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]

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.

19.2.1 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;
}

19.2.2 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.

19.2.3 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.

19.2.4 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.

19.2.5 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.

19.2.6 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.

19.2.7 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.

19.2.8 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.

19.2.9 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.

19.2.10 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]

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

19.3.1 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

19.3.2 The assert macro [assertions.assert]

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

19.4 Error numbers [errno]

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
:
The intent is to remain in close alignment with the POSIX standard.
— end note
]
A separate errno value shall be provided for each thread.

19.4.1 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]

1
#
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.
2
#
Components described in this subclause shall not change the value of errno.
Implementations should leave the error states provided by other libraries unchanged.

19.5.1 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 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.

19.5.2 Class error_­category [syserr.errcat]

19.5.2.1 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
:
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;
    strong_ordering operator<=>(const error_category& rhs) const noexcept;
  };

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

19.5.2.2 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.

19.5.2.3 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
:
compare_­three_­way ([comparisons.three.way]) provides a total ordering for pointers.
— end note
]

19.5.2.4 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.

19.5.2.5 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
:
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
]

19.5.3 Class error_­code [syserr.errcode]

19.5.3.1 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
:
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);
}

19.5.3.2 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).

19.5.3.3 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().

19.5.3.4 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.

19.5.3.5 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();

19.5.4 Class error_­condition [syserr.errcondition]

19.5.4.1 Overview [syserr.errcondition.overview]

1
#
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:
    // [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
  };
}

19.5.4.2 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).

19.5.4.3 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().

19.5.4.4 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.

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

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

19.5.5 Comparison 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();

19.5.6 System error hash support [syserr.hash]

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

19.5.7 Class system_­error [syserr.syserr]

19.5.7.1 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
:
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;
  };
}

19.5.7.2 Members [syserr.syserr.members]

🔗
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
:
The returned ntbs might be the contents of what_­arg + ": " + code.message().
— end note
]