Efficient way of finding distance between two 3D points

Question

I am writing a code in C++ and want to compute distance between two points. Question 1:

I have two points P(x1, y1, z1) and Q(x2, y2, z2) , where x,

Answer 1

The GNU Scientific Library defines gsl_hypot3 that computes exactly the distance you want in the first part of your question. Kind of overkill compiling the whole thing just for that, given Darius' suggestion, but maybe there's other stuff there you want.