19 Diagnostics library [diagnostics]

19.5 System error support [syserr]

19.5.4 Class error_code [syserr.errcode]

19.5.4.1 Overview [syserr.errcode.overview]

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The class error_code describes an object used to hold error code values, such as those originating from the operating system or other low-level application program interfaces.
[Note 1: 
Class error_code is an adjunct to error reporting by exception.
— end note]
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namespace std { class error_code { public: // [syserr.errcode.constructors], constructors error_code() noexcept; error_code(int val, const error_category& cat) noexcept; template<class ErrorCodeEnum> error_code(ErrorCodeEnum e) noexcept; // [syserr.errcode.modifiers], modifiers void assign(int val, const error_category& cat) noexcept; template<class ErrorCodeEnum> error_code& operator=(ErrorCodeEnum e) noexcept; void clear() noexcept; // [syserr.errcode.observers], observers int value() const noexcept; const error_category& category() const noexcept; error_condition default_error_condition() const noexcept; string message() const; explicit operator bool() const noexcept; private: int val_; // exposition only const error_category* cat_; // exposition only }; // [syserr.errcode.nonmembers], non-member functions error_code make_error_code(errc e) noexcept; template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const error_code& ec); }

19.5.4.2 Constructors [syserr.errcode.constructors]

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error_code() noexcept;
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Effects: Initializes val_ with 0 and cat_ with &system_category().
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error_code(int val, const error_category& cat) noexcept;
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Effects: Initializes val_ with val and cat_ with &cat.
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template<class ErrorCodeEnum> error_code(ErrorCodeEnum e) noexcept;
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Constraints: is_error_code_enum_v<ErrorCodeEnum> is true.
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Effects: Equivalent to: error_code ec = make_error_code(e); assign(ec.value(), ec.category());

19.5.4.3 Modifiers [syserr.errcode.modifiers]

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void assign(int val, const error_category& cat) noexcept;
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Postconditions: val_ == val and cat_ == &cat.
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template<class ErrorCodeEnum> error_code& operator=(ErrorCodeEnum e) noexcept;
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Constraints: is_error_code_enum_v<ErrorCodeEnum> is true.
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Effects: Equivalent to: error_code ec = make_error_code(e); assign(ec.value(), ec.category());
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Returns: *this.
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void clear() noexcept;
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Postconditions: value() == 0 and category() == system_category().

19.5.4.4 Observers [syserr.errcode.observers]

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int value() const noexcept;
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Returns: val_.
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const error_category& category() const noexcept;
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Returns: *cat_.
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error_condition default_error_condition() const noexcept;
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Returns: category().default_error_condition(value()).
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string message() const;
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Returns: category().message(value()).
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explicit operator bool() const noexcept;
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Returns: value() != 0.

19.5.4.5 Non-member functions [syserr.errcode.nonmembers]

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error_code make_error_code(errc e) noexcept;
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Returns: error_code(static_cast<int>(e), generic_category()).
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const error_code& ec);
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Effects: Equivalent to: return os << ec.category().name() << ':' << ec.value();