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#### Using macros to Change the Policy Defaults

You can use the various macros below to change any (or all) of the policies.

You can make a local change by placing a macro definition before a function or distribution #include.

| Caution
There is a danger of One-Definition-Rule violations if you add ad-hock macros to more than one source files: these must be set the same in every translation unit.

| Caution
If you place it after the #include it will have no effect, (and it will affect only any other following #includes). This is probably not what you intend!

If you want to alter the defaults for any or all of the policies for all functions and distributions, installation-wide, then you can do so by defining various macros in boost/math/tools/user.hpp.

###### BOOST_MATH_DOMAIN_ERROR_POLICY

Defines what happens when a domain error occurs, if not defined then defaults to `throw_on_error`, but can be set to any of the enumerated actions for error handing: `throw_on_error`, `errno_on_error`, `ignore_error` or `user_error`.

###### BOOST_MATH_POLE_ERROR_POLICY

Defines what happens when a pole error occurs, if not defined then defaults to `throw_on_error`, but can be set to any of the enumerated actions for error handing: `throw_on_error`, `errno_on_error`, `ignore_error` or `user_error`.

###### BOOST_MATH_OVERFLOW_ERROR_POLICY

Defines what happens when an overflow error occurs, if not defined then defaults to `throw_on_error`, but can be set to any of the enumerated actions for error handing: `throw_on_error`, `errno_on_error`, `ignore_error` or `user_error`.

###### BOOST_MATH_EVALUATION_ERROR_POLICY

Defines what happens when an internal evaluation error occurs, if not defined then defaults to `throw_on_error`, but can be set to any of the enumerated actions for error handing: `throw_on_error`, `errno_on_error`, `ignore_error` or `user_error`.

###### BOOST_MATH_UNDERFLOW_ERROR_POLICY

Defines what happens when an overflow error occurs, if not defined then defaults to `ignore_error`, but can be set to any of the enumerated actions for error handing: `throw_on_error`, `errno_on_error`, `ignore_error` or `user_error`.

###### BOOST_MATH_DENORM_ERROR_POLICY

Defines what happens when a denormalisation error occurs, if not defined then defaults to `ignore_error`, but can be set to any of the enumerated actions for error handing: `throw_on_error`, `errno_on_error`, `ignore_error` or `user_error`.

###### BOOST_MATH_DIGITS10_POLICY

Defines how many decimal digits to use in internal computations: defaults to `0` - meaning use all available digits - but can be set to some other decimal value. Since setting this is likely to have a substantial impact on accuracy, it's not generally recommended that you change this from the default.

###### BOOST_MATH_PROMOTE_FLOAT_POLICY

Determines whether `float` types get promoted to `double` internally to ensure maximum precision in the result, defaults to `true`, but can be set to `false` to turn promotion of `float`'s off.

###### BOOST_MATH_PROMOTE_DOUBLE_POLICY

Determines whether `double` types get promoted to `long double` internally to ensure maximum precision in the result, defaults to `true`, but can be set to `false` to turn promotion of `double`'s off.

###### BOOST_MATH_DISCRETE_QUANTILE_POLICY

Determines how discrete quantiles return their results: either as an integer, or as a real value, can be set to one of the enumerated values: `real`, `integer_round_outwards`, `integer_round_inwards`, `integer_round_down`, `integer_round_up`, `integer_round_nearest`. Defaults to `integer_round_outwards`.

###### BOOST_MATH_ASSERT_UNDEFINED_POLICY

Determines whether functions that are mathematically undefined for a specific distribution compile or raise a static (i.e. compile-time) assertion. Defaults to `true`: meaning that any mathematically undefined function will not compile. When set to `false` then the function will compile but return the result of a domain error: this can be useful for some generic code, that needs to work with all distributions and determine at runtime whether or not a particular property is well defined.

###### BOOST_MATH_MAX_SERIES_ITERATION_POLICY

Determines how many series iterations a special function is permitted to perform before it gives up and returns an evaluation_error: Defaults to 1000000.

###### BOOST_MATH_MAX_ROOT_ITERATION_POLICY

Determines how many root-finding iterations a special function is permitted to perform before it gives up and returns an evaluation_error: Defaults to 200.

###### Example

Suppose we want overflow errors to set `::errno` and return an infinity, discrete quantiles to return a real-valued result (rather than round to integer), and for mathematically undefined functions to compile, but return a domain error. Then we could add the following to boost/math/tools/user.hpp:

    #define BOOST_MATH_OVERFLOW_ERROR_POLICY errno_on_error
    #define BOOST_MATH_DISCRETE_QUANTILE_POLICY real
    #define BOOST_MATH_ASSERT_UNDEFINED_POLICY false

or we could place these definitions before

    #include <boost/math/distributions/normal.hpp>
      using boost::math::normal_distribution;

in a source .cpp file.