Geometry Boost List: Re: [geometry] Use same object as input and output parameter?

Subject: Re: [geometry] Use same object as input and output parameter?
From: Barend Gehrels (barend_at_[hidden])
Date: 2012-02-07 14:26:14

Hi Volker,

On 7-2-2012 19:06, Volker Schöch wrote:
>
> Hi,
>
> I would like to use algorithms like intersection or difference to 
> modify objects in-place.
>

Not really.

It is generally not possible - you have an input of two geometries 
(polygons) and a vector-of-output-objects (polygon), which can be empty, 
or have one or more polygons (intersection of two polygons can result in 
a dozen output polygons).

So for polygons, or any model which is not "multi", it is not applicable.

For multi-polygons it would be possible, and you could specify 
geometry::intersection(a,b,a) to get the output of the intersection in 
"a". But it will append to "a" and not clear it before. So that is not 
inplace-editing either.

So it is not really possible.

Actually I cannot remember that we agreed on appending - also not in the 
docs and might be considered as a bug, also because we've also an 
intersection_insert algorithm (though removed to detail). But I know the 
origin:- the output parameter of "intersection" does not have to be a 
multi-polygon, it can be anything which supports the output iterator...

> I cannot find any comment in the documentation whether it is 
> legitimate to provide one of the input parameters as output parameter 
> to these algorithms. Even if it would work with today's 
> implementation, it would be helpful to have confirmation that this is 
> not going to change in the future. If it is not allowed, I would have 
> to instantiate and then move an additional object, which I'd rather avoid.
>

So it does not work.

Regards, Barend