returning a reference

[Shea Martin]

Trying to avoid a temp object, and c-style funcitons
(i.e., void doSomethingTo(T ype* thisObject) ).

<referece: code below>

Is it safe to return a reference to an internally created object? Is
obj's destructor called at end of main or at end of fromString()?

This is how I uderstand it: o's copy constructor is called with obj as
the arg, and I have avoided a temporary instance of Object.

Is this safe?

Thanks

<code>
class Object
{
public:
static Object &fromString(std ::string str)
{
Object obj;
//process string, and set attributes of obj
return obj;
}
};

int main()
{
Object o = Object::fromStr ing("attribute1 ,attribute2");
cout << "att1 is " << o.getAtt1()
<< "att2 is " << o.getAtt1()
<< "att3 is " << o.getAtt1() << endl;
}
</code>

[red floyd]

Re: returning a reference

Shea Martin wrote:
[color=blue]
> Trying to avoid a temp object, and c-style funcitons
> (i.e., void doSomethingTo(T ype* thisObject) ).
>
> <referece: code below>
>
> Is it safe to return a reference to an internally created object? Is
> obj's destructor called at end of main or at end of fromString()?[/color]
No. It's not safe. Destructor is called at the end of fromString().
[color=blue]
> This is how I uderstand it: o's copy constructor is called with obj as
> the arg, and I have avoided a temporary instance of Object.
>
> Is this safe?
>
> Thanks
>
> <code>
> class Object
> {
> public:
> static Object &fromString(std ::string str)[/color]
Should be: static Object fromString(std: :string str)[color=blue]
> {
> Object obj;
> //process string, and set attributes of obj
> return obj;[/color]
This is ungood. obj is destroyed at the end of fromString
Scott Meyers has a good discussion of this in Effective C++, where he points
out that while return by reference is nice for efficiency, there are cases
(such as this one) where you need to return by value.
[color=blue]
> }
> };
>
> int main()
> {
> Object o = Object::fromStr ing("attribute1 ,attribute2");
> cout << "att1 is " << o.getAtt1()
> << "att2 is " << o.getAtt1()
> << "att3 is " << o.getAtt1() << endl;
> }
> </code>[/color]


It's essentially the equivalent of the C construct:

int *f()
{
int x;
return &x;
}

When you return from f, the pointer is invalid. Similarly, when you return from fromString, the
reference doesn't refer to anything valid.

red floyd

[jeffc]

Re: returning a reference


"Shea Martin" <smartin@arcis. com> wrote in message
news:4JXgb.6802 $f7.391628@loca lhost...[color=blue]
> Trying to avoid a temp object, and c-style funcitons
> (i.e., void doSomethingTo(T ype* thisObject) ).
>
> <referece: code below>
>
> Is it safe to return a reference to an internally created object?[/color]

Sometimes.
[color=blue]
> Is obj's destructor called at end of main or at end of fromString()?[/color]

Yes. This isn't one of those times :-)[color=blue]
>
> This is how I uderstand it: o's copy constructor is called with obj as
> the arg, and I have avoided a temporary instance of Object.[/color]

There is no obj to use as an arg, since as you guessed it was destroyed
after leaving fromString.

[Rolf Magnus]

Re: returning a reference

Shea Martin wrote:
[color=blue]
> Trying to avoid a temp object, and c-style funcitons
> (i.e., void doSomethingTo(T ype* thisObject) ).
> <referece: code below>
>
> Is it safe to return a reference to an internally created object?[/color]

No.
[color=blue]
> Is obj's destructor called at end of main or at end of fromString()?[/color]

At the end of fromString().
[color=blue]
> This is how I uderstand it: o's copy constructor is called with obj
> as the arg, and I have avoided a temporary instance of Object.[/color]

Wrong.
[color=blue]
> Is this safe?
>
> Thanks
>
> <code>
> class Object
> {
> public:
> static Object &fromString(std ::string str)[/color]

You have to return an Object, not a reference. Btw, the above will copy
the std::string, so you should use a reference here:

static Object fromString(cons t std::string& str)

Btw, a good compiler optimzies the extra copy of function return values
away.
[color=blue]
> {
> Object obj;
> //process string, and set attributes of obj
> return obj;
> }
> };
>
> int main()
> {
> Object o = Object::fromStr ing("attribute1 ,attribute2");
> cout << "att1 is " << o.getAtt1()
> << "att2 is " << o.getAtt1()
> << "att3 is " << o.getAtt1() << endl;
> }
> </code>[/color]

[jeffc]

Re: returning a reference


"Rolf Magnus" <ramagnus@t-online.de> wrote in message
news:bm1sh8$d4n $04$1@news.t-online.com...[color=blue][color=green]
> > static Object &fromString(std ::string str)[/color]
>
> You have to return an Object, not a reference.[/color]

You can return a reference as long as the object is allocated dynamically.
This puts the responsibility for the object (deletion) with the caller.

[Jonathan Mcdougall]

Re: returning a reference

> Trying to avoid a temp object, and c-style funcitons[color=blue]
> (i.e., void doSomethingTo(T ype* thisObject) ).[/color]

Good idea, but sometimes you can't.
[color=blue]
> <referece: code below>
>
> Is it safe to return a reference to an internally created object?[/color]

No.
[color=blue]
> Is
> obj's destructor called at end of main or at end of fromString()?[/color]

Yes, when it gets out of scope, it is destroyed.
[color=blue]
> This is how I uderstand it: o's copy constructor is called with obj as
> the arg, and I have avoided a temporary instance of Object.[/color]

It didn't have the time! 'obj' was destroyed before 'o' was ever aware
of its existence.
[color=blue]
> Is this safe?[/color]

No! 'Well it works on my computer'. That's bad since you'll have a lot of
troubles finding why it crashes 5 minutes (or an hour) after. This would
be called undefined behavior.
[color=blue]
> <code>
> class Object
> {
> public:
> static Object &fromString(std ::string str)[/color]

Never, never return a reference to something if that something did not exist
prior entering a function (except for dynamically allocated memory, but then
I hope you'll go for pointers.
[color=blue]
> {
> Object obj;[/color]

'obj' is created here.
[color=blue]
> //process string, and set attributes of obj
> return obj;
> }[/color]

'obj' is destroyed there.
[color=blue]
> };
>
> int main()
> {
> Object o = Object::fromStr ing("attribute1 ,attribute2");[/color]

fromString() returns a reference to an already destroyed object.
Undefined behavior. Don't do that!

When you want to return a object which was inexistant, do it
by value. You have got no other choices (except for dynamically
allocated memory, which is always something to avoid).


Jonathan

[jeffc]

Re: returning a reference


"Jonathan Mcdougall" <jonathanmcdoug all@DELyahoo.ca > wrote in message
news:cf%gb.9932 5$282.1682733@w eber.videotron. net...[color=blue][color=green]
> > public:
> > static Object &fromString(std ::string str)[/color]
>
> Never, never return a reference to something if that something did not[/color]
exist[color=blue]
> prior entering a function (except for dynamically allocated memory, but[/color]
then[color=blue]
> I hope you'll go for pointers.[/color]

Why would you recommend pointers rather than references in that case?

[Jonathan Mcdougall]

Re: returning a reference

> > > public:[color=blue][color=green][color=darkred]
> > > static Object &fromString(std ::string str)[/color]
> >
> > Never, never return a reference to something if that something did not[/color]
> exist[color=green]
> > prior entering a function (except for dynamically allocated memory, but[/color]
> then[color=green]
> > I hope you'll go for pointers.[/color]
>
> Why would you recommend pointers rather than references in that case?[/color]

Because it makes more sense :

int &f()
{
return *new int;
}

int *g()
{
return new int;
}


int main()
{
int &a = f();
int *b = g();

delete &a; // yurk
delete b;
}


Jonathan