Re: [boost] Preview 3 of the Geometry Library

Subject: Re: [boost] Preview 3 of the Geometry Library
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2008-10-14 21:35:45

Simonson, Lucanus J wrote:
> Mathias wrote:
>> - Why can objects be default-constructible?
>> ...
>> - centroid says it throws an exception if the polygon does not contain 
>> any point. But how can a polygon not contain any points!?
>> The constructor of a polygon should force it to have at least one
> point.
> 
> Obviously, a default constructed polygon contains no points.

Yes, those two comments were indeed linked to show they were maybe 
inconsistent.

Also, if there is a valid reason for polygons to be able to be empty, I 
think the algorithm shouldn't make the check but assume it as a 
precondition, possibly with a disactivable assert.

> Polygons
> have to be default constructible so that they can be elements of a std
> container.

Thankfully, standard containers do not require objects to be default 
constructible, but only copy constructible.
I'd personally be even happier if they required almost nothing at all, 
which hopefully they will.

> Objects that are not default constructible are a pain.

Says who?
Two-phase initialization is well known to be a pain. What's the point of 
having an object constructed that is not really initialized and usable?

Constructors are made to enforce invariants. For some objects, it 
doesn't make sense to make to allow "emptiness" or "default" as a valid 
state, so we might as well not allow those states to exist.

But anyway, that's another debate.
I think it was discussed extensively lately in comp.lang.c++.moderated, 
so there is no need to discuss it here.

> I
> have to deal with a legacy polygon type that is not default
> constructible, so I have a very clear idea about which way I prefer and
> why.  

Dealing with non default-constructible types is no more difficult than 
dealing with default-constructible ones.
If you have a very clear case where for polygons or something related to 
geometry it can lead to issues, feel free to expose it.