Re: [boost] [boost::proto] how to get the type of a proto expression?

From: Eric Niebler (eric_at_[hidden])
Date: 2007-05-07 19:14:41

Maurizio Vitale wrote:
> Eric,
>         I can get the type of a proto expression (wrapped in a user-defined skin) using
> BOOST_TYPEOF, but then I noticed BOOST_PROTO_TYPEOF in context.hpp.
> 
> Which one is supposed to be used?
> 
> BOOST_PROTO_TYPEOF fails on me when trying to emulate the way it is used inside context.hpp.
> 
> Here is code that shows what I'm trying. If BOOST_PROTO_TYPEOF is internal only, just let me know and forget
> the code below.

BOOST_PROTO_TYPEOF is internal only. I'll rename it, and #undef it at 
the end of context.hpp.

You should #include proto_typeof.hpp and use BOOST_PROTO_AUTO(), like this:

   BOOST_PROTO_AUTO( expr, a+b*c );

This is equivalent to:

   BOOST_AUTO( expr, boost::proto::deep_copy(a+b*c) );

Nodes in proto's expression tree are held by reference, so that building 
an expression tree requires no copying. But some nodes are temporaries. 
deep_copy() forces all nodes to be held by value, which is necessary if 
you're storing them in a local variable like this.

HTH,

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com