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In the previous section the error code enumerations tt(CatErr) and tt(Cond)
were developed. The values of these enumerations specify, respectively, the
direct and the platform independent causes of errors that may be encountered
in the context of the new error category developed in this section.
Classes derived from tt(std::error_category) are designed as singleton classes
and implement their own tt(name, message) and an tt(equivalent) members. Our
class tt(Category) also declares a static member tt(instance) returning a
reference to the class's singleton object, which is compile-time initialized
and is available by the time tt(instance) is called. Alternatively a dedicated
function (like tt(Category_category)), analogously to the function
tt(generic_category), returning a reference to the tt(Category) object could
be defined.
tt(CatErr) values, tt(Cond) values and textual descriptions of tt(CatErr's)
values are combined in a tt(std::unordered_map) using tt(CatErr) as key, and a
tt(struct POD) as value type. This map allows us to retrieve the platform
independent error types and the descriptions that are associated with
tt(CatErr) values.
Here is the interface of the class tt(Category):
verbinsert(-s4 //category examples/errcode/category/category.h)
Its tt(unordered_map s_map) provides the tt(Cond) values and verbal
descriptions of the tt(CatErr) values given those tt(CatErr) values:
verbinsert(-s4 //data examples/errcode/category/data.cc)
The functions tt(make_error_code) and tt(make_error_condition) return,
respectively, tt(error_code) and tt(error_condition) objects from,
respectively, tt(CatErr) values and tt(Cond) values.
Their declarations can be provided below the tt(Category) class interface and
their implementations pass the tt(Category) object to their constructors:
verbinsert(-s4 //make examples/errcode/category/makeerrorcode.cc)
verbinsert(-s4 //make examples/errcode/category/makeerrorcondition.cc)
The member tt(name) em(must) be defined by classes derived from
tt(error_category). It simply returns a short string naming the category
(e.g., tt("Category") for our tt(Category) class). Likewise, the member
tt(message) em(must) be redefined. Its implementation usually is slightly more
complex than tt(name's) implementation: it expects a (cast to an tt(int))
tt(CatErr) value and uses that value to find the corresponding textual
description in tt(s_map). If found the description is returned; if not found
then a short fallback message is returned:
verbinsert(-s4 //msg examples/errcode/category/message.cc)
The member tt(default_error_condition) receives a (cast to tt(int)) tt(CatErr)
value. That value is used to find the associated tt(Cond) value. If the
tt(int) received by the function does not represent a valid tt(CatErr) value
then the fallback value tt(Cond::NoCond) is used. The function returns an
tt(error_condition) object created by tt(make_error_condition) which receives
the determined tt(Cond) value as its argument:
verbinsert(-s4 //def examples/errcode/category/defaulterrorcondition.cc)
What's left is implementing the two tt(equivalent) members. The first
tt(equivalent) member (receiving a reference to an tt(error_code) object and a
(cast to tt(int)) tt(Cond) value) determines the equivalence of
the tt(Cond) value that is associated with the tt(error_code) object
and
the tt(Cond) value that is specified as the function's second argument.
If these values are equal and the tt(error_code) object's category is equal
to tt(Category) then the equivalence has been established and tt(true) is
returned. Here is its implementation:
verbinsert(-s4 //equiv examples/errcode/category/equivalent1.cc)
The second tt(equivalent) member (receiving (as an tt(int)) tt(CatErr) value
and an tt(error_condition) object) determines the equivalence of an
tt(error_condition) object that is constructed from the tt(Cond) value
that is associated with the tt(CatErr) value that was passed (as tt(int))
to the function
and
the tt(error_condition) object that was passed to the function as its
second argument.
Here a prerequisite for concluding equivalence is that the error condition's
category is tt(Category). If that's the case then the function returns
tt(true) if its tt(int) argument equals zero and the tt(condition) object also
indicates no error. Alternatively, if the tt(condition) argument is equal to
the tt(error_condition) object made from the tt(Cond) value associated with
the tt(CatErr) value passed to the function as its first argument the
equivalence has also been established and tt(true) is returned. Here is its
implementation:
verbinsert(-s4 //equiv examples/errcode/category/equivalent2.cc)
So, in order to define your own category:
itemization(
it() define an enumeration which is `promoted' to an tt(error_code_enum);
it() define an enumeration which is promoted to an
tt(error_condition_enum);
it() define a new tt(error_category) class by deriving a class from
tt(std::error_category), define it as a singleton class and override
its tt(default_error_condition, equivalent, message,) and tt(name)
members.
)
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