Constructor help

[Ook]

Can some kind soul explain this line? I'm not quite sure what the different
parts do and exactly how it works.


public:
// Constructors
Zoot(int size = 0) : _size(size), _data(_size ? new int[_size] : 0) { }

[Alf P. Steinbach]

Re: Constructor help

* "Ook" <Don't send me any freakin' spam>:[color=blue]
> Can some kind soul explain this line? I'm not quite sure what the different
> parts do and exactly how it works.
>
>
> public:
> // Constructors
> Zoot(int size = 0) : _size(size), _data(_size ? new int[_size] : 0) { }[/color]

Zoot

That's the name of the class.


(int size = 0)

One argument called 'size' that defaults to 0.


: _size(size),

The member '_size' is initialized with the value of 'size'.


_data(_size ? new int[_size] : 0)

The member '_data' is initialized with the value of


_size ? new int[_size] : 0

if '_size' is non-zero then 'new int[_size]' else 0.


{}

Does nothing in the constructor body.

General comments: this constructor only works if '_size' has been declared
before '_data'. Otherwise '_data' will be initialized first, using the
indeterminate value of '_size'. That is very ungood, and it's very simple to
avoid: use 'size' instead of '_size' in the '_data' initialization expression.

Since the problem is so easy to avoid and so totally unnecessary, this
constructor was either coded by a novice or as an illustration of this
problem.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Markus Becker]

Re: Constructor help

Alf P. Steinbach <alfps@start.no > schrieb:
[color=blue][color=green]
>> Zoot(int size = 0) : _size(size), _data(_size ? new int[_size] : 0) { }[/color]
>
> Zoot
>
> That's the name of the class.[/color]

At _this_ place, actually, it is the name of the constructor, which
of course is identical to the class name.
[color=blue]
> (int size = 0)
>
> One argument called 'size' that defaults to 0.[/color]

And because it has a default, it can be ommited in the call of the
c'tor, and because it can be omitted, the c'tor can serve as a de-
fault c'tor.
[color=blue]
> {}
>
> Does nothing in the constructor body.[/color]

Does it make the c'tor 'inline'? btw.: there's a ';' missing.
[color=blue]
> General comments: this constructor only works if '_size' has been declared
> before '_data'. Otherwise '_data' will be initialized first, using the
> indeterminate value of '_size'. That is very ungood, and it's very simple to
> avoid: use 'size' instead of '_size' in the '_data' initialization expression.[/color]

This is a _very_ good tip, I had overlooked that. I guess, without knowing
the 'standard' by word, that the order of the initialisations in the head
of the c'tor is not strictly defined?
[color=blue]
> Since the problem is so easy to avoid and so totally unnecessary, this
> constructor was either coded by a novice or as an illustration of this
> problem.[/color]

Or as a trap in a test.

Markus

[Alf P. Steinbach]

Re: Constructor help

* Markus Becker:[color=blue]
> Alf P. Steinbach <alfps@start.no > schrieb:
>[color=green][color=darkred]
> >> Zoot(int size = 0) : _size(size), _data(_size ? new int[_size] : 0) { }[/color]
> >
> > Zoot
> >
> > That's the name of the class.[/color]
>
> At _this_ place, actually, it is the name of the constructor, which
> of course is identical to the class name.[/color]

The Holy Standard informs us that a constructor has no name. I agree that
that is a word game. But that's how it is, formally.

[color=blue][color=green]
> > (int size = 0)
> >
> > One argument called 'size' that defaults to 0.[/color]
>
> And because it has a default, it can be ommited in the call of the
> c'tor, and because it can be omitted, the c'tor can serve as a de-
> fault c'tor.[/color]

Not only "can serve": it is by definition the (one and only) default
constructor for this class.

[color=blue][color=green]
> > {}
> >
> > Does nothing in the constructor body.[/color]
>
> Does it make the c'tor 'inline'?[/color]

Nope. But the unqualified class name means it is necessarily inline.

[color=blue]
> btw.: there's a ';' missing.[/color]

Nope.

[color=blue][color=green]
> > General comments: this constructor only works if '_size' has been declared
> > before '_data'. Otherwise '_data' will be initialized first, using the
> > indeterminate value of '_size'. That is very ungood, and it's very simple to
> > avoid: use 'size' instead of '_size' in the '_data' initialization expression.[/color]
>
> This is a _very_ good tip, I had overlooked that. I guess, without knowing
> the 'standard' by word, that the order of the initialisations in the head
> of the c'tor is not strictly defined?[/color]

It is very strictly & rigorously defined. For data members, it's the
declaration order. The textual order in the contstructor initialization list,
if any, does not matter.


[color=blue][color=green]
> > Since the problem is so easy to avoid and so totally unnecessary, this
> > constructor was either coded by a novice or as an illustration of this
> > problem.[/color]
>
> Or as a trap in a test.[/color]

Ah, I didn't think of that... ;-)

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Alf P. Steinbach]

Re: Constructor help

* Alf P. Steinbach:[color=blue][color=green]
> >[color=darkred]
> > >> Zoot(int size = 0) : _size(size), _data(_size ? new int[_size] : 0) { }[/color][/color]
>
> Not only "can serve": it is by definition the (one and only) default
> constructor for this class.[/color]

Sorry. It is by definition _a_ default constructor. And only by the
impracticality of having multiple default constructors (there's no way to
disambiguate for use as default constructor), the one and only one.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Markus Becker]

Re: Constructor help

Alf P. Steinbach <alfps@start.no > schrieb:
[color=blue][color=green]
>> Does it make the c'tor 'inline'?[/color]
>
> Nope. But the unqualified class name means it is necessarily inline.[/color]

What's meant by 'unqualified' at this point? That he just didn't
mention (or declare) it? Can I define a whole class by just defining
a constructor?
[color=blue][color=green]
>> btw.: there's a ';' missing.[/color]
>
> Nope.[/color]

Oops, quite right. I have to admit that - up to now - in _all_ my class
definitions where I defined the member function in the class definition
(like above), I have written something like this:

class klasse
{
public:
klasse(int n=0):_n(n) {}; // <- _this_ ';' I mean ...
};
[color=blue]
> It is very strictly & rigorously defined. For data members, it's the
> declaration order. The textual order in the contstructor initialization list,
> if any, does not matter.[/color]

Ah, thanks. But, as you mention 'data members' explicitly, what's the ex-
ception to this rule, or what are 'non-data members' (if there're any)?

Markus

[Alf P. Steinbach]

Re: Constructor help

* Markus Becker:[color=blue]
> Alf P. Steinbach <alfps@start.no > schrieb:
>[color=green][color=darkred]
> >> Does it make the c'tor 'inline'?[/color]
> >
> > Nope. But the unqualified class name means it is necessarily inline.[/color]
>
> What's meant by 'unqualified' at this point?[/color]

"Zoot" as opposed to "Zoot::Zoot ".

[color=blue]
> Can I define a whole class by just defining
> a constructor?[/color]

No.

[color=blue]
> [about initialization order][color=green]
> > It is very strictly & rigorously defined. For data members, it's the
> > declaration order. The textual order in the contstructor initialization list,
> > if any, does not matter.[/color]
>
> Ah, thanks. But, as you mention 'data members' explicitly, what's the ex-
> ception to this rule, or what are 'non-data members' (if there're any)?[/color]

Base class sub-objects.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?