|Re: [eigen] a few updates...|
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- To: eigen@xxxxxxxxxxxxxxxxxxx
- Subject: Re: [eigen] a few updates...
- From: Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
- Date: Wed, 22 Dec 2010 22:34:46 -0500
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2010/12/22 Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>:
> On 22.12.2010 19:33, Benoit Jacob wrote:
>> 2010/12/22 Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>:
>>> Another possibility would be:
>>> c) Pass argument to swap() by value if it is a Map/Block-expression.
>> I tried that, using a enable-if trick to select abstract expressions
>> vs. plain objects.
>> The problem is that that doesn't solve the problem at all. By passing
>> the expression by value, we completely forget its constness, so we
>> can't prevent doing
>> const Expression xpr;
> Well that's again all about bug54.
You're right, see below...
> Having, e.g., a `const ColXpr` does
> not prevent modifying its data -- the constness must be encoded directly
> in the type (as it is in `Map<const T>`). Otherwise you can do:
> const MatrixXd m;
> MatrixXd::RowXpr r(m.row(0));
Nope, that you can't do anymore with my changes, because row(Index)
const now returns a Block<const T>...
....but you're still right, because there's nothing I can do to prevent this:
const ExpressionType someConstXpr;
....so, indeed, const on ExpressionType can't be enforced, the only
enforceable constness for expression types is const on template
parameters i.e. Block<const T>.
In my local changes, all const-qualified expression-returning methods
return such "inherently const" types (like Block<const T>). I just was
under the wrong impression that we could do something to ensure that
when the user writes a function like
template<typename T> void foo(const DenseBase<T>& x)
we could guarantee that x can't be written too, but thank you for
reminding me why that isn't the case.
So, thanks for reorienting this debate in the right direction:
- this remaining const correctness issue is not specific to swap, and
not specific to Eigen, but is inherent to expression templates and
more generally, writable wrapper classes in c++.
- there is no need for what i called 'solution b', it won't help with that..
> Back to the swap-problem I can suggest another possibility:
> d) Mark most methods of Map<T> and Block<T> as const (everything that
> does not alter the Map or Block itself). Then have three possible
> parameters for swap:
> Matrix<...> &
> const Block<T> &
> const Map<T> &
Don't worry, all of that is already done in my changes, but I didn't
have to special-case Map and Block, it works for all expressions.
I'll write about my changes as soon as I push them.
> where the latter two could be combined as soon as
> direct-access-unification is done.
> Dipl.-Inf. Christoph Hertzberg
> Cartesium 0.051
> Universität Bremen
> Enrique-Schmidt-Straße 5
> 28359 Bremen
> Tel: (+49) 421-218-64252