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In section ref(ERRORCODE) the class tt(std::error_code) was introduced. One of
its constructors accepts ti(ErrorCodeEnum) values, where tt(ErrorCodeEnum) is
a template type name for enumerations that we may define ourselves containing
symbols that are used as error code values. Another constructor expects an
tt(int)-valued error code and a specification of an error category that uses
those error codes.
Several error code enumerations and error categories are predefined by bf(C++)
but it is also possible to define new tt(ErrorCodeEnums) and error
categories. In this section constructing new tt(ErrorCodeEnums) is covered, in
the next section designing new error categories is covered.
Defining new error code enumerations is an option when using tt(error_code)
objects is attractive, but standard error code values (like the values defined
by tt(enum class errc)) aren't appropriate. For example, when designing an
interactive calculator, several errors may be encountered that are related to
the way expressions are entered by the user. For those situations you might
want to develop your own error code enumeration.
In this and the next section a bare bones approach to defining error code
enumerations and error categories is adopted. No concrete, real-life like
class is developed. I think the advantage of this is that this way it's easier
to apply the principles to new real-life situations than if you first have to
abstract the content of a real-life example yourself. Here we go:
itemization(
it() Our first step consists of defining our own enumeration. The
enumeration contains symbols listing causes of errors. Newly defined
error code enumerations must not associate value 0 with any of its
symbols, as by convention value 0 indicates `no error'.
The tt(enum class CatErr) lists causes of errors that are associated
with our (as yet to be designed) error category:
verbinsert(-s4 //caterr examples/errcode/category/category.h)
it() By itself, defining an tt(enum class) does not allow us to pass its
values to tt(error_code) constructors. Before we can do that the tt(enum)
must be `promoted' to an emi(error_code_enum). This `promotion' is
realized by specializing the trait class
hi(is_error_code_enum)tt(std::is_error_code_enum) after which the
tt(error_code(ErrorCodeEnum)) member template and the tt(make_error_code)
function accept tt(CatErr) enumeration values. Interestingly, this
requires us to add code to the tt(std) namespace. Normally this is not
allowed, but in this case it is. The bf(C++) standard states:
quote(20.5.4.2.1 Namespace std
The behavior of a C++ program is undefined if it adds declarations or
definitions to namespace std or to a namespace within namespace std
unless otherwise specified.
A program may add a template specialization for any standard library
template to namespace std only if the declaration depends on a
user-defined type and the specialization meets the standard library
requirements for the original template and is not explicitly
prohibited.
)
Here is how the tt(is_error_code_enum) trait class is specialized:
verbinsert(-s4 //iscodeenum examples/errcode/category/category.h)
)
This completes the definition of our own error code enumeration, whose symbols
are now accepted by tt(error_code's) constructor.
Before we're able to design our own error category we must also have a look at
`higher order' causes of errors as represented by objects of the class
tt(std::error_condition) (cf. section ref(ERRCOND)). Error conditions
represent platform independent errors like syntax errors or non-existing
requests.
In our bare bones implementation of an error category these higher order
causes of errors are enumerated in the tt(enum class Cond) enumeration. It's
defined similarly to tt(CatErr).
itemization(
it() Our first step consists of defining the enumeration. As with error
code enums its symbols should not be given the value 0. Here is the
enumeration tt(Cond) whose symbols presumably represent platform
independent causes of errors:
verbinsert(-s4 //cond examples/errcode/category/category.h)
it() By itself, defining an tt(enum class) does not allow us to pass its
values to tt(error_condition) constructors. Before we can do that the
tt(enum) must be `promoted' to an emi(error_condition_enum). Also similar
to tt(CatErr) this `promotion' is realized by specializing the trait class
hi(is_error_condition_enum)tt(std::is_error_condition_enum)
Here is how the tt(is_error_condition_enum) trait class is specialized:
verbinsert(-s4 //iscondenum examples/errcode/category/category.h)
)
We're now ready for designing our own tt(error_category) class.
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