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Subject: Re: [proto] Best way to change tag in generator?
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2012-01-31 19:41:21
On 01/31/2012 07:01 PM, Eric Niebler wrote:
> You mean, for instance, you want Proto's operator+ to return an
> expression with a tag other than tag::plus?
Yes.
> Can I ask why?
I need to attach certain information to the type of operation that the
tag represents, and putting that information as typedefs in the tag type
itself would be fairly convenient.
Most of my tags are user-defined, except from the ones that map to
built-in C++ operators.
Of course, I could also just have a mapping of proto tags to custom tags
that have that information (that is what I'm currently doing).
I was just being curious whether another approach was possible.
> You can do
> this in a generator, but you're right to be cautious of compile times.
> Proto will instantiate a bunch of templates while assembling each
> expression before passing them to your generator, which would just rip
> them apart and throw them away.
>
> Your other option would be to use the (undocumented)
> BOOST_PROTO_DEFINE_UNARY_OPERATOR and
> BOOST_PROTO_DEFINE_BINARY_OPERATOR to define a complete alternate set
> of operator overloads that use your tags instead of proto's. See
> proto/operators.hpp. Nobody has ever done this and I don't know if it
> would work.
I guess I won't take that option then ;)
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