Sophisticated constraint checks

[Chris Jefferson]

Jonathan Wakely wrote:
> <snip>
> Or something like this is pretty clear, but not re-usable, and the extra
> "divide by zero" and "not integer constant" noise in the diagnostic might
> confuse some people more than it helps:
> 
>   template <typename T, int p, int r>
>     class discard_block
>     {
>       // constraints 0 <= r <= p
>       enum { _Require_r_not_less_than_zero = (1/(0<=r)) };
>       enum { _Require_r_not_greater_than_p = (1/(r<=p)) };
>     };

Just on this note, I remember hearing some time ago that gcc might in 
4.1 be gaining some kind of static_assert, so we could write something like:

static_assert(r>=0,"Block cannot be of negative length(%d),r);
static_assert(r<=p,"Length of block (%d) must be less than the 
maximum(%d)",r,p);

(not actually good / correct error messages!)

Does anyone know if such a thing is likely to appear? and I am alone in 
thinking such a thing would make much of the checking much easier to follow?

Unfortunatly, this doesn't fix all problems, in particular while people 
have managed to create templates to check things like if "a==b" is a 
valid expression, such templates are really, really complicated and 
perhaps could be easied by compiler support...

Chris