Re: [boost] [proto]purpose of binary_expr<T,L,R>::type ?

From: Eric Niebler (eric_at_[hidden])
Date: 2008-03-02 20:39:10

Larry Evans wrote:
> Just as:
> 
>   terminal<X>:;type
> 
> is the type of expressions matching the grammar, terminal<X>,
> I would expect that binary_expr<T,L,R>::type would match
> binary_expr<T,L,R>.  IOW, just as:
> 
>   matches<terminal<X>::type,terminal<X> >
> 
> is true, should:
> 
>   matches<binary_expr<T,L,R>::type,binary_expr<T,L,R> >
> 
> be true?

That is the intention, yes. More specifically, binary_expr<X,Y,Z> 
matches binary_expr<A,B,C>::type when Y matches B, Z matches C, and X is 
A or proto::_.

> If not, then what is the purpose of binary_expr<,,>::type?
> If yes, then why am I getting an error with the attached driver
> whose compilation output is also attached?

Your problem distills down to:

typedef terminal<x> GX; // grammar for x
typedef GX::type EX;    // expression for x

typedef shift_right<GX, GX> SRGX; // grammar for x>>x
typedef SRGX::type SREX;          // OOPS!

You then try to use SREX as a valid expression type, and it is not. It 
should be:

typedef shift_right<EX, EX>::type SREX;

The difference is that in the first case, you're creating the type of a 
right shift expression where the operands are grammars. You really want 
the operands to be expression types.

HTH,

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