Re: [Boost-users] [proto] another const issue

Subject: Re: [Boost-users] [proto] another const issue
From: Eric Niebler (eric_at_[hidden])
Date: 2009-08-25 15:17:16

Maurizio Vitale wrote:
> Eric, this may or may not be related to the other problem w/ the
> assignment operator.
> 
> In my application eventually I'll want to allow literals of builtin
> constants on the right hand side of the assignment, but not on the left
> hand side. So given:
> 
>   my_int my_i; // a proto terminal
>   int i;
> 
> The following should happen:
> 
>   my_i   = 5;           // OK
>   (i,my_i) = my_i;      // OK
>   (5,my_i) = my_i;      // NOT OK
> 
> For that I have a grammar for the left hand side and one for the right
> hand side. In the attached file, the grammar for the left hand side
> doesn't match and I'm not sure why.
> What I'm doing wrong?

Here is your grammar:

struct my_lhs_grammar : proto::or_<
   proto::terminal< int& >,
   proto::binary_expr<_, my_grammar, my_grammar>
   >
{
};

This says, only terminals that are references to ints are allowed. 
my_int is an int terminal, not an int& terminal, therefore it doesn't match.

If you change the grammar to terminal<int>, then it becomes too loose, 
because terminal<int> matches int, int& and int const&, as documented. 
This wiggle room is useful in most situations, but not in yours. What 
you want is this:

struct my_lhs_grammar : proto::or_<
   proto::terminal< int& >,
   proto::terminal< proto::exact<int> >,
   proto::binary_expr<_, my_grammar, my_grammar>
   >
{
};

This is all covered in the docs here:
http://www.boost.org/doc/libs/1_39_0/doc/html/proto/users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals

HTH,

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