Your pow function might run enlessly or return incorrect results because your y parameter is a double.

Careful: whitespace does not imply only " ".

never returns for n<0

rand() doesn't generate real random numbers, and using it with modulo introduces bias.
I recommend using utilities in the header (with std::uniform_int_distribution) if you need quality random numbers.
std::random_device can provide real random numbers if your system has entropy.

For data, you can construct a vector (or std::array) with fixed size 255.

switch is faster than if-else

look up std::bitset

look up std::bitset

hint:

1. either sum up the smallest values first for better precision, then you don't need temp
2. or stop after temp < sum * std::numeric_limits::epsilon.
   Any further term wont increase sum.

Not exception safe, if addEntry is fed with wrong values (e.g. Single)

countPrimeDiv is endless loop if n<1
cnt in consecKprimes might overflow