This subclause describes components that the standard library and C++ programs may use to report error conditions originating from the operating system or other low-level application program interfaces.
Components described in this subclause shall not change the value of errno ([errno]). Implementations should leave the error states provided by other libraries unchanged.
Header <system_error> synopsis
namespace std {
class error_category;
const error_category& generic_category() noexcept;
const error_category& system_category() noexcept;
class error_code;
class error_condition;
class system_error;
template <class T>
struct is_error_code_enum : public false_type {};
template <class T>
struct is_error_condition_enum : public false_type {};
enum class errc {
address_family_not_supported, address_in_use, address_not_available, already_connected, argument_list_too_long, argument_out_of_domain, bad_address, bad_file_descriptor, bad_message, broken_pipe, connection_aborted, connection_already_in_progress, connection_refused, connection_reset, cross_device_link, destination_address_required, device_or_resource_busy, directory_not_empty, executable_format_error, file_exists, file_too_large, filename_too_long, function_not_supported, host_unreachable, identifier_removed, illegal_byte_sequence, inappropriate_io_control_operation, interrupted, invalid_argument, invalid_seek, io_error, is_a_directory, message_size, network_down, network_reset, network_unreachable, no_buffer_space, no_child_process, no_link, no_lock_available, no_message_available, no_message, no_protocol_option, no_space_on_device, no_stream_resources, no_such_device_or_address, no_such_device, no_such_file_or_directory, no_such_process, not_a_directory, not_a_socket, not_a_stream, not_connected, not_enough_memory, not_supported, operation_canceled, operation_in_progress, operation_not_permitted, operation_not_supported, operation_would_block, owner_dead, permission_denied, protocol_error, protocol_not_supported, read_only_file_system, resource_deadlock_would_occur, resource_unavailable_try_again, result_out_of_range, state_not_recoverable, stream_timeout, text_file_busy, timed_out, too_many_files_open_in_system, too_many_files_open, too_many_links, too_many_symbolic_link_levels, value_too_large, wrong_protocol_type, };
template <> struct is_error_condition_enum<errc> : true_type { }
error_code make_error_code(errc e) noexcept;
error_condition make_error_condition(errc e) noexcept;
bool operator==(const error_code& lhs, const error_code& rhs) noexcept;
bool operator==(const error_code& lhs, const error_condition& rhs) noexcept;
bool operator==(const error_condition& lhs, const error_code& rhs) noexcept;
bool operator==(const error_condition& lhs, const error_condition& rhs) noexcept;
bool operator!=(const error_code& lhs, const error_code& rhs) noexcept;
bool operator!=(const error_code& lhs, const error_condition& rhs) noexcept;
bool operator!=(const error_condition& lhs, const error_code& rhs) noexcept;
bool operator!=(const error_condition& lhs, const error_condition& rhs) noexcept;
template <class T> struct hash;
template <> struct hash<error_code>;
}
The value of each enum errc constant shall be the same as the value of the <cerrno> macro shown in the above synopsis. Whether or not the <system_error> implementation exposes the <cerrno> macros is unspecified.
The is_error_code_enum and is_error_condition_enum may be specialized for user-defined types to indicate that such types are eligible for class error_code and class error_condition automatic conversions, respectively.
19.5.1 Class error_category [syserr.errcat] 19.5.1.1 Class error_category overview [syserr.errcat.overview]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;
} 19.5.1.2 Class error_category virtual members [syserr.errcat.virtuals]
Effects: Destroys an object of class error_category.
virtual const char* name() const noexcept = 0;
Returns: A string naming the error category.
virtual error_condition default_error_condition(int ev) const noexcept;
Returns: error_condition(ev, *this).
virtual bool equivalent(int code, const error_condition& condition) const noexcept;
Returns: default_error_condition(code) == condition.
virtual bool equivalent(const error_code& code, int condition) const noexcept;
Returns: *this == code.category() && code.value() == condition.
virtual string message(int ev) const = 0;
Returns: A string that describes the error condition denoted by ev.
19.5.1.3 Class error_category non-virtual members [syserr.errcat.nonvirtuals] constexpr error_category() noexcept;
Effects: Constructs an object of class error_category.
bool operator==(const error_category& rhs) const noexcept;
bool operator!=(const error_category& rhs) const noexcept;
Returns: !(*this == rhs).
bool operator<(const error_category& rhs) const noexcept;
Returns: less<const error_category*>()(this, &rhs).
[ Note: less ([comparisons]) provides a total ordering for pointers. — end note ]
19.5.1.4 Program defined classes derived from error_category [syserr.errcat.derived] virtual const char* name() const noexcept = 0;
Returns: A string naming the error category.
virtual error_condition default_error_condition(int ev) const noexcept;
Returns: An object of type error_condition that corresponds to ev.
virtual bool equivalent(int code, const error_condition& condition) const noexcept;
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;
Returns: true if, for the category of error represented by *this, code is considered equivalent to condition; otherwise, false.
19.5.1.5 Error category objects [syserr.errcat.objects] const error_category& generic_category() noexcept;
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.
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;
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.
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 ]
19.5.2 Class error_code [syserr.errcode] 19.5.2.1 Class error_code overview [syserr.errcode.overview]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: Class error_code is an adjunct to error reporting by exception. — end note ]
namespace std {
class error_code {
public:
error_code() noexcept;
error_code(int val, const error_category& cat) noexcept;
template <class ErrorCodeEnum>
error_code(ErrorCodeEnum e) noexcept;
void assign(int val, const error_category& cat) noexcept;
template <class ErrorCodeEnum>
error_code& operator=(ErrorCodeEnum e) noexcept;
void clear() noexcept;
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_; const error_category* cat_; };
error_code make_error_code(errc e) noexcept;
bool operator<(const error_code& lhs, const error_code& rhs) noexcept;
template <class charT, class traits>
basic_ostream<charT,traits>&
operator<<(basic_ostream<charT,traits>& os, const error_code& ec);
} 19.5.2.2 Class error_code constructors [syserr.errcode.constructors]
Effects: Constructs an object of type error_code.
Postconditions: val_ == 0 and cat_ == &system_category().
error_code(int val, const error_category& cat) noexcept;
Effects: Constructs an object of type error_code.
Postconditions: val_ == val and cat_ == &cat.
template <class ErrorCodeEnum> error_code(ErrorCodeEnum e) noexcept;
Effects: Constructs an object of type error_code.
Postconditions: *this == make_error_code(e).
Remarks: This constructor shall not participate in overload resolution unless is_error_code_enum<ErrorCodeEnum>::value is true.
19.5.2.3 Class error_code modifiers [syserr.errcode.modifiers] void assign(int val, const error_category& cat) noexcept;
Postconditions: val_ == val and cat_ == &cat.
template <class ErrorCodeEnum> error_code& operator=(ErrorCodeEnum e) noexcept;
Postconditions: *this == make_error_code(e).
Remarks: This operator shall not participate in overload resolution unless is_error_code_enum<ErrorCodeEnum>::value is true.
Postconditions: value() == 0 and category() == system_category().
19.5.2.4 Class error_code observers [syserr.errcode.observers] const error_category& category() const noexcept;
error_condition default_error_condition() const noexcept;
Returns: category().default_error_condition(value()).
Returns: category().message(value()).
explicit operator bool() const noexcept;
error_code make_error_code(errc e) noexcept;
Returns: error_code(static_cast<int>(e), generic_category()).
bool operator<(const error_code& lhs, const error_code& rhs) noexcept;
Returns: lhs.category() < rhs.category() || lhs.category() == rhs.category() && lhs.value() < rhs.value().
template <class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& os, const error_code& ec);
Effects: os << ec.category().name() << ':' << ec.value().
19.5.3 Class error_condition [syserr.errcondition] 19.5.3.1 Class error_condition overview [syserr.errcondition.overview]The class error_condition describes an object used to hold values identifying error conditions. [ Note: error_condition values are portable abstractions, while error_code values ([syserr.errcode]) are implementation specific. — end note ]
namespace std {
class error_condition {
public:
error_condition() noexcept;
error_condition(int val, const error_category& cat) noexcept;
template <class ErrorConditionEnum>
error_condition(ErrorConditionEnum e) noexcept;
void assign(int val, const error_category& cat) noexcept;
template<class ErrorConditionEnum>
error_condition& operator=(ErrorConditionEnum e) noexcept;
void clear() noexcept;
int value() const noexcept;
const error_category& category() const noexcept;
string message() const;
explicit operator bool() const noexcept;
private:
int val_; const error_category* cat_; };
bool operator<(const error_condition& lhs, const error_condition& rhs) noexcept;
} 19.5.3.2 Class error_condition constructors [syserr.errcondition.constructors]
Effects: Constructs an object of type error_condition.
Postconditions: val_ == 0 and cat_ == &generic_category().
error_condition(int val, const error_category& cat) noexcept;
Effects: Constructs an object of type error_condition.
Postconditions: val_ == val and cat_ == &cat.
template <class ErrorConditionEnum> error_condition(ErrorConditionEnum e) noexcept;
Effects: Constructs an object of type error_condition.
Postcondition: *this == make_error_condition(e).
Remarks: This constructor shall not participate in overload resolution unless is_error_condition_enum<ErrorConditionEnum>::value is true.
19.5.3.3 Class error_condition modifiers [syserr.errcondition.modifiers] void assign(int val, const error_category& cat) noexcept;
Postconditions: val_ == val and cat_ == &cat.
template <class ErrorConditionEnum> error_condition& operator=(ErrorConditionEnum e) noexcept;
Postcondition: *this == make_error_condition(e).
Remarks: This operator shall not participate in overload resolution unless is_error_condition_enum<ErrorConditionEnum>::value is true.
Postconditions: value() == 0 and category() == generic_category().
19.5.3.4 Class error_condition observers [syserr.errcondition.observers] const error_category& category() const noexcept;
Returns: category().message(value()).
explicit operator bool() const noexcept;
error_condition make_error_condition(errc e) noexcept;
Returns: error_condition(static_cast<int>(e), generic_category()).
bool operator<(const error_condition& lhs, const error_condition& rhs) noexcept;
Returns: lhs.category() < rhs.category() || lhs.category() == rhs.category() &&
lhs.value() < rhs.value().
bool operator==(const error_code& lhs, const error_code& rhs) noexcept;
Returns: lhs.category() == rhs.category() && lhs.value() == rhs.value().
bool operator==(const error_code& lhs, const error_condition& rhs) noexcept;
Returns: lhs.category().equivalent(lhs.value(), rhs) || rhs.category().equivalent(lhs, rhs.value()).
bool operator==(const error_condition& lhs, const error_code& rhs) noexcept;
Returns: rhs.category().equivalent(rhs.value(), lhs) || lhs.category().equivalent(rhs, lhs.value()).
bool operator==(const error_condition& lhs, const error_condition& rhs) noexcept;
Returns: lhs.category() == rhs.category() && lhs.value() == rhs.value().
bool operator!=(const error_code& lhs, const error_code& rhs) noexcept; bool operator!=(const error_code& lhs, const error_condition& rhs) noexcept; bool operator!=(const error_condition& lhs, const error_code& rhs) noexcept; bool operator!=(const error_condition& lhs, const error_condition& rhs) noexcept;
template <> struct hash<error_code>;
The template specialization shall meet the requirements of class template hash ([unord.hash]).
19.5.6 Class system_error [syserr.syserr] 19.5.6.1 Class system_error overview [syserr.syserr.overview]The class system_error describes an exception object used to report error conditions that have an associated error code. Such error conditions typically originate from the operating system or other low-level application program interfaces.
[ Note: If an error represents an out-of-memory condition, implementations are encouraged to throw an exception object of type bad_alloc [bad.alloc] rather than system_error. — end note ]
namespace std {
class system_error : public runtime_error {
public:
system_error(error_code ec, const string& what_arg);
system_error(error_code ec, const char* what_arg);
system_error(error_code ec);
system_error(int ev, const error_category& ecat,
const string& what_arg);
system_error(int ev, const error_category& ecat,
const char* what_arg);
system_error(int ev, const error_category& ecat);
const error_code& code() const noexcept;
const char* what() const noexcept;
};
} 19.5.6.2 Class system_error members [syserr.syserr.members]
system_error(error_code ec, const string& what_arg);
Effects: Constructs an object of class system_error.
Postconditions: code() == ec.
string(what()).find(what_arg) != string::npos.
system_error(error_code ec, const char* what_arg);
Effects: Constructs an object of class system_error.
Postconditions: code() == ec.
string(what()).find(what_arg) != string::npos.
Effects: Constructs an object of class system_error.
Postconditions: code() == ec.
system_error(int ev, const error_category& ecat, const string& what_arg);
Effects: Constructs an object of class system_error.
Postconditions: code() == error_code(ev, ecat).
string(what()).find(what_arg) != string::npos.
system_error(int ev, const error_category& ecat, const char* what_arg);
Effects: Constructs an object of class system_error.
Postconditions: code() == error_code(ev, ecat).
string(what()).find(what_arg) != string::npos.
system_error(int ev, const error_category& ecat);
Effects: Constructs an object of class system_error.
Postconditions: code() == error_code(ev, ecat).
const error_code& code() const noexcept;
Returns: ec or error_code(ev, ecat), from the constructor, as appropriate.
const char* what() const noexcept;
Returns: An ntbs incorporating the arguments supplied in the constructor.
[ Note: The returned NTBS might be the contents of what_arg + ": " + code.message(). — end note ]
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