Re: [Boost-users] [Proto] Storing an Expression Template as a Data Member

Subject: Re: [Boost-users] [Proto] Storing an Expression Template as a Data Member
From: Eric Niebler (eric_at_[hidden])
Date: 2011-01-06 11:10:40

On 1/6/2011 8:58 AM, Hossein Haeri wrote:
> Dear all,
> 
> Is there any way I can have classes with data members of type (proto)
> expression templates? Here is where I get stuck:
> 
> template<typename T> class MyClass { ... T expression_; };
> 
> What type should T be?
> 
> If you're wondering why on earth I might need that, here is an
> explanation: Instances of MyClass need to store the lazy calculation
> they will be performing later on when enough initialisation data is
> available. Each instance of MyClass has its own FORMULATION for the
> lazy calculation in terms of the initialisation data. They need to be
> fed with these formulations and I thought (proto) expression
> templates will perhaps do the job. But, then, I don't know how to
> store these formulations until it's time for them to perform...

If you use proto::make_expr to build expressions explicitly (instead of
using the operator overloads), you can have completel control over how
sub-expressions are stored. (You'll need them to be stored by value if
you're saving the ET in a variable; otherwise, they'll go out of scope.)

And if you're using proto::make_expr, you can use
proto::result_of::make_expr to compute the type of the resulting ET.

Read the docs for make_expr carefully. That should solve your problem.

HTH,

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