class object initialisation

**Ivan Vecerina** · Jul 19 '05, 05:25 PM

Re: class object initialisation

"A" <A@iprimus.com. au> wrote in message
news:3f61b50b_1 @news.iprimus.c om.au...
| 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?

Only when you are unable to, e.g. when a complex computation involving
several intermediate variables is needed and a dedicated function doesn't
make sense.

IMO the initialization list should always be preferred.

Regards,
--

Ivan Vecerina->Home

http://ivan.vecerina.com

Ivan Vecerina - expert in medical devices, software - info, links, contact information, code snippets

**jeffc** · Jul 19 '05, 05:25 PM

Re: class object initialisation

"A" <A@iprimus.com. au> wrote in message
news:3f61b50b_1 @news.iprimus.c om.au...[color=blue]
> Hi,
>
> I have always been taught to use an inialization list for initialising[/color]
data[color=blue]
> members of a class. I realize that initialsizing primitives and pointers[/color]
use[color=blue]
> 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?[/color]

The basic rule of thumb is that anything that can go in the initialize list
should go there. Obviously, you can't put something like this in there:
if (a)
i = 2;
else
i = 4;

**Erik** · Jul 19 '05, 05:26 PM

Re: class object initialisation

> > Hi,[color=blue][color=green]
> >
> > I have always been taught to use an inialization list for initialising[/color]
> data[color=green]
> > members of a class. I realize that initialsizing primitives and pointers[/color]
> use[color=green]
> > 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[/color][/color]
data[color=blue][color=green]
> > members of a class?[/color]
>
> The basic rule of thumb is that anything that can go in the initialize[/color]
list[color=blue]
> should go there. Obviously, you can't put something like this in there:
> if (a)
> i = 2;
> else
> i = 4;[/color]

Many, if not most, of those cases can be covered by the ?: operator:
MyClass(int a) : i(a ? 2 : 4) {}

/ Erik

**Gianni Mariani** · Jul 19 '05, 05:26 PM

Re: class object initialisation

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

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

**jeffc** · Jul 19 '05, 05:27 PM

Re: class object initialisation

"Gianni Mariani" <gi2nospam@mari ani.ws> wrote in message
news:bjsqne$oou @dispatch.conce ntric.net...[color=blue]
> Erik wrote:
> ...[color=green][color=darkred]
> >>should go there. Obviously, you can't put something like this in there:
> >>if (a)
> >> i = 2;
> >>else
> >> i = 4;[/color]
> >
> >
> > Many, if not most, of those cases can be covered by the ?: operator:
> > MyClass(int a) : i(a ? 2 : 4) {}
> >[/color]
>
> Yep - and sometimes it makes sense to create a function that does more
> complex things so that everything is in the initializer list.[/color]

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

**jeffc** · Jul 19 '05, 05:27 PM

Re: class object initialisation

"Erik" <no@spam.com> wrote in message news:bjspcn$r22 $1@news.lth.se. ..[color=blue][color=green]
> > The basic rule of thumb is that anything that can go in the initialize[/color]
> list[color=green]
> > should go there. Obviously, you can't put something like this in there:
> > if (a)
> > i = 2;
> > else
> > i = 4;[/color]
>
> Many, if not most, of those cases can be covered by the ?: operator:
> MyClass(int a) : i(a ? 2 : 4) {}[/color]

Yeah, maybe simple "if" cases like that, but you can't put logic or function
calls (other than base class constructors) in the initializer list. You
added an expression, not a statement.

**Ron Natalie** · Jul 19 '05, 05:27 PM

Re: class object initialisation

"jeffc" <nobody@nowhere .com> wrote in message news:3f622ea6_1 @news1.prserv.n et...[color=blue]
>[/color]
[color=blue][color=green]
> > Yep - and sometimes it makes sense to create a function that does more
> > complex things so that everything is in the initializer list.[/color]
>
> And how exactly do you propose calling such a function from the initializer
> list?[/color]

Same way you call any other function. It behooves you not to use the
yet uninitialized parts of the object being constructed though.

**Ron Natalie** · Jul 19 '05, 05:27 PM

Re: class object initialisation

"jeffc" <nobody@nowhere .com> wrote in message news:3f622fa7_4 @news1.prserv.n et...
[color=blue][color=green]
> > Many, if not most, of those cases can be covered by the ?: operator:
> > MyClass(int a) : i(a ? 2 : 4) {}[/color]
>
> Yeah, maybe simple "if" cases like that, but you can't put logic or function
> calls (other than base class constructors) in the initializer list. You
> added an expression, not a statement.[/color]

Who says you can't have function calls?

**Kevin Goodsell** · Jul 19 '05, 05:27 PM

Re: class object initialisation

Ying Yang wrote:
[color=blue][color=green]
>>
>>Never.
>>Initializatio n lists are the only way to *initialize* members of a class
>>or to specify which constructor from a base class should be used.[/color]
>
>
> 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.[/color]

No, you cannot initialize anything with an assignment operator ever.
Assignment operators assign, they don't initialize. If you don't the
difference, you should look it up.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

**Shane Beasley** · Jul 19 '05, 05:28 PM

Re: class object initialisation

Kevin Goodsell <usenet1.spamfr ee.fusion@never box.com> wrote in message news:<7tv8b.918 $UN4.802@newsre ad3.news.pas.ea rthlink.net>...
[color=blue][color=green]
> > 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.[/color][/color]

(NB: This is true for all POD types, not just primitive types.)
[color=blue]
> No, you cannot initialize anything with an assignment operator ever.
> Assignment operators assign, they don't initialize. If you don't the
> difference, you should look it up.[/color]

int n; // n is not initialized
n = 0; // 0 assigned to n

Using an uninitialized lvalue as an rvalue is undefined (4.1/1). If
assignment doesn't initialize, ever, then

printf("%d\n", n);

results in undefined behavior. I, for one, highly doubt that.

- Shane

**Kevin Goodsell** · Jul 19 '05, 05:28 PM

Re: class object initialisation

Shane Beasley wrote:
[color=blue]
>
>
> int n; // n is not initialized
> n = 0; // 0 assigned to n
>
> Using an uninitialized lvalue as an rvalue is undefined (4.1/1).[/color]

Using an indeterminate lvalue is undefined. "Uninitiali zed" is used to
mean "indetermin ate" sometimes, apparently even in the standard.
[color=blue]
> If
> assignment doesn't initialize, ever, then[/color]

It doesn't. It assigns.
[color=blue]
>
> printf("%d\n", n);
>
> results in undefined behavior. I, for one, highly doubt that.[/color]

The value is not indeterminate at this point.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

A · Jul 19 '05, 05:29 PM

Re: class object initialisation

[color=blue]
> Ying Yang wrote:
>[color=green][color=darkred]
> >>
> >>Never.
> >>Initializatio n lists are the only way to *initialize* members of a class
> >>or to specify which constructor from a base class should be used.[/color]
> >
> >
> > 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[/color][/color]
using[color=blue][color=green]
> > an initialization list.[/color]
>
> No, you cannot initialize anything with an assignment operator ever.
> Assignment operators assign, they don't initialize. If you don't the
> difference, you should look it up.[/color]

To initialize means you give a variable it's initial value. An assignment
operator can be used to give a variable it's initial value, 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: assignment or an initialisation list.
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.

**Kevin Goodsell** · Jul 19 '05, 05:29 PM

Re: class object initialisation

A wrote:
[color=blue]
>
> To initialize means you give a variable it's initial value.[/color]

Agreed.
[color=blue]
> An assignment
> operator can be used to give a variable it's initial value,[/color]

No, it can't. If you believe it can, please give an example.
[color=blue]
> 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:[/color]

Well, you can't give something its initial value twice, so I assume you
mean "one of the following ways".
[color=blue]
> assignment or an initialisation list.[/color]

....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.
[color=blue]
> 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.[/color]

Initialization happens when an object is initially constructed.
Assignment can only happen after that time (or not at all, for certain
types of objects).

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

**foo** · Jul 19 '05, 05:29 PM

Re: class object initialisation

"jeffc" <nobody@nowhere .com> wrote in message news:<3f622fa7_ 4@news1.prserv. net>...[color=blue]
> "Erik" <no@spam.com> wrote in message news:bjspcn$r22 $1@news.lth.se. ..[color=green][color=darkred]
> > > The basic rule of thumb is that anything that can go in the initialize[/color][/color]
> list[color=green][color=darkred]
> > > should go there. Obviously, you can't put something like this in there:
> > > if (a)
> > > i = 2;
> > > else
> > > i = 4;[/color]
> >
> > Many, if not most, of those cases can be covered by the ?: operator:
> > MyClass(int a) : i(a ? 2 : 4) {}[/color]
>
> Yeah, maybe simple "if" cases like that, but you can't put logic or function
> calls (other than base class constructors) in the initializer list. You
> added an expression, not a statement.[/color]
There are plenty of things you can put in the initializer list.
You can put functions, additions, conditon logic, and more.
Example:

class foo
{
public:
foo(int x, const std::string &Src)
:m_i((x > 3)?Func2(8):x),
m_data((Src == "test")?Func1(" good"):Func1(Sr c)+" test"),
i(m_i)
{
}

private:
std::string Func1(const std::string &Src){return "Hello World " + Src;}
int Func2(int y){return y + 44;}
int m_i;
public:
const std::string m_data;//Needs to be in an initialized list
const int &i; //Needs to be in an initialized list
};

int main(int argc, char* argv[])
{
foo myfoo(2, "bla bla");
cout << myfoo.m_data << endl;
cout << myfoo.i << endl;

system("pause") ;
return 0;
}

class object initialisation

class object initialisation

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