On 8/16/2012 6:44 AM, Andrew Sutton wrote:
To avoid redundant errors, you must dispatch to an empty implementation on concept check failure:
template <typename I, typename T> I find_impl(std::true_type, I first, I last, const T& value) { while (first != last && *first != value) ++first; }
template <typename I, typename T> I find_impl(std::false_type, I first, I last, const T& value) { static_assert (Input_iterator<I>(), ""); }
template <typename I, typename T> I find(I first, I last, const T& value) { find_impl(Input_iterator<I>(), first, last, value); }
Not if you use the trick above.
Interesting... That looks like it will work.
It should. It's an old trick I use in a bunch of places. I even wrote about it here: http://cpp-next.com/archive/2010/09/expressive-c-why-template-errors-suck-an... (See the section "Avoid Follow-on Errors") FWIW, the code I sent above can be improved. The static_assert should be in find(), not find_impl. That will remove one frame of the stack backtrace. And I think the find_impl that takes false_type only needs to be forward-declared. That way you don't need to mock up a return call that will never be used.
Do you still need the sfinae_error class to get shallow errors? I'm guessing not in these cases.
Depends. If you already have an easy way to implement your concept classes so that checking against types that don't model the concept doesn't cause a hard error, then you can forget about the try_call wrapper. Otherwise, it's still useful to you, even though you wouldn't be making use of the short error reporting mechanism.
You only have the guard the calls that add constraints. Other calls will simply propagate sfinae_error and don't need to be guarded. (In my example, S2 doesn't need to guard the call to S1, but S1 must guard S0 because it adds an Addable constraint.) Also, you can guard your function object once by defining it with try_call_wrapper, so it doesn't need to be guarded everywhere.
Just apply more force. :-)
Indeed :)
Side note: sfinae_error is a bad name in a worse way than how "PIN number" is redundant. It's not an error, but it is. substitution_failure or subst_failure might be better choices.
You're right. Thanks. -- Eric Niebler BoostPro Computing http://www.boostpro.com