class object initialisation

[A]

Hi,

I have always been taught to use an inialization list for initialising data
members of a class. I realize that initialsizing primitives and pointers use
an inialization list is exactly the same as an assignment, but for class
types it has a different effect - it calls the copy constructor.

My question is when to not use an initalisation list for initialising data
members of a class?
Regards
Adi

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[tom_usenet]

On Mon, 15 Sep 2003 14:14:13 +0300, "Attila Feher"
<at**********@l mf.ericsson.se> wrote:

Karl Heinz Buchegger wrote:

Ying Yang wrote:

int i; //declared but not initialised

wrong. initialized to an undetermined value. Initialization.

Wrong. This i, if it has an automatic storage, is not initialized.
Accessing it for anything else then writing (assigment or initialization) it
(well, reading it in any way) is undefined behavior.

The lvalue to rvalue conversion is undefined for indeterminate values
(use of the term "initialize d" has been recognized as a defect in the
standard).

According to other parts of the standard (see my other posts), the
above does count as initialization to an indeterminate value.

Tom

[Gary Labowitz]

"Karl Heinz Buchegger" <kb******@gasca d.at> wrote in message
news:3F******** *******@gascad. at...

Attila Feher wrote:

Karl Heinz Buchegger wrote:

Ying Yang wrote:
>
>
> int i; //declared but not initialised

wrong. initialized to an undetermined value. Initialization.
Wrong.

OK.
(it was just a wild guess, apologies for that).

This i, if it has an automatic storage, is not initialized.

Accessing it for anything else then writing (assigment or

initialization)
If it has not been initialized, how can it be initialized later?
I thought that initialization can only occour when a variable
is created. Afterwards it is always assignment.

Would this allow for assigning a value to a const variable during
construction of the object containing the variable? (Or does it have to be
initialized in the declaration statement?)
Assigning the initial value of a variable (even const, called "final") is
allow in (for example) Java. They appear to keep track of a variable's state
as "uninitiali zed" and define the assigning of a value during construction
as initialization. Does C++ do this? If it did, then calling a constructor
as part of an initialization list is perfectly valid; and I think it is!
--
Gary

[tom_usenet]

On Fri, 12 Sep 2003 16:34:29 -0400, "jeffc" <no****@nowhere .com>
wrote:

"Gianni Mariani" <gi*******@mari ani.ws> wrote in message
news:bj******* *@dispatch.conc entric.net...

Erik wrote:
...

>>should go there. Obviously, you can't put something like this in there:
>>if (a)
>> i = 2;
>>else
>> i = 4;
>
>
> Many, if not most, of those cases can be covered by the ?: operator:
> MyClass(int a) : i(a ? 2 : 4) {}
>

Yep - and sometimes it makes sense to create a function that does more
complex things so that everything is in the initializer list.

And how exactly do you propose calling such a function from the initializer
list?

How do you think?

MyClass(int a): i(someFunction( a))
{
}

someFunction might be a member (static or otherwise), or a namespace
scope function.

Tom

[Attila Feher]

tom_usenet wrote:

specified for an object, the object and its subobjects, if any, have
an indeterminate initial value;"

In other words, if you don't explicitly initialize an int (which is a
POD type), it is initialized with an indeterminate initial value.

No "in other words". They will have an "indetermin ate initial value". They
are *not* initialized. Furthermore (let's not force me to look up the
paragraph) accessing such an uninitialized variable is undefined behavior.
(BTW it is in 4.1 paragraph 1)

--
Attila aka WW

[Attila Feher]

tom_usenet wrote:

On Mon, 15 Sep 2003 14:14:13 +0300, "Attila Feher"
<at**********@l mf.ericsson.se> wrote:

Karl Heinz Buchegger wrote:

Ying Yang wrote:
int i; //declared but not initialised

wrong. initialized to an undetermined value. Initialization.

Wrong. This i, if it has an automatic storage, is not initialized.
Accessing it for anything else then writing (assigment or
initialization) it (well, reading it in any way) is undefined
behavior.

The lvalue to rvalue conversion is undefined for indeterminate values
(use of the term "initialize d" has been recognized as a defect in the
standard).

According to other parts of the standard (see my other posts), the
above does count as initialization to an indeterminate value.

If that is what the standard is being changed to, then it is pretty silly.
Initialization is an _action_ being taken. So we say (for automatic/member
PODs): if they are are not initialized then we call this initialization
which never happened as initialization to an indetermined value. Great. It
goes to the deepness of the quantum physics. Furthermore: it has a value,
but that value cannot be read... That suggests me that it is *not*
initialized. It has an indetermined, and non-determinable value. Like the
female-mind vs. male-mind. ;-)

--
Attila aka WW

[Agent Mulder]

<Shane Beasley>

it doesn't matter what your opinion (or mine) is; neither of us will
learn anything about C++ that we didn't already know before this
thread </>

I do learn from this thread. There are always people lurking. And,
for once, it's on topic!
"Arguing on the Internet is like competing in the Special Olympics...
Even if you win, you're still retarded."

:-)

-X

[tom_usenet]

On Mon, 15 Sep 2003 15:29:36 +0300, "Attila Feher"
<at**********@l mf.ericsson.se> wrote:

tom_usenet wrote:

specified for an object, the object and its subobjects, if any, have
an indeterminate initial value;"

In other words, if you don't explicitly initialize an int (which is a
POD type), it is initialized with an indeterminate initial value.

No "in other words". They will have an "indetermin ate initial value". They
are *not* initialized. Furthermore (let's not force me to look up the
paragraph) accessing such an uninitialized variable is undefined behavior.
(BTW it is in 4.1 paragraph 1)

You snipped these words from my post (reproduced in paraphase):

"When no initializer is specified for an object of class type the
object is initialized as specified in 8.5."

Yes, when you don't give an initializer, it is still initialized.
"Initialize r" and "Initialization " are different things. Even if you
don't use an initializer, initialization still takes place.

The lvalue to rvalue thing applies to things that have been
initialized too:

int* p = new int;
delete p; //p now indeterminate (but obviously initialized)
int* q = p; //error, lvalue-to-rvalue conversion

Tom

[tom_usenet]

On Mon, 15 Sep 2003 15:35:09 +0300, "Attila Feher"
<at**********@l mf.ericsson.se> wrote:

If that is what the standard is being changed to, then it is pretty silly.
Initializati on is an _action_ being taken. So we say (for automatic/member
PODs): if they are are not initialized then we call this initialization
which never happened as initialization to an indetermined value. Great. It
goes to the deepness of the quantum physics. Furthermore: it has a value,
but that value cannot be read... That suggests me that it is *not*
initialized. It has an indetermined, and non-determinable value. Like the
female-mind vs. male-mind. ;-)

If you choose your words carefully it makes more sense: "If we don't
provide an initializer for a POD, the object is initialized to an
indeterminate value."

"initialize " and "provide an initializer for" are best thought of as
different things. initialize is something the compiler (or exe) does
to a variable. It does it whether you provide an initializer or not.

Tom

[Agent Mulder]

<tom_usenet>

If you choose your words carefully it makes more sense

I am not very into this business of initialization (hard to type)
and assignment, so I try to understand it in a metaphore. The
magician with the White rabbit in the hat. First, there is no hat.
It is flat, doesn't need space. Then, the magician pops up the
hat. It takes up space and provides space. But it is 'empty'.
Is it? Is there a rabbit in the hat?

1. Yes, the rabbit was nicely folded flat in the hat.
2. No, the magician is about to put the rabbit in the hat.

For assignment, putting another rabbit in the hat, we must
visualize a flapdoor at the top of the hat. Is a rabbit POD or
is that not the question?

-X

[Agent Mulder]

Excuse me, I must add a third possibility (and a fourth)
I am not very into this business of initialization (hard to type)
and assignment, so I try to understand it in a metaphore. The
magician with the White Rabbit in the hat. First, there is no hat.
It is flat, doesn't need space. Then, the magician pops up the
hat. It takes up space and provides space. But it is 'empty'.
Is it? Is there a rabbit in the hat?

1. Yes, the rabbit was nicely folded flat in the hat.
2. No, the magician is about to put the rabbit in the hat.
3.. No, but the rabbit will jump in by itself
4. The Lovely Lady will put the rabbit in the hat

For assignment, putting another rabbit in the hat, we must
visualize a flapdoor at the top of the hat. Is a rabbit POD or
is that not the question?

-X