| ## Safe Numerics

### exception

#### Description

Here we describe the data types used to refer to exceptional conditions which might occur. Note that when we use the word "exception", we don't mean the C++ term which refers to a data type, but rather the colloquial sense of a anomaly, irregularity, deviation, special case, isolated example, peculiarity, abnormality, oddity; misfit, aberration or out of the ordinary occurrence. This concept of "exception" is more complex that one would think and hence is not manifested by a single simple type. A small number of types work together to implement this concept within the library.

We've leveraged on the std::error_code which is part of the standard library. We don't use all the facilities that it offers so it's not an exact match, but it's useful and works for our purposes.

#### enum class safe_numerics_error

The following values are those which a numeric result might return. They resemble the standard error codes used by C++ standard exceptions. This resemblance is coincidental and they are wholly unrelated to any codes of similar names. The reason for the resemblance is that the library started it's development using the standard library codes. But as development progressed it became clear that the original codes weren't sufficient so now they stand on their own. Here are a list of error codes. The description of what they mean is

Symbol | Description
--- | ---
`success` | successful operation - no error returned
`positive_overflow_error` | A positive number is too large to be represented by the data type
`negative_overflow_error` | The absolute value of a negative number is too large to be represented by the data type.
`domain_error` | the result of an operation is outside the legal range of the result.
`range_error` | an argument to a function or operator is outside the legal range - e.g. sqrt(-1).
`precision_overflow_error` | precision was lost in the course of executing the operation.
`underflow_error` | A number is too close to zero to be represented by the data type.
`uninitialized_value` | According to the C++ standard, the result may be defined by the application. e.g. 16 >> 10 will result the expected result of 0 on most machines.

The above listed codes can be transformed to a instance of type `std::error_code` with the function:

    std::error_code make_error_code(safe_numerics_error e)

This object can be

#### enum class safe_numerics_actions

The above error codes are classified into groups according to how such exceptions should be handled. The following table shows the possible actions that an error could be mapped to.

Symbol | Description
--- | ---
`no_action` | successful operation - no action action required
`uninitialized_value` | report attempt to use an uninitialized value - not currently used
`arithmetic_error` | report an arithmetic error
`implementation_defined_behavior` | report an operation which the C++ standard permits but fails to specify
`undefined_behavior` | report an operation whose result is undefined by the C++ standard.

Translation of a `safe_numerics_error` into the corresponding `safe_numerics_action` can be accomplished with the following function:

    constexpr enum  safe_numerics_actions
    make_safe_numerics_action(const safe_numerics_error & e);

#### See Also

- C++ Standard Library version The C++ standard error handling utilities.

- Thinking Asynchronously in C++ Another essential reference on the design and usage of the error_code

#### Header

`#include <boost/safe_numerics/exception.hpp>`