncopyable // no need for public inheritance
{
};


int main()
{
//A a;
//A b = a; // this fails to compile

A c = A(); // this compiles fine

return 0;
}

You may want to report the bugs to MS.

Tom

[Patrick Kowalzick]

Hello Tom,

I agree that it shouldn't compile, and for 2 separate reasons:

1: A's default constructor cannot access the default constructor for the
member copy_protection - remember that protected access only gives you
access to protected members of bases through pointers, references and
objects of the derived class, not the base class (this is to prevent one
derived class from breaking objects of other derived classes derived from
the same base). As a result, A doesn't have a well-formed default
constructor, so even "A a;" shouldn't compile. This is covered in 11.5/1
of the standard.

Delete the member copy_protection to get around this, and try a different
compiler to get the error in the first place.
Normally, I do not use a member copy_protection . It was just to test VS2005
:).
2: In the call, "c" is copy initialized from a default constructed A. The
compiler is allowed to optimize out the copy, but according to the
standard it must check that the copy could be made (e.g. that there is an
accessible copy constructor). This is covered in 12.2/1 of the standard.

Again, use a different compiler to get the error.
VS2005 is the first, where I do not get the error. I was a little bit
surprised, as this is quite basic stuff. But perhaps MS will launch a 8.1
soon :). 8.0 is a little bit annoying sometimes.

class A : public noncopyable // inherit to avoid copying
{
noncopyable copy_protection ; // make member to avoid copying
};

Should be:

class A : noncopyable // no need for public inheritance
{
};

True.
You may want to report the bugs to MS.

Yes, I want. There is antoherone I want to report, but I do not know how. I
googled a little bit, but was not successful, yet.

Regards,
Patrick

[Bruno van Dooren]

You may want to report the bugs to MS.

Yes, I want. There is antoherone I want to report, but I do not know how.
I googled a little bit, but was not successful, yet.

You can do that here:
http://lab.msdn.microsoft.com/produc...k/default.aspx

Then, after you did that you can post the url here so that we can validate
it and vote for it.

--

Kind regards,
Bruno van Dooren
br************* *********@hotma il.com
Remove only "_nos_pam"

[Patrick Kowalzick]

Hello all,

You may want to report the bugs to MS.

Yes, I want. There is antoherone I want to report, but I do not know how.
I googled a little bit, but was not successful, yet.

You can do that here:
http://lab.msdn.microsoft.com/produc...k/default.aspx

Then, after you did that you can post the url here so that we can validate
it and vote for it.

I posted it here:
http://lab.msdn.microsoft.com/Produc...ckid=FDBK47765

I wanted to refer to
http://lab.msdn.microsoft.com/produc...ckid=FDBK18668

but I did not manage it. I opened a new one, because the latter one refers
to the beta-version.

Regards,
Patrick

[Tom Widmer [VC++ MVP]]

Patrick Kowalzick wrote:

Hello all,

You may want to report the bugs to MS.

Yes, I want. There is antoherone I want to report, but I do not know how.
I googled a little bit, but was not successful, yet.

You can do that here:
http://lab.msdn.microsoft.com/produc...k/default.aspx

Then, after you did that you can post the url here so that we can validate
it and vote for it.

I posted it here:
http://lab.msdn.microsoft.com/Produc...ckid=FDBK47765

I wanted to refer to
http://lab.msdn.microsoft.com/produc...ckid=FDBK18668

but I did not manage it. I opened a new one, because the latter one refers
to the beta-version.

Just to clarify, the following program should *not* compile - if it does
in VS2005 (which I don't have handy), it's another bug which should be
reported:

class Foo
{
protected:
Foo(){}
};

class Bar: public Foo
{
Foo f;
};

int main()
{
Bar b;
}

Tom

[Carl Daniel [VC++ MVP]]

Tom Widmer [VC++ MVP] wrote:

Just to clarify, the following program should *not* compile - if it
does in VS2005 (which I don't have handy), it's another bug which should
be
reported:

class Foo
{
protected:
Foo(){}
};

class Bar: public Foo
{
Foo f;
};

int main()
{
Bar b;
}

It does compile with VC8. Looks like another bug is needed.

-cd

[Patrick Kowalzick]

> Just to clarify, the following program should *not* compile - if it does

in VS2005 (which I don't have handy), it's another bug which should be
reported:

class Foo
{
protected:
Foo(){}
};

class Bar: public Foo
{
Foo f;
};

int main()
{
Bar b;
}

I posted it here:

http://lab.msdn.microsoft.com/Produc...ckId=FDBK47867

I messed up the class namig, but it should be understandable :).

Regards,
Patrick

[Tom Widmer [VC++ MVP]]

Patrick Kowalzick wrote:

Just to clarify, the following program should *not* compile - if it does
in VS2005 (which I don't have handy), it's another bug which should be
reported:

class Foo
{
protected:
Foo(){}
};

class Bar: public Foo
{
Foo f;
};

int main()
{
Bar b;
}

I posted it here:

http://lab.msdn.microsoft.com/Produc...ckId=FDBK47867

I messed up the class namig, but it should be understandable :).

Great. It's about time I installed VC8 I think, then I could at least
validate the bug.

Tom

[Patrick Kowalzick]

Hello all,

>You may want to report the bugs to MS.

Yes, I want. There is antoherone I want to report, but I do not know
how. I googled a little bit, but was not successful, yet.

You can do that here:
http://lab.msdn.microsoft.com/produc...k/default.aspx

Then, after you did that you can post the url here so that we can
validate it and vote for it.

I posted it here:
http://lab.msdn.microsoft.com/Produc...ckid=FDBK47765

I wanted to refer to
http://lab.msdn.microsoft.com/produc...ckid=FDBK18668

but I did not manage it. I opened a new one, because the latter one
refers to the beta-version.

MS marked the bug as resolved with the note that
A a = A();

is a direct initialization. As I wondered what the difference between this
and
A a;

shall be, I just posted in comp.lang.c++.m oderated.

Regards,
Patrick