Why is that? It's done in a consistent way across all distributions. Can we remove the check inside the non member functions that are already checked in the constructor?
As I recollect, I think we thought there were circumstances when this check wasn't redundant (but I can't remember the exact case. Users changes the distribution parameters?).
And because it was believed better to be safe than sorry, and the check was very cheap (at runtime), we kept it that way.
The issue is like this:
* If the current policy (a template param) is to throw on domain errors (the default), then the second check in the non-member functions is redundant as any errors will have triggered an exception in the distribution constructor. On the other hand: * If the current policy is to not throw (return NaN on domain errors), then the constructor checks are redundant and the non-member function checks are the required ones.
If it really is a performance bottleneck, then we could add a check on the current exception policy so that those two mutually exclusive options get optimized at compile time.
From a performance point of view, suppose I wanted to have a Policy mechanism that allows me to eliminate *all* error checking machine instructions from a distribution constructor of distribution non member function. Would it be possible to move the detail::check_scale(d) etc into the Policy class? That way I could provide a different policy that get's eliminated compile time via an implementation like "inline policy::check_scale(d){return;}"