How efficient is it to make a temporary uniform random distribution each time round a loop?

Question

For example:

for (...)
{
    ... std::uniform_real_distribution<float>(min, max)(rng) ...
}

Intuitively it seems to me that the constructor can't need to do much besides store the two values, and there shouldn't be any state in the uniform_*_distribution instance. I haven't profiled it myself (I'm not at that kind of stage in the project yet), but I felt this question belonged out there :)

I am aware that this would be a bad idea for some distribution types - for example, std::normal_distribution might generate its numbers in pairs, and the second number would be wasted each time.

I feel what I have is more readable than just accessing rng() and doing the maths myself, but I'd be interested if there are any other ways to write this more straightforwardly.

Answer 1

std::uniform_real_distribution's objects are lightweight, so it's not a problem to construct them each time inside the loop.

Sometimes, the hidden internal state of distribution is important, but not in this case. reset() function is doing nothing in all popular STL implementations:

void
reset() { }

For example, it's not true for std::normal_distribution:

void
reset()
{ _M_saved_available = false; }