Re: [boost] Looking for thoughts on a new smart pointer: shared_ptr_nonnull

Subject: Re: [boost] Looking for thoughts on a new smart pointer: shared_ptr_nonnull
From: Julian Gonggrijp (j.gonggrijp_at_[hidden])
Date: 2013-10-08 03:11:05

Gavin Lambert wrote:

> On 10/8/2013 12:16 PM, Quoth Julian Gonggrijp:
>> I'm sorry to say this, but I think you're now just being evasive. A
>> few posts ago in a reply to Matt, you stated that you agreed that if
>> the assertion is to be replaced by a throw, the precondition should be
>> removed from the constructor. This means that you also agree that if
>> the precondition is *not* removed from the constructor, the assertion
>> should *not* be replaced by a throw (modus tollens).
> 
> That's not modus tollens, that's denying the antecedent.  Which is a fallacy.

No, I'm denying the consequens. If A then B. If not B then not A.

> 
>> I don't think that anyone disagrees that in the ideal world, all
>> conditions are always checked. Apparently we also agree that checks on
>> preconditions should be assertions when a check is at all possible. So
>> the discussion should be about whether not passing null to the
>> constructor is a precondition.
> 
> I'm not trying to be "evasive" or anything, I just think I possibly don't have the same definitions for some of the terms you are using.
> 
> I have I thought quite clearly stated that:
> - it should assert != null
> - it should throw if == null and it survived the assert
> 
> I don't really care what you call that behaviour, whether this is a "precondition" because it's asserted or whether it's not because it's checked for and thrown, or whatever.

So your argument is again about how the condition should be checked. 
Apparently you changed your mind and you think that precondition or not 
does not bear any relevance.

> 
> Maybe this is not what you are saying, but I get the definite impression from Matt's posts that he thinks that the act of declaring it as a precondition of the constructor means that it must not be tested for in the constructor except as an assert.

Yes, that's exactly what he said and as I explained to you, you have previously 
made the impression that you agreed with that premise.

> I do not agree with not testing this; you can read what you like into whether this means that I don't agree with the earlier assertion in this paragraph or whether it means that it shouldn't be a precondition.

I read that you want to test it no matter what.

> 
> If it doesn't fit within your worldview to do both things, then you could leave out the assert (though I think this would be a mistake).
> 
> I don't think that leaving out the throw can ever be a good idea, for reasons I have already explained.  (The only reasonable justification for not performing this sort of check is if it is hard to do -- which it isn't; or if it will cause a performance hit -- which will be negligible to nonexistent in this case.)

Yes, your opinion on checking is very clear. I'm afraid the precondition discussion 
is a dead end.

As for considerations on performance, I think the tail of the recent post by 
Rob Stewart illustrates how not everyone may agree with you on that. Also consider 
the possibility of constructing lots of non-null pointer objects.

-Julian