22 Diagnostics library [diagnostics]

22.5 System error support [syserr]

22.5.2 Class error_­category [syserr.errcat]

22.5.2.1 Class error_­category overview [syserr.errcat.overview]

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The class error_­category serves as a base class for types used to identify the source and encoding of a particular category of error code. Classes may be derived from error_­category to support categories of errors in addition to those defined in this International Standard. Such classes shall behave as specified in this subclause. [ Note: error_­category objects are passed by reference, and two such objects are equal if they have the same address. This means that applications using custom error_­category types should create a single object of each such type.  — end note ] 

namespace std {
  class error_category {
  public:
    constexpr error_category() noexcept;
    virtual ~error_category();
    error_category(const error_category&) = delete;
    error_category& operator=(const error_category&) = delete;
    virtual const char* name() const noexcept = 0;
    virtual error_condition default_error_condition(int ev) const noexcept;
    virtual bool equivalent(int code, const error_condition& condition) const noexcept;
    virtual bool equivalent(const error_code& code, int condition) const noexcept;
    virtual string message(int ev) const = 0;

    bool operator==(const error_category& rhs) const noexcept;
    bool operator!=(const error_category& rhs) const noexcept;
    bool operator<(const error_category& rhs) const noexcept;
  };

  const error_category& generic_category() noexcept;
  const error_category& system_category() noexcept;
}

22.5.2.2 Class error_­category virtual members [syserr.errcat.virtuals]

virtual ~error_category();

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Effects: Destroys an object of class error_­category.

virtual const char* name() const noexcept = 0;

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Returns: A string naming the error category.

virtual error_condition default_error_condition(int ev) const noexcept;

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Returns: error_­condition(ev, *this).

virtual bool equivalent(int code, const error_condition& condition) const noexcept;

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Returns: default_­error_­condition(code) == condition.

virtual bool equivalent(const error_code& code, int condition) const noexcept;

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Returns: *this == code.category() && code.value() == condition.

virtual string message(int ev) const = 0;

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Returns: A string that describes the error condition denoted by ev.

22.5.2.3 Class error_­category non-virtual members [syserr.errcat.nonvirtuals]

constexpr error_category() noexcept;

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Effects: Constructs an object of class error_­category.

bool operator==(const error_category& rhs) const noexcept;

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Returns: this == &rhs.

bool operator!=(const error_category& rhs) const noexcept;

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Returns: !(*this == rhs).

bool operator<(const error_category& rhs) const noexcept;

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Returns: less<const error_­category*>()(this, &rhs).

[ Note: less provides a total ordering for pointers.  — end note ]

22.5.2.4 Program defined classes derived from error_­category [syserr.errcat.derived]

virtual const char* name() const noexcept = 0;

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Returns: A string naming the error category.

virtual error_condition default_error_condition(int ev) const noexcept;

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Returns: An object of type error_­condition that corresponds to ev.

virtual bool equivalent(int code, const error_condition& condition) const noexcept;

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Returns: true if, for the category of error represented by *this, code is considered equivalent to condition; otherwise, false.

virtual bool equivalent(const error_code& code, int condition) const noexcept;

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Returns: true if, for the category of error represented by *this, code is considered equivalent to condition; otherwise, false.

22.5.2.5 Error category objects [syserr.errcat.objects]

const error_category& generic_category() noexcept;

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Returns: A reference to an object of a type derived from class error_­category. All calls to this function shall return references to the same object.

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Remarks: The object's default_­error_­condition and equivalent virtual functions shall behave as specified for the class error_­category. The object's name virtual function shall return a pointer to the string "generic".

const error_category& system_category() noexcept;

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Returns: A reference to an object of a type derived from class error_­category. All calls to this function shall return references to the same object.

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Remarks: The object's equivalent virtual functions shall behave as specified for class error_­category. The object's name virtual function shall return a pointer to the string "system". The object's default_­error_­condition virtual function shall behave as follows:

If the argument ev corresponds to a POSIX errno value posv, the function shall return error_­condition(posv, generic_­category()). Otherwise, the function shall return error_­condition(ev, system_­category()). What constitutes correspondence for any given operating system is unspecified. [ Note: The number of potential system error codes is large and unbounded, and some may not correspond to any POSIX errno value. Thus implementations are given latitude in determining correspondence.  — end note ]