[section:error_handling Error Handling]

[def __format [@../../../../format/index.html Boost.Format]]

[heading Quick Reference]

Handling of errors by this library is split into two orthogonal parts:

* What kind of error has been raised?
* What should be done when the error is raised?

The kinds of errors that can be raised are:

[variablelist
[[Domain Error][Occurs when one or more arguments to a function 
  are out of range.]]
[[Pole Error][Occurs when the particular arguments cause the function to be
   evaluated at a pole with no well defined residual value.  For example if
   __tgamma is evaluated at exactly -2, the function approaches different limiting
   values depending upon whether you approach from just above or just below
   -2.  Hence the function has no well defined value at this point and a 
   Pole Error will be raised.]]
[[Overflow Error][Occurs when the result is either infinite, or too large
   to represent in the numeric type being returned by the function.]]
[[Underflow Error][Occurs when the result is not zero, but is too small
   to be represented by any other value in the type being returned by 
   the function.]]
[[Denormalisation Error][Occurs when the returned result would be a denormalised value.]]
[[Evaluation Error][Occurs when an internal error occured that prevented the
   result from being evaluated: this should never occur, but if it does, then
   it's likely to be due to an iterative method not converging fast enough.]]
]

The action undertaken by each error condition is determined by the current
__Policy in effect.  This can be changed program-wide by setting some 
configuration macros, or at namespace scope, or at the call site (by
specifying a specific policy in the function call).

The available actions are:

[variablelist
[[throw_on_error][Throws the exception most appropriate to the error condition.]]
[[errno_on_error][Sets ::errno to an appropriate value, and then returns the most
appropriate result]]
[[ignore_error][Ignores the error and simply the returns the most appropriate result.]]
[[user_error][Calls a 
   [link math_toolkit.policy.pol_tutorial.user_def_err_pol user-supplied error handler].]]
]

The following tables show all the permutations of errors and actions, 
with the default action for each error shown in bold:

[table Possible Actions for Domain Errors
[[Action]         [Behaviour]]
[[throw_on_error][[*Throws `std::domain_error`]]]
[[errno_on_error][Sets `::errno` to `EDOM` and returns `std::numeric_limits<T>::quiet_NaN()`]]
[[ignore_error][Returns `std::numeric_limits<T>::quiet_NaN()`]]
[[user_error][Returns the result of `boost::math::policies::user_domain_error`: 
            [link math_toolkit.policy.pol_tutorial.user_def_err_pol 
            this function must be defined by the user].]]
]
    
[table Possible Actions for Pole Errors
[[Action]   [Behaviour]]
[[throw_on_error]   [[*Throws `std::domain_error`]]]
[[errno_on_error][Sets `::errno` to `EDOM` and returns `std::numeric_limits<T>::quiet_NaN()`]]
[[ignore_error][Returns `std::numeric_limits<T>::quiet_NaN()`]]
[[user_error][Returns the result of `boost::math::policies::user_pole_error`: 
            [link math_toolkit.policy.pol_tutorial.user_def_err_pol 
            this function must be defined by the user].]]
]

[table Possible Actions for Overflow Errors
[[Action]   [Behaviour]]
[[throw_on_error][[*Throws `std::overflow_error`]]]
[[errno_on_error][Sets `::errno` to `ERANGE` and returns `std::numeric_limits<T>::infinity()`]]
[[ignore_error][Returns `std::numeric_limits<T>::infinity()`]]
[[user_error][Returns the result of `boost::math::policies::user_overflow_error`: 
            [link math_toolkit.policy.pol_tutorial.user_def_err_pol 
            this function must be defined by the user].]]
]

[table Possible Actions for Underflow Errors
[[Action]   [Behaviour]]
[[throw_on_error][Throws `std::underflow_error`]]
[[errno_on_error][Sets `::errno` to `ERANGE` and returns 0.]]
[[ignore_error][[*Returns 0]]]
[[user_error][Returns the result of `boost::math::policies::user_underflow_error`: 
            [link math_toolkit.policy.pol_tutorial.user_def_err_pol 
            this function must be defined by the user].]]
]

[table Possible Actions for Denorm Errors
[[Action]   [Behaviour]]
[[throw_on_error][Throws `std::underflow_error`]]
[[errno_on_error][Sets `::errno` to `ERANGE` and returns the denormalised value.]]
[[ignore_error][[*Returns the denormalised value.]]]
[[user_error][Returns the result of `boost::math::policies::user_denorm_error`: 
            [link math_toolkit.policy.pol_tutorial.user_def_err_pol 
            this function must be defined by the user].]]
]

[table Possible Actions for Internal Evaluation Errors
[[Action]   [Behaviour]]
[[throw_on_error][[*Throws `boost::math::evaluation_error`]]]
[[errno_on_error][Sets `::errno` to `EDOM` and returns `std::numeric_limits<T>::infinity()`.]]
[[ignore_error][Returns `std::numeric_limits<T>::infinity()`.]]
[[user_error][Returns the result of `boost::math::policies::user_evaluation_error`: 
            [link math_toolkit.policy.pol_tutorial.user_def_err_pol 
            this function must be defined by the user].]]
]

[heading Rationale]

The flexibility of the current implementation should be reasonably obvious, the
default behaviours were chosen based on feedback during the formal review of 
this library.  It was felt that: 

* Genuine errors should be flagged with exceptions
rather than following C-compatible behaviour and setting ::errno.
* Numeric underflow and denormalised results were not considered to be
fatal errors in most cases, so it was felt that these should be ignored.

[heading Finding More Information]

There are some pre-processor macro defines that can be used to
[link math_toolkit.policy.pol_ref.policy_defaults
change the policy defaults].  See also the [link math_toolkit.policy 
policy section].

An example is at the Policy tutorial in
[link math_toolkit.policy.pol_tutorial.changing_policy_defaults 
Changing the Policy Defaults].

Full source code of this typical example of passing a 'bad' argument
(negative degrees of freedom) to Student's t distribution 
is [link math_toolkit.dist.stat_tut.weg.error_eg in the error handling example].

The various kind of errors are described in more detail below.

[heading [#domain_error]Domain Errors]

When a special function is passed an argument that is outside the range
of values for which that function is defined, then the function returns
the result of:

   boost::math::policies::raise_domain_error<T>(FunctionName, Message, Val, __Policy);
   
Where
`T` is the floating-point type passed to the function, `FunctionName` is the 
name of the function, `Message` is an error message describing the problem, 
Val is the value that was out of range, and __Policy is the current policy
in use for the function that was called.

The default policy behaviour of this function is to throw a 
std::domain_error C++ exception.  But if the __Policy is to ignore
the error, or set global ::errno, then a NaN will be returned.

This behaviour is chosen to assist compatibility with the behaviour of 
['ISO/IEC 9899:1999 Programming languages - C]
and with the
[@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf Draft Technical Report on C++ Library Extensions, 2005-06-24, section 5.2.1, paragraph 6]:

[:['"Each of the functions declared above shall return a NaN (Not a Number)
if any argument value is a NaN, but it shall not report a domain error.
Otherwise, each of the functions declared above shall report a domain error
for just those argument values for which:]]

[:['"the function description's Returns clause explicitly specifies a domain, and those arguments fall outside the specified domain; or]

['"the corresponding mathematical function value has a non-zero imaginary component; or]

['"the corresponding mathematical function is not mathematically defined.]]

[:['"Note 2: A mathematical function is mathematically defined
for a given set of argument values if it is explicitly defined
for that set of argument values or
if its limiting value exists and does not depend on the direction of approach."]]

Note that in order to support information-rich error messages when throwing
exceptions, `Message` must contain
a __format recognised format specifier: the argument `Val` is inserted into
the error message according to the specifier used.

For example if `Message` contains a "%1%" then it is replaced by the value of
`Val` to the full precision of T, where as "%.3g" would contain the value of
`Val` to 3 digits.  See the __format documentation for more details.

[heading [#pole_error]Evaluation at a pole]

When a special function is passed an argument that is at a pole
without a well defined residual value, then the function returns
the result of:

   boost::math::policies::raise_pole_error<T>(FunctionName, Message, Val, __Policy);
   
Where
`T` is the floating point type passed to the function, `FunctionName` is the 
name of the function, `Message` is an error message describing the problem, 
`Val` is the value of the argument that is at a pole, and __Policy is the 
current policy in use for the function that was called.

The default behaviour of this function is to throw a std::domain_error exception.
But __error_policy can be used to change this, for example to `ignore_error`
and return NaN.

Note that in order to support information-rich error messages when throwing
exceptions, `Message` must contain
a __format recognised format specifier: the argument `val` is inserted into
the error message according to the specifier used.

For example if `Message` contains a "%1%" then it is replaced by the value of
`val` to the full precision of T, where as "%.3g" would contain the value of
`val` to 3 digits.  See the __format documentation for more details.

[heading [#overflow_error]Numeric Overflow]

When the result of a special function is too large to fit in the argument
floating-point type, then the function returns the result of:

   boost::math::policies::raise_overflow_error<T>(FunctionName, Message, __Policy);
   
Where
`T` is the floating-point type passed to the function, `FunctionName` is the 
name of the function, `Message` is an error message describing the problem,
and __Policy is the current policy
in use for the function that was called.

The default policy for this function is that `std::overflow_error` 
C++ exception is thrown. But if, for example, an `ignore_error` policy 
is used, then returns `std::numeric_limits<T>::infinity()`.
In this situation if the type `T` doesn't support infinities,
the maximum value for the type is returned.

[heading [#underflow_error]Numeric Underflow]

If the result of a special function is known to be non-zero, but the
calculated result underflows to zero, then the function returns the result of:

   boost::math::policies::raise_underflow_error<T>(FunctionName, Message, __Policy);
   
Where
`T` is the floating point type passed to the function, `FunctionName` is the 
name of the function, `Message` is an error message describing the problem,
and __Policy is the current policy
in use for the called function.

The default version of this function returns zero.
But with another policy, like `throw_on_error`, 
throws an `std::underflow_error` C++ exception.  

[heading [#denorm_error]Denormalisation Errors]

If the result of a special function is a denormalised value /z/ then the function
returns the result of:

   boost::math::policies::raise_denorm_error<T>(z, FunctionName, Message, __Policy);
   
Where
`T` is the floating point type passed to the function, `FunctionName` is the 
name of the function, `Message` is an error message describing the problem,
and __Policy is the current policy
in use for the called function.

The default version of this function returns /z/.
But with another policy, like `throw_on_error` 
throws an `std::underflow_error` C++ exception.

[heading [#evaluation_error]Evaluation Errors]

When a special function calculates a result that is known to be erroneous,
or where the result is incalculable then it calls:

   boost::math::policies::raise_evaluation_error<T>(FunctionName, Message, Val, __Policy);
   
Where
`T` is the floating point type passed to the function, `FunctionName` is the 
name of the function, `Message` is an error message describing the problem,
`Val` is the erroneous value,
and __Policy is the current policy
in use for the called function.

The default behaviour of this function is to throw a `boost::math::evaluation_error`.

Note that in order to support information rich error messages when throwing
exceptions, `Message` must contain
a __format recognised format specifier: the argument `val` is inserted into
the error message according to the specifier used.

For example if `Message` contains a "%1%" then it is replaced by the value of
`val` to the full precision of T, where as "%.3g" would contain the value of
`val` to 3 digits.  See the __format documentation for more details.

[heading [#checked_narrowing_cast]Errors from typecasts]

Many special functions evaluate their results at a higher precision
than their arguments in order to ensure full machine precision in 
the result: for example, a function passed a float argument may evaluate
its result using double precision internally.  Many of the errors listed
above may therefore occur not during evaluation, but when converting 
the result to the narrower result type.  The function:

   template <class T, class __Policy, class U>
   T checked_narrowing_cast(U const& val, const char* function);
   
Is used to perform these conversions, and will call the error handlers
listed above on [link overflow_error overflow], 
[link underflow_error underflow] or [link denorm_error denormalisation].

[endsect][/section:error_handling Error Handling]

[/ 
  Copyright 2006 John Maddock and Paul A. Bristow.
  Distributed under 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).
]

