Proto for Boost mailing list page: Re: [proto] Adding stuff in proto operator

Subject: Re: [proto] Adding stuff in proto operator
From: Eric Niebler (eric_at_[hidden])
Date: 2010-12-28 17:13:36

On 12/28/2010 5:05 PM, Joel Falcou wrote:
> Here i smy use case. I guess Eric answer will be "do this at
> evaluation time" 

Do this at evaluation time. Just kidding.

> but let's I have some array/matrix DSEL going on. I
> want to test if two expression containing said matrix has compatible
> size before creating a proto ast node.
> 
> e.g if a,b are matrices, a + b should assert if size(a) != size(b)
> (in the matlab meaning of size).
> 
> Now i can do the check when evaluating the expression before trying
> to assign it BUT it irks me that the assert triggers inside the
> matrix expression evaluator instead of at the line said + was wrongly
> called.
> 
> Could we have some way to specify code to call before returning the
> a new operator AST node, shoudl I overload operators myself ? Should
> I stick with the "assert in eval" policy and try to come up with way
> to tell the user which operators faile din which expression, did I
> miss the obvious ?

You missed the "Generator" parameter to proto::domain. It's a unary
function object that accepts all new proto expressions and does
something to it. That something can include asserting if matrix/vector
sizes don't match.

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com