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

From: Larry Evans (cppljevans_at_[hidden])
Date: 2008-03-02 21:34:28

On 03/02/08 20:19, Eric Niebler wrote:
> Larry Evans wrote:
>> On 03/02/08 19:39, Eric Niebler wrote:
>> [snip]
>>> 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.
[snip]
> 
>> I'd be more comfortable if the validity of the nested type
>> didn't depend on particular type of the template parameter.
> 
> It doesn't.
> 

If the template arguments are grammars then shift_right<L,R>::type is
not an expression type. AFAICT, it's of no use.  OTOH, If the template 
arguments are expression types then shift_right<L,R>::type is an 
expression type, at least according to what you say above.  So maybe
I should have said the "usefulness" instead of "validity".