• Chris Warwick (4/4) Mar 09 2007 Errm so what does "if (object)" and "Assert(object)" do??
• Jarrett Billingsley (21/22) Mar 09 2007 "if(object)" is the same as "if(object !is null)".
• Hasan Aljudy (4/34) Mar 09 2007 Really?
• torhu (16/33) Mar 10 2007 It's mentioned here:
• Derek Parnell (7/12) Mar 10 2007 Yes. That would be a very good idea.
• Jarrett Billingsley (3/6) Mar 10 2007 votes = votes + 1; // add one to votes
• Max Samukha (3/9) Mar 10 2007 vote++ for obj.invariant

"Chris Warwick" <sp m.me.not> writes:

Errm so what does "if (object)" and "Assert(object)" do??

Are they the same as (object !is null)?

cheers,

cw 

"Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:

"Chris Warwick" <sp m.me.not> wrote in message 
news:est5n5$1krh$1 digitalmars.com...
 Errm so what does "if (object)" and "Assert(object)" do??

"if(object)" is the same as "if(object !is null)".

But "assert(object)" is a little tricky.  It's a little-known feature.  If 
you use "assert(object)", it will call any invariants defined for the 
object, i.e.

class C
{
    invariant
    {
        writefln("foo");
    }
}

...
scope c = new C();
assert(c); // prints foo since it calls the invariant

This means that if you use "assert(objectReference)" on a null reference, 
you will (like with '==' !) get an access violation, since it tries to look 
up the invariant from a null reference.

Therefore, when doing an assert on an object reference, you should always 
use "assert(c is null)" or "assert(c !is null)". 

Hasan Aljudy <hasan.aljudy gmail.com> writes:

Jarrett Billingsley wrote:
 "Chris Warwick" <sp m.me.not> wrote in message 
 news:est5n5$1krh$1 digitalmars.com...
 Errm so what does "if (object)" and "Assert(object)" do??

 
 "if(object)" is the same as "if(object !is null)".
 
 But "assert(object)" is a little tricky.  It's a little-known feature.  If 
 you use "assert(object)", it will call any invariants defined for the 
 object, i.e.
 
 class C
 {
     invariant
     {
         writefln("foo");
     }
 }
 
 ...
 scope c = new C();
 assert(c); // prints foo since it calls the invariant
 
 This means that if you use "assert(objectReference)" on a null reference, 
 you will (like with '==' !) get an access violation, since it tries to look 
 up the invariant from a null reference.
 
 Therefore, when doing an assert on an object reference, you should always 
 use "assert(c is null)" or "assert(c !is null)". 
 
 

Really?
I thought assert(object) just throws an assert error if the reference is 
null!

torhu <fake address.dude> writes:

Hasan Aljudy wrote:
 
 Jarrett Billingsley wrote:
 scope c = new C();
 assert(c); // prints foo since it calls the invariant
 
 This means that if you use "assert(objectReference)" on a null reference, 
 you will (like with '==' !) get an access violation, since it tries to look 
 up the invariant from a null reference.
 
 Therefore, when doing an assert on an object reference, you should always 
 use "assert(c is null)" or "assert(c !is null)". 
 
 

 
 Really?
 I thought assert(object) just throws an assert error if the reference is 
 null!

It's mentioned here:
http://www.digitalmars.com/d/class.html#invariants

The funny part is that assert(obj !is null && obj) does *not* call the 
invariant.  So assert(obj) is a special case.

I think assert(obj) being the same as assert(obj !is null), and then 
being able to do 'obj.invariant' if you really want to explicitly check 
the invariant would a lot clearer.  Or even 'invariant(obj)'.

Calling the invariant explicitly might be useful in the middle of 
methods.  Then I think this.invariant or invariant(this), is a lot more 
readable than assert(this).

You want both a null check and an invariant check, you could then do 
'assert(obj && invariant(obj))' or assert(obj && obj.invariant)'.  Seems 
pretty obvious to me what that would do.

Maybe obj.invariant is the best way, since it doesn't try to hide that 
invariant really is a method?

Derek Parnell <derek psych.ward> writes:

On Sat, 10 Mar 2007 11:05:34 +0100, torhu wrote:

 So assert(obj) is a special case.
 
 I think assert(obj) being the same as assert(obj !is null), and then 
 being able to do 'obj.invariant' if you really want to explicitly check 
 the invariant would a lot clearer.  Or even 'invariant(obj)'.

Yes. That would be a very good idea.

-- 
Derek Parnell
Melbourne, Australia
"Justice for David Hicks!"
skype: derek.j.parnell

"Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:

"torhu" <fake address.dude> wrote in message 
news:estvsu$2pd9$1 digitalmars.com...
 I think assert(obj) being the same as assert(obj !is null), and then being 
 able to do 'obj.invariant' if you really want to explicitly check the 
 invariant would a lot clearer.  Or even 'invariant(obj)'.

votes = votes + 1; // add one to votes 

Max Samukha <samukha voliacable.com> writes:

On Sat, 10 Mar 2007 12:43:22 -0500, "Jarrett Billingsley"
<kb3ctd2 yahoo.com> wrote:

"torhu" <fake address.dude> wrote in message 
news:estvsu$2pd9$1 digitalmars.com...
 I think assert(obj) being the same as assert(obj !is null), and then being 
 able to do 'obj.invariant' if you really want to explicitly check the 
 invariant would a lot clearer.  Or even 'invariant(obj)'.

votes = votes + 1; // add one to votes 

vote++ for obj.invariant