结合之前博客中的代码， 产生新的求根函数

using dFun        = function<double(const double&)>;
using cdouble     = const double;

cdouble TOLERANCE = 0.00001;

bool CloseEnough (cdouble &val1, cdouble &val2)
{
    return (abs(val1 - val2) < TOLERANCE);
}

double FixedPoint (dFun f, cdouble &guess)
{
    const auto next = f (guess);
    if (CloseEnough (guess, next)) {
        return next;
    }
    else {
        return FixedPoint (f, next);
    }
}

dFun AverageDamp (dFun f)
{
    return [f] (cdouble &x)
    {return (x + f(x)) / 2;};
}

double Sqrt (cdouble &x)
{
    return FixedPoint (AverageDamp(
                                 [x] (cdouble &y)
                                     {return (x / y); })
                               , 1.0);
}

int main ()
{
    cout << Sqrt(121.0);
    cout << endl;
    return 0;
}