Passing zero to a 'const reference'

[mikaelhc@gmail.com]

Playing a bit around with Trolltech's Qt library I noticed the
following constructor:

QProgressDialog ( const QString & labelText, const QString &
cancelButtonTex t, ...)

In the documentation they state that setting cancelButtonTex t to 0
prevents the button from being shown (and it works as stated).

But shouldn't references always point to a well-defined object? Is this
valid c++?

/Mikael.

[Ben Pope]

Re: Passing zero to a 'const reference'

mikaelhc@gmail. com wrote:[color=blue]
> Playing a bit around with Trolltech's Qt library I noticed the
> following constructor:
>
> QProgressDialog ( const QString & labelText, const QString &
> cancelButtonTex t, ...)
>
> In the documentation they state that setting cancelButtonTex t to 0
> prevents the button from being shown (and it works as stated).
>
> But shouldn't references always point to a well-defined object? Is this
> valid c++?[/color]

My guess is that you can do anything you like inside QString.

Perhaps there is a constructor from int, and it does something special
with a 0 value.

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[Bob Hairgrove]

Re: Passing zero to a 'const reference'

On 15 Feb 2006 03:18:01 -0800, mikaelhc@gmail. com wrote:
[color=blue]
>Playing a bit around with Trolltech's Qt library I noticed the
>following constructor:
>
>QProgressDialo g ( const QString & labelText, const QString &
>cancelButtonTe xt, ...)
>
>In the documentation they state that setting cancelButtonTex t to 0
>prevents the button from being shown (and it works as stated).[/color]

How is it possible to set something to 0 inside the function when it
is passed as a const reference? Perhaps there is a const assignment
function somewhere in the definition of QString?
[color=blue]
>But shouldn't references always point to a well-defined object? Is this
>valid c++?
>
>/Mikael.[/color]

References are a kind of alias for the object they refer to. Indeed,
they must always "point to" (but watch out, references are not
pointers!) a well-defined object. When you assign something to a
reference, you are actually assigning it to the underlying object.

--
Bob Hairgrove
NoSpamPlease@Ho me.com

[Ben Pope]

Re: Passing zero to a 'const reference'

Bob Hairgrove wrote:[color=blue]
> On 15 Feb 2006 03:18:01 -0800, mikaelhc@gmail. com wrote:
>[color=green]
>> Playing a bit around with Trolltech's Qt library I noticed the
>> following constructor:
>>
>> QProgressDialog ( const QString & labelText, const QString &
>> cancelButtonTex t, ...)
>>
>> In the documentation they state that setting cancelButtonTex t to 0
>> prevents the button from being shown (and it works as stated).[/color]
>
> How is it possible to set something to 0 inside the function when it
> is passed as a const reference? Perhaps there is a const assignment
> function somewhere in the definition of QString?[/color]

http://doc.trolltech.com/3.3/qstring.html

There is a constructor from a char*:

QString::QStrin g ( const char * str )

Which would be a valid candidate? So a temporary Qstring is constructed
from 0 (resulting in a null string), and a reference to that temporary
is accepted as the argument.

Is that how it works?

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

[mikaelhc@gmail.com]

Re: Passing zero to a 'const reference'

It does not seems there is a constructor from int:
QString s = 5; // This won't compile
QString s = 0; // This is OK

But QString behaves very much like a pointer (though it is defined as a
class), i.e.
if (s) Debug("The string is defined");
works.

I'll look at the Qt source, and see if I can figure out what is
happening.

[mikaelhc@gmail.com]

Re: Passing zero to a 'const reference'

Oh, I missed the char* constructor! I'm pretty sure you are right about
that, Ben.

This combined with the:
QString::operat or const char * () const

would also allow for implicit conversions to a pointer type - if I
understand it right. (Which would explain why
if (s) Debug("The string is defined");
works).

Thanks!