Return by reference

[Michael]

Ok,
When can i return by reference? For the sake of simplicity:
lets say I have a class MLine:

class mv3
{
public:
float x,y,z;


};

class MLine
{
mv3 PtOnLine;
mv3 Direction

public:
MLine();
const mv3& GetDirection() const
{
return Direction;
}

};

is this OK? I appreciate that in nots of cases this can be dangerous, eg if
I returned (Direction *4) as a tempory object would be created with local
scope then returning a reference to an invalid object.

Thanks
Mike

[Victor Bazarov]

Re: Return by reference

Michael wrote:[color=blue]
> When can i return by reference?[/color]

When the object a reference to which you're returning is still alive
and well after the function returns.
[color=blue]
> For the sake of simplicity:
> lets say I have a class MLine:
>
> class mv3
> {
> public:
> float x,y,z;
>
>
> };
>
> class MLine
> {
> mv3 PtOnLine;
> mv3 Direction
>
> public:
> MLine();
> const mv3& GetDirection() const
> {
> return Direction;
> }
>
> };
>
> is this OK?[/color]

Yes, perfectly OK.
[color=blue]
> I appreciate that in nots of cases this can be dangerous, eg if
> I returned (Direction *4) as a tempory object would be created with local
> scope then returning a reference to an invalid object.[/color]

Absolutely.

V

[JKop]

Re: Return by reference

Michael posted:
[color=blue]
> Ok,
> When can i return by reference? For the sake of simplicity:
> lets say I have a class MLine:
>
> class mv3
> {
> public:
> float x,y,z;
>
>
> };
>
> class MLine
> {
> mv3 PtOnLine;
> mv3 Direction
>
> public:
> MLine();
> const mv3& GetDirection() const
> {
> return Direction;
> }
>
> };
>
> is this OK? I appreciate that in nots of cases this can be dangerous,
> eg if I returned (Direction *4) as a tempory object would be created
> with local scope then returning a reference to an invalid object.
>
> Thanks
> Mike[/color]


You can use a reference where you can use a pointer.

Your code in the above is valid (save for a typo or two).


There are also other things you can do with references:

int main()
{
std::string const &blah = std::string("He llo!");
//blah is now valid until the end of main


std::string const &poo = FuncThatReturns StringByVALUE() ; //Note: by
value
//poo is now valid until the end of main
}

The above is referred (no pun intended) to as "binding a temporary to a
reference".


-JKop


-JKop

[Michael]

Re: Return by reference


"Michael" <slick_mick_00@ hotmail.com> wrote in message
news:ckk4t0$ohd $1@hercules.bti nternet.com...[color=blue]
> Ok,
> When can i return by reference? For the sake of simplicity:
> lets say I have a class MLine:
>
> class mv3
> {
> public:
> float x,y,z;
>
>
> };
>
> class MLine
> {
> mv3 PtOnLine;
> mv3 Direction
>
> public:
> MLine();
> const mv3& GetDirection() const
> {
> return Direction;
> }
>
> };
>
> is this OK? I appreciate that in nots of cases this can be dangerous, eg[/color]
if[color=blue]
> I returned (Direction *4) as a tempory object would be created with local
> scope then returning a reference to an invalid object.
>
> Thanks
> Mike
>
>[/color]

Sorry I didn't include all the mv3 operators but one of then allows me to
multiply by floats!

[Michael]

Re: Return by reference


"JKop" <NULL@NULL.NULL > wrote in message
news:iPgbd.3335 5$Z14.13494@new s.indigo.ie...[color=blue]
> Michael posted:
>[color=green]
> > Ok,
> > When can i return by reference? For the sake of simplicity:
> > lets say I have a class MLine:
> >
> > class mv3
> > {
> > public:
> > float x,y,z;
> >
> >
> > };
> >
> > class MLine
> > {
> > mv3 PtOnLine;
> > mv3 Direction
> >
> > public:
> > MLine();
> > const mv3& GetDirection() const
> > {
> > return Direction;
> > }
> >
> > };
> >
> > is this OK? I appreciate that in nots of cases this can be dangerous,
> > eg if I returned (Direction *4) as a tempory object would be created
> > with local scope then returning a reference to an invalid object.
> >
> > Thanks
> > Mike[/color]
>
>
> You can use a reference where you can use a pointer.
>
> Your code in the above is valid (save for a typo or two).
>
>
> There are also other things you can do with references:
>
> int main()
> {
> std::string const &blah = std::string("He llo!");
> //blah is now valid until the end of main
>
>
> std::string const &poo = FuncThatReturns StringByVALUE() ; //Note: by
> value
> //poo is now valid until the end of main
> }
>
> The above is referred (no pun intended) to as "binding a temporary to a
> reference".
>
>
> -JKop
>
>
> -JKop[/color]

that is cool. Thanks!

[Ioannis Vranos]

Re: Return by reference

Michael wrote:
[color=blue]
> Ok,
> When can i return by reference? For the sake of simplicity:
> lets say I have a class MLine:
>
> class mv3
> {
> public:
> float x,y,z;
>
>
> };
>
> class MLine
> {
> mv3 PtOnLine;
> mv3 Direction
>
> public:
> MLine();
> const mv3& GetDirection() const
> {
> return Direction;
> }
>
> };
>
> is this OK? I appreciate that in nots of cases this can be dangerous, eg if
> I returned (Direction *4) as a tempory object would be created with local
> scope then returning a reference to an invalid object.[/color]



Yes, but don't do it with regular functions by returning temporaries.



--
Ioannis Vranos

http://www23.brinkster.com/noicys

[Siemel Naran]

Re: Return by reference

"Michael" <slick_mick_00@ hotmail.com> wrote in message news:ckk4t0
[color=blue]
> class MLine
> {
> mv3 PtOnLine;
> mv3 Direction
>
> public:
> MLine();
> const mv3& GetDirection() const
> {
> return Direction;
> }
>
> };
>
> is this OK? I appreciate that in nots of cases this can be dangerous, eg[/color]
if[color=blue]
> I returned (Direction *4) as a tempory object would be created with local
> scope then returning a reference to an invalid object.[/color]

It's fine and done often, as in vector<T>::oper ator[] which returns a T& or
const T&. Be aware of code like this and don't write it:

const mv3& f() {
MLine m;
return m.GetDirection( );
}

int main() {
cout << f().x; // crash, memory access violation!
}