That's unfair! The algorithm is in the function "escape", not the function "fractal".

escape's function signature is much simpler:

    int escape(std::complex<double> c, int iter_max, const std::function<std::complex<double>(std::complex<double>, 
	std::complex<double>)> &func)

At least we get to use "auto" when we define func, though:

    auto func = [] (std::complex <double> z, std::complex<double> c) -> std::complex<double> {return z * z * z + c; };