A C++ implementation of log that one can specify the base for? [duplicate]

Question

This question already has an answer here:

• Logarithm function of an arbitrary integer base in C 1 answer

I've been playing around with some math recently and I would like to know if anyone has written/seen a C++ implementation of log that one can specify the base (root..?) for? As in:

Mathematical function definition http://i1091.photobucket.com/albums/i383/dannydeth1/forumla.png

Obviously I would prefer giving the base as an argument: double d = log(b,x);

Thank you for your time and any answers are much appreciated. :}

EDIT: Also, I take it would use Taylor Series?

Answer 1

log_b_(x) = log(x) / log(b). Just do this:

double log(double base, double x)
{
    return std::log(x) / std::log(base);
}

Answer 2

It's straightforward to implement yourself:

double
logb( double n, double b )
{
    return log(n) / log(b);
}

Is it generally useful? Or are practically all of the uses subsumed by log, log10 and log2?