Round double to nearest uint64_t

Question

I need to round a double to the nearest valid uint64_t .

So

uint64_t Round(double);
Round(std::numeric_limits<double>::max())==std::numeric_limits<uint64_t>::max();
Round(std::numeric_limits<double>::lowest())==std::numeric_limits<uint64_t>::min();
Round(1.1)==1;

The Round should be equivalent this to but for uint64_t rather than signed integral

auto Round(double d){
    std::fesetround(FE_TONEAREST);
    return llrint(d);
}

Are there any functions in std && boost to enable this?

Answer 1

A double cannot hold all the values of uint64_t , since both are usually 64-bits and the double needs to set aside bits for the exponent.

However, it's not too hard to get the closest value:

uint64_t Round(double x){
    double rounded=round(x);
    if(rounded<0)
        return 0;
    if(rounded>=pow(2,64))
       return std::numeric_limits<uint64_t>::max();
    return static_cast<uint64_t>(rounded);
}