A constructor question

[Jorn Attermann]

I have a simple problem which I don't know how to handle. I have two
classes, X and Y, and in each I have a constructor which takes a reference
to the other as an argument as sketched below.

class X {
private:
int a;
public:
X();
X(Y&);
};


class Y {
private:
int b;
public:
Y();
Y(X&);
};


Naturally, the compiler complains because Y is undeclared when it parses the
definition of class X.

Any help is appreciated.

-----
Jorn Attermann,

[Ron Natalie]

Re: A constructor question


"Jorn Attermann" <jornat@nospam. dk> wrote in message news:3f8c4e5c$0 $54792$edfadb0f @dread11.news.t ele.dk...[color=blue]
> I have a simple problem which I don't know how to handle. I have two
> classes, X and Y, and in each I have a constructor which takes a reference
> to the other as an argument as sketched below.
>[/color]
add
class Y;
before the definition of X and vice versa.

http://www.parashift.com/c++-faq-lite/misc-technical-issues.html#faq-38.11

[grejdanospam@pacbell.net]

Re: A constructor question

On Tue, 14 Oct 2003 21:27:38 +0200, Jorn Attermann <jornat@nospam. dk>
wrote:
[color=blue]
> I have a simple problem which I don't know how to handle. I have two
> classes, X and Y, and in each I have a constructor which takes a
> reference
> to the other as an argument as sketched below.
>
> class X {
> private:
> int a;
> public:
> X();
> X(Y&);
> };
>
>
> class Y {
> private:
> int b;
> public:
> Y();
> Y(X&);
> };
>
>
> Naturally, the compiler complains because Y is undeclared when it parses
> the
> definition of class X.
>
> Any help is appreciated.
>
> -----
> Jorn Attermann,
>
>
>[/color]


Forward declaration is needed.

class X;

class Y { declarations/ definitions ... };
class X { declarations/defintiions ... };

If , let's say , you want to use X as a member of Y , then X should be
defined first.
If reverse then reverse.
It's nonsense to have type X member in Y and type Y member in X at the
same type.



--
grzegorz

[jeffc]

Re: A constructor question


<grejdanospam@p acbell.net> wrote in message
news:oprw1srfle 6u0rcr@news.sf. sbcglobal.net.. .[color=blue]
>
> If , let's say , you want to use X as a member of Y , then X should be
> defined first.
> If reverse then reverse.
> It's nonsense to have type X member in Y and type Y member in X at the
> same type.[/color]

Not necessarily. I might put a box in car, or I might put a car in a box.
I might have a record that has a container, or I might have a container that
has a record.

[Rolf Magnus]

Re: A constructor question

jeffc wrote:
[color=blue]
>
> <grejdanospam@p acbell.net> wrote in message
> news:oprw1srfle 6u0rcr@news.sf. sbcglobal.net.. .[color=green]
>>
>> If , let's say , you want to use X as a member of Y , then X should
>> be
>> defined first.
>> If reverse then reverse.
>> It's nonsense to have type X member in Y and type Y member in X at
>> the same type.[/color]
>
> Not necessarily.[/color]

Yes, necessarily.
[color=blue]
> I might put a box in car, or I might put a car in a box.[/color]

Right. The important part is _or_. Not every box contains a car and not
every car contains a box (that in turn would then contain a car, which
would contain a box, which .... you get the point).
[color=blue]
> I might have a record that has a container, or I might have a
> container that has a record.[/color]

[jeffc]

Re: A constructor question


"Rolf Magnus" <ramagnus@t-online.de> wrote in message
news:bmi0ti$rvb $02$2@news.t-online.com...[color=blue]
> jeffc wrote:
>[color=green]
> >
> > <grejdanospam@p acbell.net> wrote in message
> > news:oprw1srfle 6u0rcr@news.sf. sbcglobal.net.. .[color=darkred]
> >>
> >> If , let's say , you want to use X as a member of Y , then X should
> >> be
> >> defined first.
> >> If reverse then reverse.
> >> It's nonsense to have type X member in Y and type Y member in X at
> >> the same type.[/color]
> >
> > Not necessarily.[/color]
>
> Yes, necessarily.
>[color=green]
> > I might put a box in car, or I might put a car in a box.[/color]
>
> Right. The important part is _or_. Not every box contains a car and not
> every car contains a box (that in turn would then contain a car, which
> would contain a box, which .... you get the point).[/color]

Who says they have to be mutually exclusive? Let's just say for sake of
argument that all cars are normal size, but box size varies greatly. All
cars *can* have a box in them, and all boxes *can* have a car in them.

class Box;

class Car
{
private:
Box* b;
public:
Car():b(0) {}
Car(Box& box):b(&box){}
};

class Box
{
private:
int cubicFeet;
Car* c;
public:
Box(int size):cubicFeet (size) {}
Box(Car& car, int size):cubicFeet (size), c(&car){}
};

int main()
{
Car car1;
Box automobileShipp ingCrate(car1, 250);
Box containerForThi ngs(1);
Car car2(containerF orThings);

return 0;
}

[jeffc]

Re: A constructor question


"Rolf Magnus" <ramagnus@t-online.de> wrote in message
news:bmi0ti$rvb $02$2@news.t-online.com...[color=blue]
>
> Right. The important part is _or_. Not every box contains a car and not
> every car contains a box (that in turn would then contain a car, which
> would contain a box, which .... you get the point).[/color]

grejdan said " It's nonsense to have type X member in Y and type Y member in
X at the same type."
Maybe he meant at the same "time". If so I see your point - yes, you can't
do a recursive inclusion like that.

[Karl Heinz Buchegger]

Re: A constructor question



grejdanospam@pa cbell.net wrote:[color=blue]
>[/color]
[color=blue]
> If , let's say , you want to use X as a member of Y , then X should be
> defined first.
> If reverse then reverse.
> It's nonsense to have type X member in Y and type Y member in X at the
> same type.[/color]

Right.
But this is not what the OP is doing.
The OP is doing the equivalent of:
Every parent knows about its child and
every child knows about its parent.

--
Karl Heinz Buchegger
kbuchegg@gascad .at

[Rolf Magnus]

Re: A constructor question

jeffc wrote:
[color=blue]
>
> "Rolf Magnus" <ramagnus@t-online.de> wrote in message
> news:bmi0ti$rvb $02$2@news.t-online.com...[color=green]
>>
>> Right. The important part is _or_. Not every box contains a car and
>> not every car contains a box (that in turn would then contain a car,
>> which would contain a box, which .... you get the point).[/color]
>
> grejdan said " It's nonsense to have type X member in Y and type Y
> member in X at the same type."
> Maybe he meant at the same "time".[/color]

That's what I read anyway :-)
[color=blue]
> If so I see your point - yes, you can't do a recursive inclusion like
> that.[/color]

I don't know if he meant that, but at least it was my point.

[jeffc]

Re: A constructor question


"Karl Heinz Buchegger" <kbuchegg@gasca d.at> wrote in message
news:3F8D14DA.D 0E354DE@gascad. at...[color=blue]
>
>
> grejdanospam@pa cbell.net wrote:[color=green]
> >[/color]
>[color=green]
> > If , let's say , you want to use X as a member of Y , then X should be
> > defined first.
> > If reverse then reverse.
> > It's nonsense to have type X member in Y and type Y member in X at the
> > same type.[/color]
>
> Right.
> But this is not what the OP is doing.
> The OP is doing the equivalent of:
> Every parent knows about its child and
> every child knows about its parent.[/color]

Huh? I don't understand your terminology. There is no subclassing involved
here.

[Jorn Attermann]

Re: A constructor question


"Karl Heinz Buchegger" <kbuchegg@gasca d.at> wrote in message
news:3F8D14DA.D 0E354DE@gascad. at...[color=blue]
>
>
> grejdanospam@pa cbell.net wrote:[color=green]
> >[/color]
>[color=green]
> > If , let's say , you want to use X as a member of Y , then X should be
> > defined first.
> > If reverse then reverse.
> > It's nonsense to have type X member in Y and type Y member in X at the
> > same type.[/color]
>
> Right.
> But this is not what the OP is doing.
> The OP is doing the equivalent of:
> Every parent knows about its child and
> every child knows about its parent.
>
> --
> Karl Heinz Buchegger
> kbuchegg@gascad .at[/color]

I really appreciate the help from the group; it solved my problem. I also
find the other responses interesting and please let me explain what my
original problem was:

I have two classes (from Template Numerical Toolkit -
http://math.nist.gov/tnt/), an Array1D and an Array2D for numerical
computing. And I wanted to have a method for conversion between these. The
way I solved this was to include a constructor in each class which take as
argument a reference to the other class. So, there *is* actually a practical
use for it.

Thanks again,

Jorn Attermann