ceph/src/boost/libs/math/example/autodiff_fourth_power.cpp

   1 //           Copyright Matthew Pulver 2018 - 2019.
   2 // Distributed under the Boost Software License, Version 1.0.
   3 //      (See accompanying file LICENSE_1_0.txt or copy at
   4 //           https://www.boost.org/LICENSE_1_0.txt)
   5
   6 #include <boost/math/differentiation/autodiff.hpp>
   7 #include <iostream>
   8
   9 template <typename T>
  10 T fourth_power(T const& x) {
  11   T x4 = x * x;  // retval in operator*() uses x4's memory via NRVO.
  12   x4 *= x4;      // No copies of x4 are made within operator*=() even when squaring.
  13   return x4;     // x4 uses y's memory in main() via NRVO.
  14 }
  15
  16 int main() {
  17   using namespace boost::math::differentiation;
  18
  19   constexpr unsigned Order = 5;                  // Highest order derivative to be calculated.
  20   auto const x = make_fvar<double, Order>(2.0);  // Find derivatives at x=2.
  21   auto const y = fourth_power(x);
  22   for (unsigned i = 0; i <= Order; ++i)
  23     std::cout << "y.derivative(" << i << ") = " << y.derivative(i) << std::endl;
  24   return 0;
  25 }
  26 /*
  27 Output:
  28 y.derivative(0) = 16
  29 y.derivative(1) = 32
  30 y.derivative(2) = 48
  31 y.derivative(3) = 48
  32 y.derivative(4) = 24
  33 y.derivative(5) = 0
  34 **/