C++11: Compile-time value checking of constructor arguments possible?

[Zack Weinberg]

On Thu, Mar 15, 2012 at 2:34 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Thu, 15 Mar 2012, Zack Weinberg wrote:
>> const K good(1);   // should compile
>> const K bad(999);  // should not compile
>
> You'ld want constexpr on those too (if there was any chance).

... why?

> constexpr has nothing to do with const or volatile, it is closer to static
> if you want a comparison...

I admit I don't fully understand constexpr, but this doesn't clarify
things _at all_ for me.

>> Any advice?  Solutions that work with 4.6 *strongly* preferred.
>
> Use a factory: make_K<1>() or make_K<999>() ?

This class already has too much boilerplate.  Also, it does not help
with the other situation where I need constructor value checks: an
class B whose instances do _not_ represent constants, but which should
only be initializable from K instances, B instances, or the integer
literal 0.  (I have a kludge for that involving std::nullptr_t, but I
don't like it.)

zw