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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[Ying Yang]

"Kevin Goodsell" <us************ *********@never box.com> wrote in message
news:f6******** *********@newsr ead3.news.pas.e arthlink.net...

A wrote:

To initialize means you give a variable it's initial value.
Agreed.

An assignment
operator can be used to give a variable it's initial value,

No, it can't. If you believe it can, please give an example.

int i; //declared but not initialised
i =5; // variable i is initialised with value 5.

and it can also
be used to re-assign a variable with a different value, providing the
variable is not defined as constant. The point is, you can initialise data members via two of the following ways:

Well, you can't give something its initial value twice, so I assume you
mean "one of the following ways".

what are you smoking?
...But you're still wrong. Assignment does not give an initial value to
something. Assignment can only happen to an existing object. Since the
object must exist prior to the assignment, it clearly has already
received its initial value.

Assignment can be used for both objects types and primitive types. And i was
referring to initialising data members of a class type. Again, what are you
smoking?

However, the behaviour of the two is different depending whether is it a
primitive or class type data member you want to initialize. If you don't
agree with my definition of assignment and initialisation then I like to
hear your definitions.

[Ying Yang]

> > 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

You need to have an item in the initialize list if it's a constant or
a reference data member.
You also need to have the base class constructor in the initializer
list if the base class does not have a default contructor.

If my class does not extend any class then there would be no base class your
referring to. Maybe you mean a default constructor of a class must always be
present when you use an initialisation list. Can you be more clear?
PPP

[Karl Heinz Buchegger]

Ying Yang wrote:

A wrote:

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.

No. It calls whatever constructor you specify in the initializer list.

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

Never.
Initialization lists are the only way to *initialize* members of a class
or to specify which constructor from a base class should be used.

No, it's not the only way - as i mentioned you can use an assignment
operator for initialising primitive data members, which is the same as using
an initialization list.

But then it is no longer *initialization *.
It is assignment.

Those are different things.

--
Karl Heinz Buchegger
kb******@gascad .at

[Karl Heinz Buchegger]

Ying Yang wrote:

int i; //declared but not initialised
wrong. initialized to an undetermined value. Initialization.
i =5; // variable i is initialised with value 5.

replacing the undetermined value with a determined value. Assignment.

Initialization happens when a variable comes to live. Note: A variable
can also be initialized to an undetermined value! Once a variable
is 'alive', you can't initialize it any more. But of course you
can assign values to it (except in some cases, eg. references).

--
Karl Heinz Buchegger
kb******@gascad .at

[Ying Yang]

> Ying Yang wrote:

int i; //declared but not initialised

wrong. initialized to an undetermined value. Initialization.

disagree.

Initialization of an object or primitive means to give a determined value to
them for the first time.

i =5; // variable i is initialised with value 5.

replacing the undetermined value with a determined value. Assignment.

Initialization happens when a variable comes to live. Note: A variable
can also be initialized to an undetermined value! Once a variable
is 'alive', you can't initialize it any more. But of course you
can assign values to it (except in some cases, eg. references).

In light of the above, this is no longer valid.

[Attila Feher]

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.

--
Attila aka WW

[Karl Heinz Buchegger]

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.

--
Karl Heinz Buchegger
kb******@gascad .at

[tom_usenet]

On Mon, 15 Sep 2003 20:41:24 +0930, "Ying Yang" <Yi******@hotma il.com>
wrote:

Ying Yang wrote:

>
>
> int i; //declared but not initialised

wrong. initialized to an undetermined value. Initialization.

disagree.

Initializati on of an object or primitive means to give a determined value to
them for the first time.

Your "definition " of initialization is at odds with the C++ standard:

"1 When no initializer is specified for an object of (possibly
cv*qualified) class type (or array thereof), or the
initializer has the form (), the object is initialized as specified in
8.5. [Note: if the class is a non*POD, it is
default*initial ized. ]"

Here's the bit in 8.5:
"9 If no initializer is specified for an object, and the object is of
(possibly cv*qualified) non*POD class type (or array thereof), the
object shall be default*initial ized; if the object is of
const*qualified type, the underlying class type shall have a
user*declared default constructor. Otherwise, if no initializer is
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.

Tom

[Attila Feher]

Karl Heinz Buchegger wrote:

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.

I was not clear. Of course it can only be assigned later on. Except of
course if someone is "clever enough" (remember! we talk about an int, POD)
and call placement new on its address. It would be an initialization,
although a pretty questionable one. :-)

--
Attila aka WW

[tom_usenet]

On 14 Sep 2003 19:07:41 -0700, sb******@cs.uic .edu (Shane Beasley)
wrote:

Kevin Goodsell <us************ *********@never box.com> wrote in message news:<dw******* **********@news read4.news.pas. earthlink.net>. ..

Please show me where in the standard this definition is given.

As previously quoted, 4.1/1 clearly states that an lvalue may not be
used as an rvalue unless it was previously initialized. Apparently,

int n;
n = 0;

qualifies as initialization for these purposes. But it's not done via
direct- or copy-initialization syntax in the declaration of n; rather,
it's done via an assignment expression, so called because it performs
assignment.

That is a defect in the standard. It should say "indetermin ate value":

http://std.dkuug.dk/jtc1/sc22/wg21/d...ctive.html#240

The intent of the standard is clear: initialization and assignment are
not ever meant to mean the same thing.

Tom