why it's not possible calling constructor from constructor?

[Giulio]

why definition of two constructors like these is not possible in c++???

-----------------------
date::date(cons t int d, const int m, const int y, const int ora, const int
mi, const int se){
day_ = d;
month_ = m;
year_ = y;

secondo_ = se;
minuto_ = mi;
ora_ = ora;
}

date::date(){ date(0,0,0,0,0, 0); }
---------------

thanx
Giulio

[Simon Saunders]

Re: why it's not possible calling constructor from constructor?

"Giulio" <giulio.gL E V A@email.it> wrote in message
news:c8eKa.3929 3$Fr5.850871@to rnado.fastwebne t.it...[color=blue]
> why definition of two constructors like these is not possible in c++???
>
> -----------------------
> date::date(cons t int d, const int m, const int y, const int ora, const int
> mi, const int se){
> day_ = d;
> month_ = m;
> year_ = y;
>
> secondo_ = se;
> minuto_ = mi;
> ora_ = ora;
> }
>
> date::date(){ date(0,0,0,0,0, 0); }
> ---------------
>[/color]

The second constructor creates an unnamed temporary object; that's just the
way C++ works. The usual workaround is to write a private member function to
do the initialization and call that from both constructors, for example:

date::date(int d, int m, int y, int ora, int mi, int se)
{
init(d, m, y, ora, mi, se);
}

date::date()
{
init(0, 0, 0, 0, 0, 0);
}

void date::init(int d, int m, int y, int ora, int mi, int se)
{
day_ = d;
month_ = m;
year_ = y;
secondo_ = se;
minuto_ = mi;
ora_ = ora;
}

[Patrick Kowalzick]

Re: why it's not possible calling constructor from constructor?

> class date {[color=blue]
> public:
> date(const int & d=0,const int & m=0, const int & y=0, const int & ora=0,
> const int & mi=0, const int & se=0)
> : day(d), month(m),year(y ),secondo(se),m inuto(mi),ora(o ra) {}
> private:
> int day,month,year, secondo,minuto, ora;
> };[/color]
Argh, forgotten something

with this constructor you have a lot constructors - perhaps too much. You
have to check this in each case:

date();
date(int);
date(int,int);
date(int,int,in t);
date(int,int,in t,int);
date(int,int,in t,int,int);
date(int,int,in t,int,int,int);

Greetings,
Patrick

[tom_usenet]

Re: why it's not possible calling constructor from constructor?

On Wed, 25 Jun 2003 11:42:20 +0200, "Giulio" <giulio.gL E V
A@email.it> wrote:
[color=blue]
>why definition of two constructors like these is not possible in c++???[/color]

The construction model in C++ is quite strict (mostly due to the fact
that C++ has "value-based" UDTs rather than just pointers, as in
Java). Basically, by the time you enter the body of your constructor,
all of your base classes and all of your member variables will already
have been constructed. So there is no way to invoke another
constructor from the body of your constructor, because that would
involve double construction of all your variables.

If you just want to share just the body of the constructor, then
refactor it out into a separate function that both of your
constructors call.
[color=blue]
>
>-----------------------
>date::date(con st int d, const int m, const int y, const int ora, const int
>mi, const int se){
> day_ = d;
> month_ = m;
> year_ = y;
>
> secondo_ = se;
> minuto_ = mi;
> ora_ = ora;
>}[/color]

Normally that would be done using an initializer list:
date::date(cons t int d, const int m, const int y, const int ora, const
int mi, const int se)
:day_(d),
month_(m),
year_(y),
secondo_(se),
minuto_(mi),
ora_(ora)
{
}

[color=blue]
>date::date() { date(0,0,0,0,0, 0); }[/color]

date::date()
:day_(0),
month_(0),
year_(0),
secondo_(0),
minuto_(0),
ora_(0)
{
}

As you can see, there is no code in the body at all, so there's no
common code to share anyway.

If you want to do it your way, do it like this:

void date::initialis e(const int d, const int m, const int y, const int
ora, const int mi, const int se)
{
day_ = d;
month_ = m;
year_ = y;

secondo_ = se;
minuto_ = mi;
ora_ = ora;
//any checking?
}

date::date()
{
initialise(0,0, 0,0,0,0);
}

date::date(cons t int d, const int m, const int y, const int ora, const
int mi, const int se)
{
initialise(d, m, y, ora, mi, se);
}

but for the level of code sharing involved, it doesn't seem worth it.

Tom

[Karl Heinz Buchegger]

Re: why it's not possible calling constructor from constructor?



Patrick Kowalzick wrote:[color=blue]
>
>
> An answer to your question. Every call of a constructor instanciates an
> object. So calling the constructor twice should create two objects. While
> this makes no sense it is forbidden to call a constructor from a
> constructor. (95% sure ;-) )[/color]

Strictly speaking this is not correct. In fact you can't call
a constructor, since a constructor is a function with no name.

It is the other way round: Whenever you create an object, a
constructor is called.
Therefore the call of the constructor is a side effect of createing
an object, not the other way round.

--
Karl Heinz Buchegger
kbuchegg@gascad .at

[Victor Bazarov]

Re: why it's not possible calling constructor from constructor?

"Chandra Shekhar Kumar" <chandra.kumar@ oracle.com> wrote...[color=blue]
>
> it is possible in C++.[/color]

Would you stop making those bogus statements? I can understand
how it may be amusing to you, but it is definitely confusing to
those who expect a correct answer.

It is impossible to _call_ a contstructor. There is no way in
the language to "forward" the construction of the object from one
constructor to another like in Java. If there is some common
code to be executed by both constructors, the best place to put
it is a separate function which will be called by the constructors.

As to the OP's problem, the simplest thing to do is to have one
constructor with all arguments (d,m,y,ora,mi,s e) with default
argument values set to 0:

date::date(int d = 0, int m = 0, int y = 0,
int ora = 0, int mi = 0, int se = 0) :
day_(d), month_(m), year_(y),
ora_(ora), minuto_(mi), secondo_(se)
{
}
[color=blue]
> the only thing imp here is what do u want to do with this....
> i think u want to keep the date(const int d, const int m, const int y,[/color]
const[color=blue]
> int ora, ...) as private so the client can't use it directly...and then u[/color]
want[color=blue]
> to have a public date() which calls the above private stuff with some[/color]
specific[color=blue]
> values.....
>[color=green]
> > why definition of two constructors like these is not possible in c++???
> >
> > -----------------------
> > date::date(cons t int d, const int m, const int y, const int ora, const[/color][/color]
int[color=blue][color=green]
> > mi, const int se){
> > day_ = d;
> > month_ = m;
> > year_ = y;
> >
> > secondo_ = se;
> > minuto_ = mi;
> > ora_ = ora;
> > }
> >
> > date::date(){ date(0,0,0,0,0, 0); }
> >[/color]
>[/color]

[Ron Natalie]

Re: why it's not possible calling constructor from constructor?


"Chandra Shekhar Kumar" <chandra.kumar@ oracle.com> wrote in message news:3EF8D82F.6 42702AC@oracle. com...[color=blue]
>
> it is possible in C++.[/color]

It is NOT possible in C++.
[color=blue]
> i think u want to keep the date(const int d, const int m, const int y, const
> int ora, ...) as private so the client can't use it directly...and then u want
> to have a public date() which calls the above private stuff with some specific
> values.....[/color]

What makes you think he wants to do that.
[color=blue][color=green]
> > date::date(){ date(0,0,0,0,0, 0); }
> >[/color]
>[/color]
This does not call the constructor. You can NOT call constructors at all.
The syntax above that appears to be a call is the creation of a temporary date
object in a simple expression that ceases to be at the end of the statement.
It insidiously compiles, but does nothing.

[Giulio]

Re: why it's not possible calling constructor from constructor?

thanx to all for the exhaustive answers, in the future I'll use separate
functions used as constructors -init(...)- in this case because there are
only two constructors for class date I'll leave them separate.

this is a great NG
Giulio

[tom_usenet]

Re: why it's not possible calling constructor from constructor?

On Wed, 25 Jun 2003 11:42:20 +0200, "Giulio" <giulio.gL E V
A@email.it> wrote:
[color=blue]
>why definition of two constructors like these is not possible in c++???
>
>-----------------------
>date::date(con st int d, const int m, const int y, const int ora, const int
>mi, const int se){
> day_ = d;
> month_ = m;
> year_ = y;
>
> secondo_ = se;
> minuto_ = mi;
> ora_ = ora;
>}
>
>date::date() { date(0,0,0,0,0, 0); }[/color]

Ahh, I forgot the super hacky, completely illegal solution:

date::date(){
new (this) date(0,0,0,0,0, 0);
}

It will probably work though! ;)

Tom

[Patrick Kowalzick]

Re: why it's not possible calling constructor from constructor?

> >. (95% sure ;-) )[color=blue]
>
> Strictly speaking this is not correct.[/color]

These are the famous outliers. I wanted to write 99% first. PUH ;-)

Thanks
Patrick