Re: [Boost-users] [PROTO] Custom functions, scalar terminal and make_expr vs details::as_expr_if

Subject: Re: [Boost-users] [PROTO] Custom functions, scalar terminal and make_expr vs details::as_expr_if
From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2008-11-30 05:15:59

Eric Niebler a écrit :
> The first question to ask is if it is really absolutely necessary to 
> prevent users from creating invalid expressions with 
> my_custom_binary_func(), or if you can catch the error someplace else; 
> e.g., when users try to evaluate the expression. Reporting a hard 
> error later can often yield a better error message then using SFINAE 
> to prune overload sets earlier.
OK then, but I don't really want my user face a large template error 
because they mismatched some binary function arguments when they can 
have a nice 'no such function F(A,B)' error message instead.

The solution of enumerating the (expr,expr) (expr,scalar) (scalar,expr) 
variants is maybe what's needed then ?
The only problem I see is that enumerating the ternary one will be real 
long.
> If you want to improve compile times even more, you can avoid 
> make_expr entirely and code by-hand what make_expr would do. So, e.g., 
> the second overload above would be:
> <snip>
Why is this faster than calling make_expr ? Even worse, doesn't it 
defeat the proto purpose ?
Or am I in a situation I have to write dirty code like this and only use 
Proto as a expression builder ?

-- 
___________________________________________
Joel Falcou - Assistant Professor
PARALL Team - LRI - Universite Paris Sud XI
Tel : [phone]