How to find nearest next/previous double value (numeric_limits::epsilon for given number)

Question

The title is quite self-explanatory, input is given double value, and I want to add/substract the smallest amount possible.

Answer 1

Here's a very dirty trick that isn't actually legal and only works if your platform uses IEEE754 floats: The binary representation of the float is ordered in the same way as the float value, so you can increment the binary representation:

double x = 1.25;

uint64_t * const p = reinterpret_cast(&x);

++*p;   // undefined behaviour! but it gets the next value

// now x has the next value

You can achieve the same effect entirely legally by doing the usual binary copy gymnastics to obtain a proper uint64_t value. Make sure to check for zero, infinity and NaN properly, too.