Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.
PrevUpHomeNext

Changing the Policy on an Ad Hoc Basis for the Special Functions

All of the special functions in this library come in two overloaded forms, one with a final "policy" parameter, and one without. For example:

namespace boost{ namespace math{

template <class RealType, class Policy>
RealType tgamma(RealType, const Policy&);

template <class RealType>
RealType tgamma(RealType);

}} // namespaces

Normally, the second version is just a forwarding wrapper to the first like this:

template <class RealType>
inline RealType tgamma(RealType x)
{
   return tgamma(x, policies::policy<>());
}

So calling a special function with a specific policy is just a matter of defining the policy type to use and passing it as the final parameter. For example, suppose we want tgamma to behave in a C-compatible fashion and set ::errno when an error occurs, and never throw an exception:

#include <boost/math/special_functions/gamma.hpp>
using boost::math::tgamma;

// Define the policy to use:
using namespace boost::math::policies; // may be convenient, or

using boost::math::policies::policy;
// Types of error whose action can be altered by policies:.
using boost::math::policies::evaluation_error;
using boost::math::policies::domain_error;
using boost::math::policies::overflow_error;
using boost::math::policies::domain_error;
using boost::math::policies::pole_error;
// Actions on error (in enum error_policy_type):
using boost::math::policies::errno_on_error;
using boost::math::policies::ignore_error;
using boost::math::policies::throw_on_error;
using boost::math::policies::user_error;

typedef policy<
   domain_error<errno_on_error>,
   pole_error<errno_on_error>,
   overflow_error<errno_on_error>,
   evaluation_error<errno_on_error>
> c_policy;
//
// Now use the policy when calling tgamma:

// http://msdn.microsoft.com/en-us/library/t3ayayh1.aspx 
// Microsoft errno declared in STDLIB.H as "extern int errno;" 

int main()
{
   errno = 0; // Reset.
   cout << "Result of tgamma(30000) is: "
      << tgamma(30000, c_policy()) << endl; // Too big parameter
   cout << "errno = " << errno << endl; // errno 34 Numerical result out of range.
   cout << "Result of tgamma(-10) is: "
      << boost::math::tgamma(-10, c_policy()) << endl; // Negative parameter.
   cout << "errno = " << errno << endl; // error 33 Numerical argument out of domain.
} // int main()

which outputs:

Result of tgamma(30000) is: 1.#INF
errno = 34
Result of tgamma(-10) is: 1.#QNAN
errno = 33

Alternatively, for ad hoc use, we can use the make_policy helper function to create a policy for us: this usage is more verbose, so is probably only preferred when a policy is going to be used once only:

#include <boost/math/special_functions/gamma.hpp>
using boost::math::tgamma;

int main()
{
   // using namespace boost::math::policies; // or
   using boost::math::policies::errno_on_error;
   using boost::math::policies::make_policy;
   using boost::math::policies::pole_error;
   using boost::math::policies::domain_error;
   using boost::math::policies::overflow_error;
   using boost::math::policies::evaluation_error;

   errno = 0;
   std::cout << "Result of tgamma(30000) is: "
      << boost::math::tgamma(
         30000,
         make_policy(
            domain_error<errno_on_error>(),
            pole_error<errno_on_error>(),
            overflow_error<errno_on_error>(),
            evaluation_error<errno_on_error>()
         )
      ) << std::endl;
   // Check errno was set:
   std::cout << "errno = " << errno << std::endl;
   // and again with evaluation at a pole:
   std::cout << "Result of tgamma(-10) is: "
      << boost::math::tgamma(
         -10,
         make_policy(
            domain_error<errno_on_error>(),
            pole_error<errno_on_error>(),
            overflow_error<errno_on_error>(),
            evaluation_error<errno_on_error>()
         )
      ) << std::endl;
   // Check errno was set:
   std::cout << "errno = " << errno << std::endl;
}

PrevUpHomeNext