19 Diagnostics library [diagnostics]

19.1 General [diagnostics.general]

1
#
This Clause describes components that C++ programs may use to detect and report error conditions.
2
#
The following subclauses describe components for reporting several kinds of exceptional conditions, documenting program assertions, and a global variable for error number codes, as summarized in Table 40.
Table 40: Diagnostics library summary [tab:diagnostics.summary]
Subclause
Header
Exception classes
<stdexcept>
Assertions
<cassert>
Error numbers
<cerrno>
System error support
<system_­error>

19.2 Exception classes [std.exceptions]

19.2.1 General [std.exceptions.general]

1
#
The C++ standard library provides classes to be used to report certain errors ([res.on.exception.handling]) in C++ programs.
In the error model reflected in these classes, errors are divided into two broad categories: logic errors and runtime errors.
2
#
The distinguishing characteristic of logic errors is that they are due to errors in the internal logic of the program.
In theory, they are preventable.
3
#
By contrast, runtime errors are due to events beyond the scope of the program.
They cannot be easily predicted in advance.
The header <stdexcept> defines several types of predefined exceptions for reporting errors in a C++ program.
These exceptions are related by inheritance.

19.2.2 Header <stdexcept> synopsis [stdexcept.syn]

namespace std { class logic_error; class domain_error; class invalid_argument; class length_error; class out_of_range; class runtime_error; class range_error; class overflow_error; class underflow_error; }

19.2.3 Class logic_­error [logic.error]

namespace std { class logic_error : public exception { public: explicit logic_error(const string& what_arg); explicit logic_error(const char* what_arg); }; }
1
#
The class logic_­error defines the type of objects thrown as exceptions to report errors presumably detectable before the program executes, such as violations of logical preconditions or class invariants.
🔗
logic_error(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
logic_error(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.2.4 Class domain_­error [domain.error]

namespace std { class domain_error : public logic_error { public: explicit domain_error(const string& what_arg); explicit domain_error(const char* what_arg); }; }
1
#
The class domain_­error defines the type of objects thrown as exceptions by the implementation to report domain errors.
🔗
domain_error(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
domain_error(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.2.5 Class invalid_­argument [invalid.argument]

namespace std { class invalid_argument : public logic_error { public: explicit invalid_argument(const string& what_arg); explicit invalid_argument(const char* what_arg); }; }
1
#
The class invalid_­argument defines the type of objects thrown as exceptions to report an invalid argument.
🔗
invalid_argument(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
invalid_argument(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.2.6 Class length_­error [length.error]

namespace std { class length_error : public logic_error { public: explicit length_error(const string& what_arg); explicit length_error(const char* what_arg); }; }
1
#
The class length_­error defines the type of objects thrown as exceptions to report an attempt to produce an object whose length exceeds its maximum allowable size.
🔗
length_error(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
length_error(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.2.7 Class out_­of_­range [out.of.range]

namespace std { class out_of_range : public logic_error { public: explicit out_of_range(const string& what_arg); explicit out_of_range(const char* what_arg); }; }
1
#
The class out_­of_­range defines the type of objects thrown as exceptions to report an argument value not in its expected range.
🔗
out_of_range(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
out_of_range(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.2.8 Class runtime_­error [runtime.error]

namespace std { class runtime_error : public exception { public: explicit runtime_error(const string& what_arg); explicit runtime_error(const char* what_arg); }; }
1
#
The class runtime_­error defines the type of objects thrown as exceptions to report errors presumably detectable only when the program executes.
🔗
runtime_error(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
runtime_error(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.2.9 Class range_­error [range.error]

namespace std { class range_error : public runtime_error { public: explicit range_error(const string& what_arg); explicit range_error(const char* what_arg); }; }
1
#
The class range_­error defines the type of objects thrown as exceptions to report range errors in internal computations.
🔗
range_error(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
range_error(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.2.10 Class overflow_­error [overflow.error]

namespace std { class overflow_error : public runtime_error { public: explicit overflow_error(const string& what_arg); explicit overflow_error(const char* what_arg); }; }
1
#
The class overflow_­error defines the type of objects thrown as exceptions to report an arithmetic overflow error.
🔗
overflow_error(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
overflow_error(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.2.11 Class underflow_­error [underflow.error]

namespace std { class underflow_error : public runtime_error { public: explicit underflow_error(const string& what_arg); explicit underflow_error(const char* what_arg); }; }
1
#
The class underflow_­error defines the type of objects thrown as exceptions to report an arithmetic underflow error.
🔗
underflow_error(const string& what_arg);
2
#
Postconditions: strcmp(what(), what_­arg.c_­str()) == 0.
🔗
underflow_error(const char* what_arg);
3
#
Postconditions: strcmp(what(), what_­arg) == 0.

19.3 Assertions [assertions]

19.3.1 General [assertions.general]

1
#
The header <cassert> provides a macro for documenting C++ program assertions and a mechanism for disabling the assertion checks.

19.3.2 Header <cassert> synopsis [cassert.syn]

#define assert(E) see below
1
#
The contents are the same as the C standard library header <assert.h>, except that a macro named static_­assert is not defined.
See also: ISO C 7.2

19.3.3 The assert macro [assertions.assert]

1
#
An expression assert(E) is a constant subexpression, if

19.4 Error numbers [errno]

19.4.1 General [errno.general]

1
#
The contents of the header <cerrno> are the same as the POSIX header <errno.h>, except that errno shall be defined as a macro.
[Note 1:
The intent is to remain in close alignment with the POSIX standard.
— end note]
A separate errno value shall be provided for each thread.

19.4.2 Header <cerrno> synopsis [cerrno.syn]

#define errno see below #define E2BIG see below #define EACCES see below #define EADDRINUSE see below #define EADDRNOTAVAIL see below #define EAFNOSUPPORT see below #define EAGAIN see below #define EALREADY see below #define EBADF see below #define EBADMSG see below #define EBUSY see below #define ECANCELED see below #define ECHILD see below #define ECONNABORTED see below #define ECONNREFUSED see below #define ECONNRESET see below #define EDEADLK see below #define EDESTADDRREQ see below #define EDOM see below #define EEXIST see below #define EFAULT see below #define EFBIG see below #define EHOSTUNREACH see below #define EIDRM see below #define EILSEQ see below #define EINPROGRESS see below #define EINTR see below #define EINVAL see below #define EIO see below #define EISCONN see below #define EISDIR see below #define ELOOP see below #define EMFILE see below #define EMLINK see below #define EMSGSIZE see below #define ENAMETOOLONG see below #define ENETDOWN see below #define ENETRESET see below #define ENETUNREACH see below #define ENFILE see below #define ENOBUFS see below #define ENODATA see below #define ENODEV see below #define ENOENT see below #define ENOEXEC see below #define ENOLCK see below #define ENOLINK see below #define ENOMEM see below #define ENOMSG see below #define ENOPROTOOPT see below #define ENOSPC see below #define ENOSR see below #define ENOSTR see below #define ENOSYS see below #define ENOTCONN see below #define ENOTDIR see below #define ENOTEMPTY see below #define ENOTRECOVERABLE see below #define ENOTSOCK see below #define ENOTSUP see below #define ENOTTY see below #define ENXIO see below #define EOPNOTSUPP see below #define EOVERFLOW see below #define EOWNERDEAD see below #define EPERM see below #define EPIPE see below #define EPROTO see below #define EPROTONOSUPPORT see below #define EPROTOTYPE see below #define ERANGE see below #define EROFS see below #define ESPIPE see below #define ESRCH see below #define ETIME see below #define ETIMEDOUT see below #define ETXTBSY see below #define EWOULDBLOCK see below #define EXDEV see below
1
#
The meaning of the macros in this header is defined by the POSIX standard.
See also: ISO C 7.5

19.5 System error support [syserr]

19.5.1 General [syserr.general]

1
#
Subclause [syserr] 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.
2
#
Components described in [syserr] shall not change the value of errno ([errno]).
Implementations should leave the error states provided by other libraries unchanged.

19.5.2 Header <system_­error> synopsis [system.error.syn]

#include <compare> // see [compare.syn] 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, // EAFNOSUPPORT address_in_use, // EADDRINUSE address_not_available, // EADDRNOTAVAIL already_connected, // EISCONN argument_list_too_long, // E2BIG argument_out_of_domain, // EDOM bad_address, // EFAULT bad_file_descriptor, // EBADF bad_message, // EBADMSG broken_pipe, // EPIPE connection_aborted, // ECONNABORTED connection_already_in_progress, // EALREADY connection_refused, // ECONNREFUSED connection_reset, // ECONNRESET cross_device_link, // EXDEV destination_address_required, // EDESTADDRREQ device_or_resource_busy, // EBUSY directory_not_empty, // ENOTEMPTY executable_format_error, // ENOEXEC file_exists, // EEXIST file_too_large, // EFBIG filename_too_long, // ENAMETOOLONG function_not_supported, // ENOSYS host_unreachable, // EHOSTUNREACH identifier_removed, // EIDRM illegal_byte_sequence, // EILSEQ inappropriate_io_control_operation, // ENOTTY interrupted, // EINTR invalid_argument, // EINVAL invalid_seek, // ESPIPE io_error, // EIO is_a_directory, // EISDIR message_size, // EMSGSIZE network_down, // ENETDOWN network_reset, // ENETRESET network_unreachable, // ENETUNREACH no_buffer_space, // ENOBUFS no_child_process, // ECHILD no_link, // ENOLINK no_lock_available, // ENOLCK no_message_available, // ENODATA no_message, // ENOMSG no_protocol_option, // ENOPROTOOPT no_space_on_device, // ENOSPC no_stream_resources, // ENOSR no_such_device_or_address, // ENXIO no_such_device, // ENODEV no_such_file_or_directory, // ENOENT no_such_process, // ESRCH not_a_directory, // ENOTDIR not_a_socket, // ENOTSOCK not_a_stream, // ENOSTR not_connected, // ENOTCONN not_enough_memory, // ENOMEM not_supported, // ENOTSUP operation_canceled, // ECANCELED operation_in_progress, // EINPROGRESS operation_not_permitted, // EPERM operation_not_supported, // EOPNOTSUPP operation_would_block, // EWOULDBLOCK owner_dead, // EOWNERDEAD permission_denied, // EACCES protocol_error, // EPROTO protocol_not_supported, // EPROTONOSUPPORT read_only_file_system, // EROFS resource_deadlock_would_occur, // EDEADLK resource_unavailable_try_again, // EAGAIN result_out_of_range, // ERANGE state_not_recoverable, // ENOTRECOVERABLE stream_timeout, // ETIME text_file_busy, // ETXTBSY timed_out, // ETIMEDOUT too_many_files_open_in_system, // ENFILE too_many_files_open, // EMFILE too_many_links, // EMLINK too_many_symbolic_link_levels, // ELOOP value_too_large, // EOVERFLOW wrong_protocol_type, // EPROTOTYPE }; template<> struct is_error_condition_enum<errc> : true_type {}; // [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); // [syserr.errcondition.nonmembers], non-member functions error_condition make_error_condition(errc e) noexcept; // [syserr.compare], comparison operator functions 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_condition& rhs) noexcept; strong_ordering operator<=>(const error_code& lhs, const error_code& rhs) noexcept; strong_ordering operator<=>(const error_condition& lhs, const error_condition& rhs) noexcept; // [syserr.hash], hash support template<class T> struct hash; template<> struct hash<error_code>; template<> struct hash<error_condition>; // [syserr], system error support template<class T> inline constexpr bool is_error_code_enum_v = is_error_code_enum<T>::value; template<class T> inline constexpr bool is_error_condition_enum_v = is_error_condition_enum<T>::value; }
1
#
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.
2
#
The is_­error_­code_­enum and is_­error_­condition_­enum may be specialized for program-defined types to indicate that such types are eligible for class error_­code and class error_­condition automatic conversions, respectively.

19.5.3 Class error_­category [syserr.errcat]

19.5.3.1 Overview [syserr.errcat.overview]

1
#
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 document.
Such classes shall behave as specified in subclause [syserr.errcat].
[Note 1:
error_­category objects are passed by reference, and two such objects are equal if they have the same address.
If there is more than a single object of a custom error_­category type, such equality comparisons can evaluate to false even for objects holding the same value.
— 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; strong_ordering operator<=>(const error_category& rhs) const noexcept; }; const error_category& generic_category() noexcept; const error_category& system_category() noexcept; }

19.5.3.2 Virtual members [syserr.errcat.virtuals]

🔗
virtual const char* name() const noexcept = 0;
1
#
Returns: A string naming the error category.
🔗
virtual error_condition default_error_condition(int ev) const noexcept;
2
#
Returns: error_­condition(ev, *this).
🔗
virtual bool equivalent(int code, const error_condition& condition) const noexcept;
3
#
Returns: default_­error_­condition(code) == condition.
🔗
virtual bool equivalent(const error_code& code, int condition) const noexcept;
4
#
Returns: *this == code.category() && code.value() == condition.
🔗
virtual string message(int ev) const = 0;
5
#
Returns: A string that describes the error condition denoted by ev.

19.5.3.3 Non-virtual members [syserr.errcat.nonvirtuals]

🔗
bool operator==(const error_category& rhs) const noexcept;
1
#
Returns: this == &rhs.
🔗
strong_ordering operator<=>(const error_category& rhs) const noexcept;
2
#
Returns: compare_­three_­way()(this, &rhs).
[Note 1:
compare_­three_­way ([comparisons.three.way]) provides a total ordering for pointers.
— end note]

19.5.3.4 Program-defined classes derived from error_­category [syserr.errcat.derived]

🔗
virtual const char* name() const noexcept = 0;
1
#
Returns: A string naming the error category.
🔗
virtual error_condition default_error_condition(int ev) const noexcept;
2
#
Returns: An object of type error_­condition that corresponds to ev.
🔗
virtual bool equivalent(int code, const error_condition& condition) const noexcept;
3
#
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;
4
#
Returns: true if, for the category of error represented by *this, code is considered equivalent to condition; otherwise, false.

19.5.3.5 Error category objects [syserr.errcat.objects]

🔗
const error_category& generic_category() noexcept;
1
#
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.
2
#
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;
3
#
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.
4
#
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 1:
The number of potential system error codes is large and unbounded, and some might not correspond to any POSIX errno value.
Thus implementations are given latitude in determining correspondence.
— end note]

19.5.4 Class error_­code [syserr.errcode]

19.5.4.1 Overview [syserr.errcode.overview]

1
#
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]
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]

🔗
error_code() noexcept;
1
#
Postconditions: val_­ == 0 and cat_­ == &system_­category().
🔗
error_code(int val, const error_category& cat) noexcept;
2
#
Postconditions: val_­ == val and cat_­ == &cat.
🔗
template<class ErrorCodeEnum> error_code(ErrorCodeEnum e) noexcept;
3
#
Constraints: is_­error_­code_­enum_­v<ErrorCodeEnum> is true.
4
#
Postconditions: *this == make_­error_­code(e).

19.5.4.3 Modifiers [syserr.errcode.modifiers]

🔗
void assign(int val, const error_category& cat) noexcept;
1
#
Postconditions: val_­ == val and cat_­ == &cat.
🔗
template<class ErrorCodeEnum> error_code& operator=(ErrorCodeEnum e) noexcept;
2
#
Constraints: is_­error_­code_­enum_­v<ErrorCodeEnum> is true.
3
#
Postconditions: *this == make_­error_­code(e).
4
#
Returns: *this.
🔗
void clear() noexcept;
5
#
Postconditions: value() == 0 and category() == system_­category().

19.5.4.4 Observers [syserr.errcode.observers]

🔗
int value() const noexcept;
1
#
Returns: val_­.
🔗
const error_category& category() const noexcept;
2
#
Returns: *cat_­.
🔗
error_condition default_error_condition() const noexcept;
3
#
Returns: category().default_­error_­condition(value()).
🔗
string message() const;
4
#
Returns: category().message(value()).
🔗
explicit operator bool() const noexcept;
5
#
Returns: value() != 0.

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

🔗
error_code make_error_code(errc e) noexcept;
1
#
Returns: error_­code(static_­cast<int>(e), generic_­category()).
🔗
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const error_code& ec);
2
#
Effects: Equivalent to: return os << ec.category().name() << ':' << ec.value();

19.5.5 Class error_­condition [syserr.errcondition]

19.5.5.1 Overview [syserr.errcondition.overview]

1
#
The class error_­condition describes an object used to hold values identifying error conditions.
[Note 1:
error_­condition values are portable abstractions, while error_­code values ([syserr.errcode]) are implementation specific.
— end note]
namespace std { class error_condition { public: // [syserr.errcondition.constructors], constructors error_condition() noexcept; error_condition(int val, const error_category& cat) noexcept; template<class ErrorConditionEnum> error_condition(ErrorConditionEnum e) noexcept; // [syserr.errcondition.modifiers], modifiers void assign(int val, const error_category& cat) noexcept; template<class ErrorConditionEnum> error_condition& operator=(ErrorConditionEnum e) noexcept; void clear() noexcept; // [syserr.errcondition.observers], observers int value() const noexcept; const error_category& category() const noexcept; string message() const; explicit operator bool() const noexcept; private: int val_; // exposition only const error_category* cat_; // exposition only }; }

19.5.5.2 Constructors [syserr.errcondition.constructors]

🔗
error_condition() noexcept;
1
#
Postconditions: val_­ == 0 and cat_­ == &generic_­category().
🔗
error_condition(int val, const error_category& cat) noexcept;
2
#
Postconditions: val_­ == val and cat_­ == &cat.
🔗
template<class ErrorConditionEnum> error_condition(ErrorConditionEnum e) noexcept;
3
#
Constraints: is_­error_­condition_­enum_­v<ErrorConditionEnum> is true.
4
#
Postconditions: *this == make_­error_­condition(e).

19.5.5.3 Modifiers [syserr.errcondition.modifiers]

🔗
void assign(int val, const error_category& cat) noexcept;
1
#
Postconditions: val_­ == val and cat_­ == &cat.
🔗
template<class ErrorConditionEnum> error_condition& operator=(ErrorConditionEnum e) noexcept;
2
#
Constraints: is_­error_­condition_­enum_­v<ErrorConditionEnum> is true.
3
#
Postconditions: *this == make_­error_­condition(e).
4
#
Returns: *this.
🔗
void clear() noexcept;
5
#
Postconditions: value() == 0 and category() == generic_­category().

19.5.5.4 Observers [syserr.errcondition.observers]

🔗
int value() const noexcept;
1
#
Returns: val_­.
🔗
const error_category& category() const noexcept;
2
#
Returns: *cat_­.
🔗
string message() const;
3
#
Returns: category().message(value()).
🔗
explicit operator bool() const noexcept;
4
#
Returns: value() != 0.

19.5.5.5 Non-member functions [syserr.errcondition.nonmembers]

🔗
error_condition make_error_condition(errc e) noexcept;
1
#
Returns: error_­condition(static_­cast<int>(e), generic_­category()).

19.5.6 Comparison operator functions [syserr.compare]

🔗
bool operator==(const error_code& lhs, const error_code& rhs) noexcept;
1
#
Returns: lhs.category() == rhs.category() && lhs.value() == rhs.value()
🔗
bool operator==(const error_code& lhs, const error_condition& rhs) noexcept;
2
#
Returns: lhs.category().equivalent(lhs.value(), rhs) || rhs.category().equivalent(lhs, rhs.value())
🔗
bool operator==(const error_condition& lhs, const error_condition& rhs) noexcept;
3
#
Returns: lhs.category() == rhs.category() && lhs.value() == rhs.value()
🔗
strong_ordering operator<=>(const error_code& lhs, const error_code& rhs) noexcept;
4
#
Effects: Equivalent to: if (auto c = lhs.category() <=> rhs.category(); c != 0) return c; return lhs.value() <=> rhs.value();
🔗
strong_ordering operator<=>(const error_condition& lhs, const error_condition& rhs) noexcept;
5
#
Returns: if (auto c = lhs.category() <=> rhs.category(); c != 0) return c; return lhs.value() <=> rhs.value();

19.5.7 System error hash support [syserr.hash]

🔗
template<> struct hash<error_code>; template<> struct hash<error_condition>;
1
#
The specializations are enabled ([unord.hash]).

19.5.8 Class system_­error [syserr.syserr]

19.5.8.1 Overview [syserr.syserr.overview]

1
#
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.
2
#
[Note 1:
If an error represents an out-of-memory condition, implementations are encouraged to throw an exception object of type 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 override; }; }

19.5.8.2 Members [syserr.syserr.members]

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system_error(error_code ec, const string& what_arg);
1
#
Postconditions: code() == ec and
string_­view(what()).find(what_­arg.c_­str()) != string_­view​::​npos.
🔗
system_error(error_code ec, const char* what_arg);
2
#
Postconditions: code() == ec and string_­view(what()).find(what_­arg) != string_­view​::​npos.
🔗
system_error(error_code ec);
3
#
Postconditions: code() == ec.
🔗
system_error(int ev, const error_category& ecat, const string& what_arg);
4
#
Postconditions: code() == error_­code(ev, ecat) and
string_­view(what()).find(what_­arg.c_­str()) != string_­view​::​npos.
🔗
system_error(int ev, const error_category& ecat, const char* what_arg);
5
#
Postconditions: code() == error_­code(ev, ecat) and
string_­view(what()).find(what_­arg) != string_­view​::​npos.
🔗
system_error(int ev, const error_category& ecat);
6
#
Postconditions: code() == error_­code(ev, ecat).
🔗
const error_code& code() const noexcept;
7
#
Returns: ec or error_­code(ev, ecat), from the constructor, as appropriate.
🔗
const char* what() const noexcept override;
8
#
Returns: An ntbs incorporating the arguments supplied in the constructor.
[Note 1:
The returned ntbs might be the contents of what_­arg + ": " + code.message().
— end note]