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


// example_error_handling.cpp

// Copyright Paul A. Bristow 2007.
// Copyright John Maddock 2007.

// Use, modification and distribution are subject to the
// Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt
// or copy at

// Note that this file contains quickbook markup as well as code
// and comments, don't change any of the special comment markups!

The following example demonstrates the effect of
when an invalid argument is encountered.  For the
purposes of this example, we'll pass a negative
degrees of freedom parameter to the student's t

Since we know that this is a single file program we could
just add:

   #define BOOST_MATH_DOMAIN_ERROR_POLICY ignore_error

to the top of the source file to change the default policy
to one that simply returns a NaN when a domain error occurs.
Alternatively we could use:

   #define BOOST_MATH_DOMAIN_ERROR_POLICY errno_on_error

To ensure the `::errno` is set when a domain error occurs
as well as returning a NaN.

This is safe provided the program consists of a single
translation unit /and/ we place the define /before/ any
#includes.  Note that should we add the define after the includes
then it will have no effect!  A warning such as:

[pre warning C4005: 'BOOST_MATH_OVERFLOW_ERROR_POLICY' : macro redefinition]

is a certain sign that it will /not/ have the desired effect.

We'll begin our sample program with the needed includes:

// Boost
#include <boost/math/distributions/students_t.hpp>
   using boost::math::students_t;  // Probability of students_t(df, t).

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

#include <stdexcept>
   using std::exception;


Next we'll define the program's main() to call the student's t
distribution with an invalid degrees of freedom parameter, the program
is set up to handle either an exception or a NaN:


int main()
   cout << "Example error handling using Student's t function. " << endl;
   cout << "BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "

   double degrees_of_freedom = -1; // A bad argument!
   double t = 10;

      errno = 0;
      students_t dist(degrees_of_freedom); // exception is thrown here if enabled
      double p = cdf(dist, t);
      // test for error reported by other means:
         cout << "cdf returned a NaN!" << endl;
         cout << "errno is set to: " << errno << endl;
         cout << "Probability of Student's t is " << p << endl;
   catch(const std::exception& e)
      std::cout <<
         "\n""Message from thrown exception was:\n   " << e.what() << std::endl;

   return 0;
} // int main()


Here's what the program output looks like with a default build
(one that does throw exceptions):

Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: throw_on_error

Message from thrown exception was:
   Error in function boost::math::students_t_distribution<double>::students_t_distribution:
   Degrees of freedom argument is -1, but must be > 0 !

Alternatively let's build with:

   #define BOOST_MATH_DOMAIN_ERROR_POLICY ignore_error

Now the program output is:

Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: ignore_error
cdf returned a NaN!
errno is set to: 0

And finally let's build with:

   #define BOOST_MATH_DOMAIN_ERROR_POLICY errno_on_error

Which gives the output:

Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: errno_on_error
cdf returned a NaN!
errno is set to: 33


//] [error_handling_eg end quickbook markup]