A trick like Incorrect Values Creating Expression From Other Expressions - #2 by francesco-ballarin may be of help. Define two symbols x0 = sympy.var("x[0]") and x1 = sympy.var("x[1]"), use sympy to compute theta and r starting from x0 and x1, write your expression in theta and r, and finally convert them to a C++ code with sympy.printing.ccode.