boost_1_45_0/libs/math/example/policy_eg_6.cpp

//  Copyright John Maddock 2007.
//  Copyright Paul A. Bristow 2010
//  Use, modification and distribution are subject to the
//  Boost Software License, Version 1.0. (See accompanying file
//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

// Note that this file contains quickbook mark-up as well as code
// and comments, don't change any of the special comment mark-ups!

#include <iostream>
using std::cout;  using std::endl;
#include <cerrno> // for ::errno

//[policy_eg_6

/*`
Suppose we want a set of distributions to behave as follows:

* Return infinity on overflow, rather than throwing an exception.
* Don't perform any promotion from double to long double internally.
* Return the closest integer result from the quantiles of discrete
distributions.

We'll begin by including the needed header for all the distributions:
*/

#include <boost/math/distributions.hpp>

/*`

Open up an appropriate namespace, calling it `my_distributions`,
for our distributions, and define the policy type we want.
Any policies we don't specify here will inherit the defaults:

*/

namespace my_distributions
{
  using namespace boost::math::policies;
  // using boost::math::policies::errno_on_error; // etc.

  typedef policy<
     // return infinity and set errno rather than throw:
     overflow_error<errno_on_error>,
     // Don't promote double -> long double internally:
     promote_double<false>,
     // Return the closest integer result for discrete quantiles:
     discrete_quantile<integer_round_nearest>
  > my_policy;

/*`

All we need do now is invoke the BOOST_MATH_DECLARE_DISTRIBUTIONS
macro passing the floating point type `double` and policy types `my_policy` as arguments:

*/

BOOST_MATH_DECLARE_DISTRIBUTIONS(double, my_policy)

} // close namespace my_namespace

/*`

We now have a set of typedefs defined in namespace my_distributions
that all look something like this:

``
typedef boost::math::normal_distribution<double, my_policy> normal;
typedef boost::math::cauchy_distribution<double, my_policy> cauchy;
typedef boost::math::gamma_distribution<double, my_policy> gamma;
// etc
``

So that when we use my_distributions::normal we really end up using
`boost::math::normal_distribution<double, my_policy>`:

*/

int main()
{
   // Construct distribution with something we know will overflow
  // (using double rather than if promoted to long double):
   my_distributions::normal norm(10, 2);

   errno = 0;
   cout << "Result of quantile(norm, 0) is: "
      << quantile(norm, 0) << endl; // -infinity.
   cout << "errno = " << errno << endl;
   errno = 0;
   cout << "Result of quantile(norm, 1) is: "
      << quantile(norm, 1) << endl; // +infinity.
   cout << "errno = " << errno << endl;

   // Now try a discrete distribution.
   my_distributions::binomial binom(20, 0.25);
   cout << "Result of quantile(binom, 0.05) is: "
      << quantile(binom, 0.05) << endl; // To check we get integer results.
   cout << "Result of quantile(complement(binom, 0.05)) is: "
      << quantile(complement(binom, 0.05)) << endl;
}

/*`

Which outputs:

[pre
Result of quantile(norm, 0) is: -1.#INF
errno = 34
Result of quantile(norm, 1) is: 1.#INF
errno = 34
Result of quantile(binom, 0.05) is: 1
Result of quantile(complement(binom, 0.05)) is: 8
]

This mechanism is particularly useful when we want to define a
project-wide policy, and don't want to modify the Boost source
or set  project wide build macros (possibly fragile and easy to forget).

*/
//] //[/policy_eg_6]