It means it isn't worth converting it to C++ ;) Use numerical methods to find the length.
The length of a curve y = f(x) between x = a and x = b is
integral(from a to b)(sqrt(1 + (dy/dx)2)) which is not a nice thing to integrate analytically. (EDIT: Because of the square root. It would be easy for polynomials if the sqrt wasn't there).
Wikipedia page about it is here: https://en.wikipedia.org/wiki/Arc_length
EDIT2: So, for the simple function y = x3 the integration involves complex numbers, inverse hyperbolic sin function, and elliptic integrals. Not nice at all ;)
The arc length for a parabola y = x2 is a little bit nicer, http://www.wolframalpha.com/input/?i=integral%28sqrt%281+%2B+4x^2%29%29 , it only involves inverse hyperbolic sin (asinh)... which is now standard in cmath with C++11, but it wasn't in cmath as standard before that.
EDIT3: It's easy for y = x though, here is the full working for that.
Arc length for y = x:
dy/dx = 1
integral(sqrt(1 + (dy/dx)2)) = integral(sqrt(1 + 12)) = integral(sqrt(1 + 1)) = integral(sqrt(2)) = x * sqrt(2) + constant. (note: the integral is with respect to x)
so for arc length between a and b the answer is integrate(a to b)(sqrt(2)) = F(b) - F(a) where F(x) = integral(sqrt(2)) = x * sqrt(2) + c
= b * sqrt(2) + c - (a * sqrt(2) + c) = (b-a)sqrt(2) + (c - c) = (b-a)sqrt(2)
so arc length from 0 to 1 of y = x is (1 - 0)sqrt(2) = sqrt(2) as expected.
EDIT4: Lots of edits ;)
EDIT5: Last edit, I promise. The derivation of the arc length for a parabola is explained here http://planetmath.org/arclengthofparabola