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### Changing the Policy Defaults

The default policies used by the library are changed by the usual configuration macro method.

For example, passing `-DBOOST_MATH_DOMAIN_ERROR_POLICY=errno_on_error` to your compiler will cause domain errors to set `::errno` and return a NaN rather than the usual default behaviour of throwing a `std::domain_error` exception.

| Tip
For Microsoft Visual Studio,you can add to the Project Property Page, C/C++, Preprocessor, Preprocessor definitions like:
    BOOST_MATH_ASSERT_UNDEFINED_POLICY=0
    BOOST_MATH_OVERFLOW_ERROR_POLICY=errno_on_error
This may be helpful to avoid complications with pre-compiled headers that may mean that the equivalent definitions in source code:
    #define BOOST_MATH_ASSERT_UNDEFINED_POLICY false
    #define BOOST_MATH_OVERFLOW_ERROR_POLICY errno_on_error
may be ignored. The compiler command line shows:
    /D "BOOST_MATH_ASSERT_UNDEFINED_POLICY=0"
    /D "BOOST_MATH_OVERFLOW_ERROR_POLICY=errno_on_error"

There is however a very important caveat to this:

| Important
Default policies changed by setting configuration macros must be changed uniformly in every translation unit in the program. Failure to follow this rule may result in violations of the "One Definition Rule (ODR)" and result in unpredictable program behaviour.

That means there are only two safe ways to use these macros:

- Edit them in boost/math/tools/user.hpp, so that the defaults are set on an installation-wide basis. Unfortunately this may not be convenient if you are using a pre-installed Boost distribution (on Linux for example).
- Set the defines in your project's Makefile or build environment, so that they are set uniformly across all translation units.

What you should not do is:

- Set the defines in the source file using `#define` as doing so almost certainly will break your program, unless you're absolutely certain that the program is restricted to a single translation unit.

And, yes, you will find examples in our test programs where we break this rule: but only because we know there will always be a single translation unit only: don't say that you weren't warned!

The following example demonstrates the effect of setting the macro BOOST_MATH_DOMAIN_ERROR_POLICY 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 distribution.

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:

    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:

       #define BOOST_MATH_DOMAIN_ERROR_POLICY ignore_error

    // 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>

    #include <cstddef>
       // using ::errno

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: "
          << BOOST_STRINGIZE(BOOST_MATH_DOMAIN_ERROR_POLICY) << endl;

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

       try
       {
          errno = 0; // Clear/reset.
          students_t dist(degrees_of_freedom); // exception is thrown here if enabled.
          double p = cdf(dist, t);
          // Test for error reported by other means:
          if((boost::math::isnan)(p))
          {
             cout << "cdf returned a NaN!" << endl;
             if (errno != 0)
             { // So errno has been set.
               cout << "errno is set to: " << errno << endl;
             }
          }
          else
             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!

And finally let's build with:

    #define BOOST_MATH_DOMAIN_ERROR_POLICY errno_on_error

Which gives the output show errno:

    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