////
Copyright 2003-2017 Beman Dawes
Copyright 2018-2022 Peter Dimov

Distributed under the Boost Software License, Version 1.0.

See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt
////

[#reference]
# Reference
:idprefix: ref_

## Macros

When `BOOST_SYSTEM_ENABLE_DEPRECATED` is defined, the library provides
deprecated features for compatibility. These features are bound to eventually
disappear.

When `BOOST_SYSTEM_USE_UTF8` is defined, on Windows the library returns
UTF-8 messages using code page `CP_UTF8` instead of the default `CP_ACP`.
This macro has no effect on POSIX.

When `BOOST_SYSTEM_DISABLE_THREADS` is defined, the library assumes that
the current platform doesn't support multiple threads and disables the use
of the standard header `<mutex>`, eliminating the mutex locks. The single
threaded `libstdc++` is one such platform.

## Deprecated Names

In the process of adding Boost.System to the {cpp}11 standard library, the
{cpp} committee changed some names. To ease transition, Boost.System deprecates
the old names, but will provide them when the macro `BOOST_SYSTEM_ENABLE_DEPRECATED` is defined.

|===
|Old usage, now deprecated|Replacement

|`get_generic_category()`|`generic_category()`
|`get_system_category()`|`system_category()`
|`namespace posix`|`namespace errc`
|`namespace posix_error`|`namespace errc`
|`get_posix_category()`|`generic_category()`
|`posix_category`|`generic_category()`
|`errno_ecat`|`generic_category()`
|`native_ecat`|`system_category()`
|===

## <boost/system/{zwsp}is_error_code_enum.hpp>

### is_error_code_enum

```
namespace boost {
namespace system {

template<class T>
  struct is_error_code_enum { static const bool value = false; };

} // namespace system
} // namespace boost
```

Users may specialize `is_error_code_enum` for their error enumeration
types to indicate that they should be eligible for automatic conversions
to `error_code`. This conversion calls `make_error_code(e)`, which should
be provided in the same namespace as the enumeration type.

## <boost/system/{zwsp}is_error_condition_enum.hpp>

### is_error_condition_enum

```
namespace boost {
namespace system {

template<class T>
  struct is_error_condition_enum { static const bool value = false; };

} // namespace system
} // namespace boost
```

Users may specialize `is_error_condition_enum` for their error enumeration
types to indicate that they should be eligible for automatic conversions
to `error_condition`. This conversion calls `make_error_condition(e)`, which
should be provided in the same namespace as the enumeration type.

## <boost/system/{zwsp}errc.hpp>

### errc

```
namespace boost {
namespace system {

namespace errc {
  enum errc_t
  {
    success = 0,
    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
  };

} // namespace errc

template<> struct is_error_condition_enum<errc::errc_t>
  { static const bool value = true; };

constexpr error_condition make_error_condition( errc::errc_t e ) noexcept;

constexpr error_code make_error_code( errc::errc_t e ) noexcept;

error_code make_error_code( errc::errc_t e,
  boost::source_location const * loc ) noexcept;

} // namespace system
} // namespace boost
```

The predefined enumeration type `errc::errc_t` provides named constants
corresponding to the values of the `<cerrno>` macros.

```
constexpr error_condition make_error_condition( errc::errc_t e ) noexcept;
```
[none]
* {blank}
+
Returns: :: `error_condition( e, generic_category() )`.

Since `errc::errc_t` provides a specialization of `is_error_condition_enum`
and an overload of `make_error_condition`, it can be converted implicitly to
an `error_condition`. This is typically useful when comparing `error_code`
values returned from APIs to a portable condition, as in the below example:
* {blank}
+
```
void api_function( boost::system::error_code& ec );

void my_function()
{
    boost::system::error_code ec;
    api_function( ec );

    if( ec == boost::system::errc::no_such_file_or_directory )
    {
        // an entity wasn't found (ENOENT)
        // handle this condition
    }
}
```

```
constexpr error_code make_error_code( errc::errc_t e ) noexcept;
```
[none]
* {blank}
+
Returns: :: `error_code( e, generic_category() )`.

In addition to `make_error_condition`, `errc::errc_t` provides an overload of
`make_error_code`. This allows the creation of generic error codes, an
operation typically useful when a function needs to signal a generic failure
that does not come from an underlying API, such as for instance an out of
memory condition:
* {blank}
+
```
void my_api_function( boost::system::error_code& ec )
{
    void* p = std::malloc( 16 );

    if( p == 0 )
    {
        // return ENOMEM
        ec = make_error_code( boost::system::errc::out_of_memory );
        return;
    }

    // use p
}
```

```
constexpr error_code make_error_code( errc::errc_t e,
  boost::source_location const * loc ) noexcept;
```
[none]
* {blank}
+
Returns: :: `error_code( e, generic_category(), loc )`.

Same as the above overload, but takes a source location.
* {blank}
+
```
void my_api_function( boost::system::error_code& ec )
{
    void* p = std::malloc( 16 );

    if( p == 0 )
    {
        // return ENOMEM

        BOOST_STATIC_CONSTEXPR boost::source_location loc =
          BOOST_CURRENT_LOCATION;

        ec = make_error_code( boost::system::errc::out_of_memory, &loc );
        return;
    }

    // use p
}
```

## <boost/system/{zwsp}error_category.hpp>

### error_category

The class `error_category` defines the 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 Boost.System.

```
namespace boost {
namespace system {

class error_category
{
public: // noncopyable

    error_category( error_category const & ) = delete;
    error_category& operator=( error_category const & ) = delete;

protected:

    ~error_category() = default;

    constexpr error_category() noexcept;
    explicit constexpr error_category( unsigned long long id ) noexcept;

public:

    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 std::string message( int ev ) const = 0;
    virtual char const * message( int ev, char * buffer, std::size_t len )
      const noexcept;

    virtual bool failed( int ev ) const noexcept;

    friend constexpr bool operator==( const error_category & lhs,
      const error_category & rhs ) noexcept;
    friend constexpr bool operator!=( const error_category & lhs,
      const error_category & rhs ) noexcept;

    friend constexpr bool operator< ( const error_category & lhs,
      const error_category & rhs ) noexcept;

    operator std::error_category const & () const;

private:

    unsigned long long id_; // exposition only
};

} // namespace system
} // namespace boost
```

#### Constructors

```
constexpr error_category() noexcept;
```
[none]
* {blank}
+
Effects: :: Initializes `id_` to 0.
Remarks: :: Since equivalence for categories that do not have an identifier is
  based on comparing object addresses, a user-defined derived category of type
  `C` that uses this constructor should ensure that only one object of type `C`
  exists in the program.

```
explicit constexpr error_category( unsigned long long id ) noexcept;
```
[none]
* {blank}
+
Effects: :: Initializes `id_` to `id`.
Remarks: :: User-defined derived categories that use this constructor are considered
  equivalent when their identifiers match. Therefore, those categories may have more
  than one instance existing in a program, but to minimize the possibility of
  collision, their identifiers must be randomly chosen (at the time the category
  is implemented, not at runtime). One way of generating a 64 bit random identifier
  is https://www.random.org/cgi-bin/randbyte?nbytes=8&format=h.

#### Virtuals

```
virtual const char * name() const noexcept = 0;
```
[none]
* {blank}
+
Returns: :: In derived classes, a character literal naming the error category.

```
virtual error_condition default_error_condition( int ev ) const noexcept;
```
[none]
* {blank}
+
Returns: ::
- In derived classes, an error condition corresponding to `ev`.
  The returned error condition will typically come from the generic category.
- In the default implementation, `error_condition( ev, *this )`.

```
virtual bool equivalent( int code, const error_condition & condition )
  const noexcept;
```
[none]
* {blank}
+
Returns: ::
- In derived classes, `true` when `error_code( code, *this )` is equivalent to `condition`.
- In the default implementation, `default_error_condition( code ) == condition`.

```
virtual bool equivalent( const error_code & code, int condition )
  const noexcept;
```
[none]
* {blank}
+
Returns: ::
- In derived classes, `true` when `code` is equivalent to `error_condition( condition, *this )`.
- In the default implementation, `*this == code.category() && code.value() == condition`.

```
virtual std::string message( int ev ) const = 0;
```
[none]
* {blank}
+
Returns: :: In derived classes, a string that describes the error denoted by `ev`.

```
virtual char const * message( int ev, char * buffer, std::size_t len )
  const noexcept;
```
[none]
* {blank}
+
Effects: ::
** Derived classes should either
  *** return a pointer to a character literal describing the error denoted by `ev`, or
  *** copy a string describing the error into `buffer`, truncating it to `len-1`
      characters and storing a null terminator, and return `buffer`. If `len` is 0,
      nothing is copied, but the function still returns `buffer`. Note that
      when `len` is 0, `buffer` may be `nullptr`.
** The default implementation calls `message(ev)` and copies the result into
   `buffer`, truncating it to `len-1` characters and storing a null terminator.
   If `len` is 0, copies nothing. Returns `buffer`. If `message(ev)` throws an
   exception, the string `"Message text unavailable"` is used.
Example: ::
+
```
const char* my_category::message(int ev, char* buffer, size_t len) const noexcept
{
    switch(ev)
    {
    case 0: return "no error";
    case 1: return "voltage out of range";
    case 2: return "impedance mismatch";

    case 31:
    case 32:
    case 33:

        std::snprintf(buffer, len, "component %d failure", ev-30);
        return buffer;

    default:

        std::snprintf(buffer, len, "unknown error %d", ev);
        return buffer;
    }
}
```

```
virtual bool failed( int ev ) const noexcept;
```
[none]
* {blank}
+
Returns: ::
- In derived classes, `true` when `ev` represents a failure.
- In the default implementation, `ev != 0`.
Remarks: ::
  All calls to this function with the same `ev` must return the same value.

#### Comparisons

```
friend constexpr bool operator==( const error_category & lhs,
  const error_category & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `rhs.id_ == 0? &lhs == &rhs: lhs.id_ == rhs.id_`.
Remarks: :: Two category objects are considered equivalent when they have
  matching nonzero identifiers, or are the same object.

```
friend constexpr bool operator!=( const error_category & lhs,
  const error_category & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `!( lhs == rhs )`.

```
constexpr bool operator< ( const error_category & lhs,
  const error_category & rhs ) const noexcept;
```
[none]
* {blank}
+
Returns: ::
** If `lhs.id_ < rhs.id_`, `true`;
** Otherwise, if `lhs.id_ > rhs.id_`, `false`;
** Otherwise, if `rhs.id_ != 0`, `false`;
** Otherwise, `std::less<error_category const *>()( &lhs, &rhs )`.

#### Conversions

```
operator std::error_category const & () const;
```
[none]
* {blank}
+
Returns: :: A reference to an `std::error_category` object corresponding
  to `*this`.

## <boost/system/{zwsp}system_category.hpp>

### system_category

```
namespace boost {
namespace system {

constexpr const error_category & system_category() noexcept;

} // namespace system
} // namespace boost
```

```
constexpr const error_category & system_category() noexcept;
```
[none]
* {blank}
+
Returns: :: A reference to a predefined `error_category` object identifying
  errors originating from the operating system.

## <boost/system/{zwsp}generic_category.hpp>

### generic_category

```
namespace boost {
namespace system {

constexpr const error_category & generic_category() noexcept;

} // namespace system
} // namespace boost
```

```
constexpr const error_category & generic_category() noexcept;
```
[none]
* {blank}
+
Returns: :: A reference to a predefined `error_category` object identifying
  portable error codes and conditions.

## <boost/system/{zwsp}error_code.hpp>

### error_code

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. It's an adjunct to error reporting
by exception.

```
namespace boost {
namespace system {

class error_code {
public:

    // constructors

    constexpr error_code() noexcept;
    constexpr error_code( int val, const error_category & cat ) noexcept;

    error_code( int val, const error_category & cat,
      boost::source_location const * loc ) noexcept;

    template<class ErrorCodeEnum>
      constexpr error_code( ErrorCodeEnum e ) noexcept;

    error_code( error_code const& ec,
      boost::source_location const * loc ) noexcept;

    error_code( std::error_code const& ec ) noexcept;

    // modifiers

    constexpr void assign( int val, const error_category & cat ) noexcept;

    void assign( int val, const error_category & cat,
      boost::source_location const * loc ) noexcept;

    template<class ErrorCodeEnum>
      constexpr error_code & operator=( ErrorCodeEnum e ) noexcept;

    void assign( error_code const& ec,
      boost::source_location const * loc ) noexcept;

    constexpr void clear() noexcept;

    // observers

    constexpr int value() const noexcept;
    constexpr const error_category & category() const noexcept;

    error_condition default_error_condition() const noexcept;

    std::string message() const;
    char const * message( char * buffer, std::size_t len ) const noexcept;

    constexpr bool failed() const noexcept;
    constexpr explicit operator bool() const noexcept;

    bool has_location() const noexcept;
    boost::source_location const & location() const noexcept;

    // comparisons

    friend constexpr bool operator==( const error_code & lhs,
      const error_code & rhs ) noexcept;

    friend constexpr bool operator!=( const error_code & lhs,
      const error_code & rhs ) noexcept;

    friend constexpr bool operator<( const error_code & lhs,
      const error_code & rhs ) noexcept;

    friend bool operator==( const error_code & code,
      const error_condition & condition ) noexcept;
    friend bool operator==( const error_condition & condition,
      const error_code & code ) noexcept;

    friend bool operator!=( const error_code & code,
      const error_condition & condition ) noexcept;
    friend bool operator!=( const error_condition & condition,
      const error_code & code ) noexcept;

    friend bool operator==( const error_code & lhs,
      const std::error_code & rhs ) noexcept;
    friend bool operator==( const std::error_code & lhs,
      const error_code & rhs ) noexcept;

    friend bool operator!=( const error_code & lhs,
      const std::error_code & rhs ) noexcept;
    friend bool operator!=( const std::error_code & lhs,
      const error_code & rhs ) noexcept;

    template<class E>
      friend constexpr bool operator==( const error_code & lhs, E rhs ) noexcept;
    template<class E>
      friend constexpr bool operator==( E lhs, const error_code & rhs ) noexcept;

    template<class E>
      friend constexpr bool operator!=( const error_code & lhs, E rhs ) noexcept;
    template<class E>
      friend constexpr bool operator!=( E lhs, const error_code & rhs ) noexcept;

    // conversions

    operator std::error_code() const;
    operator std::error_code();
    template<class T> operator T& (); // only when T=std::error_code

    // to_string

    std::string to_string() const;

    // stream insertion

    template<class charT, class traits>
      friend std::basic_ostream<charT, traits>&
        operator<<( basic_ostream<charT, traits>& os, const error_code & ec );

    // what

    std::string what() const;
};

// non-member functions

std::size_t hash_value( const error_code & ec );

} // namespace system
} // namespace boost
```

#### Constructors

```
constexpr error_code() noexcept;
```
[none]
* {blank}
+
Ensures: :: `value() == 0`; `category() == system_category()`; `!has_location()`.

```
constexpr error_code( int val, const error_category & cat ) noexcept;
```
[none]
* {blank}
+
Ensures: :: `value() == val`; `category() == cat`; `!has_location()`.

```
error_code( int val, const error_category & cat,
  boost::source_location const * loc ) noexcept;
```
[none]
* {blank}
+
Requires: :: `loc` points to a valid `boost::source_location` object with static storage duration.
Ensures: :: `value() == val`; `category() == cat`; `has_location()`; `&location() == loc`.

```
template<class ErrorCodeEnum>
  constexpr error_code( ErrorCodeEnum e ) noexcept;
```
[none]
* {blank}
+
Ensures: :: `*this == make_error_code( e )`.
Remarks: :: This constructor is only enabled when `is_error_code_enum<ErrorCodeEnum>::value` is `true`.

```
error_code( error_code const& ec,
  boost::source_location const * loc ) noexcept;
```
[none]
* {blank}
+
Requires: :: `loc` points to a valid `boost::source_location` object with static storage duration, or is `nullptr`.
Ensures: :: `*this == ec`.
Remarks: :: When `ec` is a default-constructed `error_code` or wraps a `std::error_code`,
  or when `loc` is `nullptr`, `*this` stores no location (`has_location()` is `false`).
  Otherwise, `*this` stores `loc` (`has_location()` is `true` and `&location()` is `loc`.)

```
error_code( std::error_code const & ec ) noexcept;
```
[none]
* {blank}
+
Effects: :: Construct an `error_code` that wraps `ec`.
Remarks: :: `value()` and `category()` are unspecified. `has_location()` is `false`.

#### Modifiers

```
constexpr void assign( int val, const error_category & cat ) noexcept;
```
[none]
* {blank}
+
Effects: :: `*this = error_code( val, cat )`.

```
void assign( int val, const error_category & cat,
  boost::source_location const * loc ) noexcept;
```
[none]
* {blank}
+
Effects: :: `*this = error_code( val, cat, loc )`.

```
template<class ErrorCodeEnum>
  constexpr error_code & operator=( ErrorCodeEnum e ) noexcept;
```
[none]
* {blank}
+
Ensures: :: `*this == make_error_code( e )`.
Remarks: :: This operator is only enabled when `is_error_code_enum<ErrorCodeEnum>::value` is `true`.

```
void assign( error_code const& ec,
  boost::source_location const * loc ) noexcept;
```
[none]
* {blank}
+
Effects: :: `*this = error_code( ec, loc )`.

```
constexpr void clear() noexcept;
```
[none]
* {blank}
+
Effects: ::
  `*this = error_code()`.

#### Observers

```
constexpr int value() const noexcept;
```
[none]
* {blank}
+
Returns: :: the error value.

```
constexpr const error_category & category() const noexcept;
```
[none]
* {blank}
+
Returns: :: the error category.

```
error_condition default_error_condition() const noexcept;
```
[none]
* {blank}
+
Returns: :: `category().default_error_condition( value() )`.

```
std::string message() const;
```
[none]
* {blank}
+
Returns: :: If `*this` wraps a `std::error_code` object `ec`, `ec.message()`.
  Otherwise, `category().message( value() )`.

```
char const * message( char * buffer, std::size_t len ) const noexcept;
```
[none]
* {blank}
+
Effects: :: If `*this` wraps a `std::error_code` object `ec`, copies the
  string returned from `ec.message()` into `buffer` and returns `buffer`.
  Otherwise, returns `category().message( value(), buffer, len )`.

```
constexpr bool failed() const noexcept;
```
[none]
* {blank}
+
Returns: :: If `*this` wraps a `std::error_code` object `ec`, `ec.value() != 0`.
  Otherwise, `category().failed( value() )`.

```
constexpr explicit operator bool() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `failed()`.

```
bool has_location() const noexcept;
```
[none]
* {blank}
+
Returns: :: `true` if `*this` has been constructed with a pointer to a source
  location, `false` otherwise.

```
boost::source_location const & location() const noexcept;
```
[none]
* {blank}
+
Returns: :: `*loc` if `*this` has been constructed with a pointer to a source
  location `loc`, a reference to a default-constructed `boost::source_location` otherwise.

#### Comparisons

```
friend constexpr bool operator==( const error_code & lhs,
  const error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: If both `lhs` and `rhs` wrap `std::error_code` objects `e1` and `e2`, `e1 == e2`.
  Otherwise, `lhs.value() == rhs.value() && lhs.category() == rhs.category()`.

```
friend constexpr bool operator!=( const error_code & lhs,
  const error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `!( lhs == rhs )`.

```
friend constexpr bool operator<( const error_code & lhs,
  const error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: If both `lhs` and `rhs` wrap `std::error_code` objects `e1` and `e2`, `e1 < e2`.
  Otherwise, `lhs.category() < rhs.category() || (lhs.category() == rhs.category() && lhs.value() < rhs.value())`.

```
friend bool operator==( const error_code & code,
  const error_condition & condition ) noexcept;
```
```
friend bool operator==( const error_condition & condition,
  const error_code & code ) noexcept;
```
[none]
* {blank}
+
Returns: :: If `code` wraps a `std::error_code` object `ec`, `ec == static_cast<std::error_condition>( condition )`.
  Otherwise, `code.category().equivalent( code.value(), condition ) || condition.category().equivalent( code, condition.value() )`.

```
friend bool operator!=( const error_code & lhs,
  const error_condition & rhs ) noexcept;
```
```
friend bool operator!=( const error_condition & lhs,
  const error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `!( lhs == rhs )`.

```
friend bool operator==( const error_code & lhs,
  const std::error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `static_cast<std::error_code>(lhs) == rhs`.

```
friend bool operator==( const std::error_code & lhs,
  const error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `lhs == static_cast<std::error_code>(rhs)`.

```
friend bool operator!=( const error_code & lhs,
  const std::error_code & rhs ) noexcept;
```
```
friend bool operator!=( const std::error_code & lhs,
  const error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: ::
  `!( lhs == rhs )`.

```
template<class E>
  friend constexpr bool operator==( const error_code & lhs, E rhs ) noexcept;
```
[none]
* {blank}
+
Effects: ::
[disc]
** When `is_error_code_enum<E>::value` is `true`, returns `lhs == make_error_code(rhs)`;
** When `is_error_condition_enum<E>::value` is `true`, returns `lhs == make_error_condition(rhs)`;
** Otherwise, this overload is disabled.

```
template<class E>
  friend constexpr bool operator==( E lhs, const error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Effects: ::
[disc]
** When `is_error_code_enum<E>::value` is `true`, returns `make_error_code(lhs) == rhs`;
** When `is_error_condition_enum<E>::value` is `true`, returns `make_error_condition(lhs) == rhs`;
** Otherwise, this overload is disabled.

```
template<class E>
  friend constexpr bool operator!=( const error_code & lhs, E rhs ) noexcept;
```
```
template<class E>
  friend constexpr bool operator!=( E lhs, const error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `!( lhs == rhs )`.
Remarks: :: These overloads are only enabled when
  `is_error_code_enum<E>::value` is `true` or
  `is_error_condition_enum<E>::value` is `true`.

#### Conversions

```
operator std::error_code() const;
```
```
operator std::error_code();
```
[none]
* {blank}
+
Returns: :: If `*this` wraps a `std::error_code` object `ec`, `ec`.
  Otherwise, `std::error_code( value(), category() )`.

```
template<class T> operator T&();
```
[none]
* {blank}
+
Effects: :: If `*this` wraps a `std::error_code` object `ec`,
  returns a reference to `ec`.
  Otherwise, makes `*this` wrap `std::error_code( *this )`,
  then returns a reference to it.
Remarks: ::
  This operator is only enabled when `T` is `std::error_code`.

#### to_string

```
std::string to_string() const;
```
[none]
* {blank}
+
Returns: :: If `*this` wraps a `std::error_code` object `e2`, a string that
  is the concatenation of `"std:"`, `e2.category().name()`, `':'`, and the
  string representation of `e2.value()`. Otherwise, the concatenation of
  `category().name()`, `':'`, and the string representation of `value()`.

#### Stream Insertion

```
template <class charT, class traits>
  std::basic_ostream<charT, traits>&
    operator<<( basic_ostream<charT, traits>& os, const error_code & ec );
```
[none]
* {blank}
+
Effects: :: `os << to_string()`.
Returns: :: `os`.

#### what

```
std::string what() const;
```
[none]
* {blank}
+
Returns: :: A string representation of `*this`, suitable for logging and
  diagnostic output. Typically incorporates `message()`, `to_string()`, and
  `location().to_string()` (if available.)

#### Nonmembers

```
std::size_t hash_value( const error_code & ec );
```
[none]
* {blank}
+
Returns: :: If `ec` wraps a `std::error_code` object `e2`, `std::hash<std::error_code>()(e2)`.
  Otherwise, a hash value representing `ec`.

## <boost/system/{zwsp}error_condition.hpp>

### error_condition

```
namespace boost {
namespace system {

class error_condition {
public:

    // constructors

    constexpr error_condition() noexcept;
    constexpr error_condition( int val, const error_category & cat ) noexcept;

    template <class ErrorConditionEnum>
      constexpr error_condition( ErrorConditionEnum e ) noexcept;

    // modifiers

    constexpr void assign( int val, const error_category & cat ) noexcept;

    template<typename ErrorConditionEnum>
      constexpr error_condition & operator=( ErrorConditionEnum e ) noexcept;

    constexpr void clear() noexcept;

    // observers

    constexpr int value() const noexcept;
    constexpr const error_category & category() const noexcept;

    std::string message() const;
    char const * message( char * buffer, std::size_t len ) const noexcept;

    constexpr bool failed() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // comparisons

    friend constexpr bool operator==( const error_condition & lhs,
      const error_condition & rhs ) noexcept;

    friend constexpr bool operator!=( const error_condition & lhs,
      const error_condition & rhs ) noexcept;

    friend constexpr bool operator<( const error_condition & lhs,
      const error_condition & rhs ) noexcept;

    friend bool operator==( const std::error_code & code,
      const error_condition & condition ) noexcept;
    friend bool operator==( const error_condition & condition,
      const std::error_code & code ) noexcept;

    friend bool operator!=( const std::error_code & code,
      const error_condition & condition ) noexcept;
    friend bool operator!=( const error_condition & condition,
      const std::error_code & code ) noexcept;

    // conversions

    operator std::error_condition() const;

    // to_string

    std::string to_string() const;

    // stream insertion

    template <class charT, class traits>
      friend std::basic_ostream<charT, traits>&
        operator<<( basic_ostream<charT, traits>& os, const error_condition & en );
};

} // namespace system
} // namespace boost
```

#### Constructors

```
constexpr error_condition() noexcept;
```
[none]
* {blank}
+
Ensures: :: `value() == 0`; `category() == generic_category()`.

```
constexpr error_condition( int val, const error_category & cat ) noexcept;
```
[none]
* {blank}
+
Ensures: :: `value() == val`; `category() == cat`.

```
template <class ErrorConditionEnum>
  constexpr error_condition( ErrorConditionEnum e ) noexcept;
```
[none]
* {blank}
+
Ensures: :: `*this == make_error_condition( e )`.
Remarks: ::
  This constructor is only enabled when `is_error_condition_enum<ErrorConditionEnum>::value` is `true`.

#### Modifiers

```
constexpr void assign( int val, const error_category & cat ) noexcept;
```
[none]
* {blank}
+
Ensures: :: `value() == val`; `category() == cat`.

```
template <class ErrorConditionEnum>
  constexpr error_condition & operator=( ErrorConditionEnum e ) noexcept;
```
[none]
* {blank}
+
Ensures: :: `*this == make_error_condition( e )`.
Remarks: :: This operator is only enabled when `is_error_condition_enum<ErrorConditionEnum>::value` is `true`.

```
constexpr void clear() noexcept;
```
[none]
* {blank}
+
Ensures: ::
  `value() == 0`; `category() == generic_category()`.

#### Observers

```
constexpr int value() const noexcept;
```
[none]
* {blank}
+
Returns: :: the error value.

```
constexpr const error_category & category() const noexcept;
```
[none]
* {blank}
+
Returns: :: the error category.

```
std::string message() const;
```
[none]
* {blank}
+
Returns: :: `category().message( value() )`.

```
char const * message( char * buffer, std::size_t len ) const noexcept;
```
[none]
* {blank}
+
Returns: :: `category().message( value(), buffer, len )`.

```
constexpr bool failed() const noexcept;
```
[none]
* {blank}
+
Returns: :: `category().failed( value() )`.

```
constexpr explicit operator bool() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `failed()`.

#### Comparisons

```
friend constexpr bool operator==( const error_condition & lhs,
  const error_condition & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `lhs.value() == rhs.value() && lhs.category() == rhs.category()`.

```
friend constexpr bool operator!=( const error_condition & lhs,
  const error_condition & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: :: `!( lhs == rhs )`.

```
friend constexpr bool operator<( const error_condition & lhs,
  const error_condition & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: ::
  `lhs.category() < rhs.category() || (lhs.category() == rhs.category() && lhs.value() < rhs.value())`.

```
friend bool operator==( const std::error_code & code,
  const error_condition & condition ) noexcept;
```
```
friend bool operator==( const error_condition & condition,
  const std::error_code & code ) noexcept;
```
[none]
* {blank}
+
Returns: :: `code == static_cast<std::error_condition>( rhs )`.

```
friend constexpr bool operator!=( const std::error_code & lhs,
  const error_condition & rhs ) noexcept;
```
```
friend constexpr bool operator!=( const error_condition & lhs,
  const std::error_code & rhs ) noexcept;
```
[none]
* {blank}
+
Returns: ::
  `!( lhs == rhs )`.

#### Conversions

```
operator std::error_condition() const;
```
[none]
* {blank}
+
Returns: ::
  `std::error_condition( value(), category() )`.

#### to_string

```
std::string to_string() const;
```
[none]
* {blank}
+
Returns: :: The concatenation of `"cond:"`, `category().name()`, `':'`,
  and the string representation of `value()`.

#### Stream Insertion

```
template <class charT, class traits>
  std::basic_ostream<charT, traits>&
    operator<<( basic_ostream<charT, traits>& os, const error_condition & en );
```
[none]
* {blank}
+
Effects: :: `os << en.to_string()`.
Returns: ::
  `os`.

## <boost/system/{zwsp}system_error.hpp>

### system_error

The class `system_error` describes an exception object used to
report errors that have an associated `error_code`. Such errors
typically originate from operating system or other low-level
application program interfaces.

```
namespace boost {
namespace system {

class system_error: public std::runtime_error
{
public:

    explicit system_error( error_code ec );
    system_error( error_code ec, const char * what_arg );
    system_error( error_code ec, const std::string & what_arg );

    system_error( int ev, const error_category & ecat );
    system_error( int ev, const error_category & ecat,
      const char * what_arg );
    system_error( int ev, const error_category & ecat,
      const std::string & what_arg );

    error_code code() const noexcept;
    const char * what() const noexcept;
};

} // namespace system
} // namespace boost
```

#### Constructors

```
explicit system_error( error_code ec );
system_error( error_code ec, const char * what_arg );
system_error( error_code ec, const std::string & what_arg );
```
[none]
* {blank}
+
Ensures: :: `code() == ec`.

```
system_error( int ev, const error_category & ecat,
  const char * what_arg );
system_error( int ev, const error_category & ecat,
  const std::string & what_arg );
system_error( int ev, const error_category & ecat );
```
[none]
* {blank}
+
Ensures: ::
  `code() == error_code( ev, ecat )`.

#### Observers

```
error_code code() const noexcept;
```
[none]
* {blank}
+
Returns: :: `ec` or `error_code( ev, ecat )`, from the constructor, as appropriate.

```
const char * what() const noexcept;
```
[none]
* {blank}
+
Returns: :: A null-terminated character string incorporating the arguments supplied
  in the constructor, typically of the form `what_arg + ": " + code().message()`.

## <boost/system/result.hpp>

This header defines the class template `result<T, E>`. Unlike the rest of the library,
it requires {cpp}11.

### Synopsis

```
namespace boost {
namespace system {

// throw_exception_from_error

BOOST_NORETURN inline void throw_exception_from_error( error_code const & e,
  boost::source_location const & loc );

BOOST_NORETURN inline void throw_exception_from_error( std::error_code const & e,
  boost::source_location const & loc );

BOOST_NORETURN inline void throw_exception_from_error( errc::errc_t const & e,
  boost::source_location const & loc );

BOOST_NORETURN inline void throw_exception_from_error( std::errc const & e,
  boost::source_location const & loc );

BOOST_NORETURN inline void throw_exception_from_error( std::exception_ptr & e,
  boost::source_location const & loc );

// in_place_*

using in_place_value_t = /*unspecified*/;
constexpr in_place_value_t in_place_value{};

using in_place_error_t = /*unspecified*/;
constexpr in_place_error_t in_place_error{};

// result

template<class T, class E = error_code> class result;
template<class E> class result<void, E>;
template<class U, class E> class result<U&, E>;

// operator|

template<class T, class E, class U> T operator|( result<T, E> const& r, U&& u );
template<class T, class E, class U> T operator|( result<T, E>&& r, U&& u );

template<class T, class E, class F> T operator|( result<T, E> const& r, F&& f );
template<class T, class E, class F> T operator|( result<T, E>&& r, F&& f );

template<class T, class E, class F, class R = ...> R operator|( result<T, E> const& r, F&& f );
template<class T, class E, class F, class R = ...> R operator|( result<T, E>&& r, F&& f );
template<class E, class F, class R = ...> R operator|( result<void, E> const& r, F&& f );
template<class E, class F, class R = ...> R operator|( result<void, E>&& r, F&& f );

// operator&

template<class T, class E, class F, class U = ...>
  result<U, E> operator&( result<T, E> const& r, F&& f );
template<class T, class E, class F, class U = ...>
  result<U, E> operator&( result<T, E>&& r, F&& f );

template<class T, class E, class F, class R = ...> R operator&( result<T, E> const& r, F&& f );
template<class T, class E, class F, class R = ...> R operator&( result<T, E>&& r, F&& f );

// operator&=

template<class T, class E, class F, class U = ...>
  result<T, E>& operator&=( result<T, E>& r, F&& f );

template<class T, class E, class F, class R = ...>
  result<T, E>& operator&=( result<T, E>& r, F&& f );

} // namespace system
} // namespace boost
```

### throw_exception_from_error

The function `throw_exception_from_error` is called by `result<T, E>::value()` when
the result holds an error. Its purpose is to throw an exception that represents the
error held in the result.

An implementation for the common and default case where `E` is `error_code` is
already provided. It throws `system_error(e)`.

If `result<T, E>` is used with other error types, the user is expected to provide
an appropriate overload of `throw_exception_from_error` in the namespace of `E`.

```
BOOST_NORETURN inline void throw_exception_from_error( error_code const & e,
  boost::source_location const & loc );
```
[none]
* {blank}
+
Effects: ::
  `boost::throw_with_location( system_error( e ), loc )`.

```
BOOST_NORETURN inline void throw_exception_from_error( std::error_code const & e,
  boost::source_location const & loc );
```
[none]
* {blank}
+
Effects: ::
  `boost::throw_with_location( std::system_error( e ), loc )`.

```
BOOST_NORETURN inline void throw_exception_from_error( errc::errc_t const & e,
  boost::source_location const & loc );
```
[none]
* {blank}
+
Effects: ::
  `boost::throw_with_location( system_error( make_error_code( e ) ), loc )`.

```
BOOST_NORETURN inline void throw_exception_from_error( std::errc const & e,
  boost::source_location const & loc );
```
[none]
* {blank}
+
Effects: ::
  `boost::throw_with_location( std::system_error( make_error_code( e ) ), loc )`.

```
BOOST_NORETURN inline void throw_exception_from_error( std::exception_ptr & e,
  boost::source_location const & loc );
```
[none]
* {blank}
+
Effects: ::
+
[disc]
** If `e` isn't null, `std::rethrow_exception( e )`.
** Otherwise, `boost::throw_with_location( std::bad_exception(), loc )`.

### result<T, E>

`result<T, E>` stores either a value of type `T`, or an error of type `E`. `E` defaults
to `error_code`. In a typical use, functions that can fail return `result<T>`.

```
namespace boost {
namespace system {

template<class T, class E = error_code> class result
{
public:

    using value_type = T;
    using error_type = E;

    static constexpr in_place_value_t in_place_value{};
    static constexpr in_place_error_t in_place_error{};

    // constructors

    constexpr result();

    template<class... A>
      constexpr result( A&&... a );

    template<class... A>
      constexpr result( in_place_value_t, A&&... a );

    template<class... A>
      constexpr result( in_place_error_t, A&&... a );

    template<class T2, class E2>
      constexpr result( result<T2, E2> const& r2 );

    template<class T2, class E2>
      constexpr result( result<T2, E2>&& r2 );

    // queries

    constexpr bool has_value() const noexcept;
    constexpr bool has_error() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // checked value access

    constexpr T& value( boost::source_location const & loc =
      BOOST_CURRENT_LOCATION ) & ;

    constexpr T const& value( boost::source_location const & loc =
      BOOST_CURRENT_LOCATION ) const& ;

    constexpr T&& value( boost::source_location const & loc =
      BOOST_CURRENT_LOCATION ) && ;

    constexpr T const&& value( boost::source_location const & loc =
      BOOST_CURRENT_LOCATION ) const&& ;

    // unchecked value access

    constexpr T* operator->() noexcept;
    constexpr T const* operator->() const noexcept;

    constexpr T& operator*() & noexcept;
    constexpr T const& operator*() const & noexcept;
    constexpr T&& operator*() && noexcept;
    constexpr T const&& operator*() const && noexcept;

    // error access

    constexpr E error() const &;
    constexpr E error() &&;

    // emplace

    template<class... A>
      constexpr T& emplace( A&&... a );

    // swap

    constexpr void swap( result& r );
    friend constexpr void swap( result & r1, result & r2 );

    // equality

    friend constexpr bool operator==( result const & r1, result const & r2 );
    friend constexpr bool operator!=( result const & r1, result const & r2 );
};

// stream insertion

template<class Ch, class Tr, class T, class E>
  std::basic_ostream<Ch, Tr>&
    operator<<( std::basic_ostream<Ch, Tr>& os, result<T, E> const & r );

} // namespace system
} // namespace boost
```

#### Constructors

```
constexpr result();
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the value `T()`.
Remarks: ::
  This constructor is only enabled when `std::is_default_constructible<T>::value` is `true`.

```
template<class... A>
  constexpr result( A&&... a );
```
[none]
* {blank}
+
Effects: ::
  - If `std::is_constructible<T, A...>::value && !std::is_constructible<E, A...>::value`,
    ensures that `*this` holds the value `T( std::forward<A>(a)... )`.
  - If `std::is_constructible<E, A...>::value && !std::is_constructible<T, A...>::value`,
    ensures that `*this` holds the error `E( std::forward<A>(a)... )`.
  - Otherwise, this constructor does not participate in overload resolution.
Remarks: ::
  This constructor is only enabled when `sizeof...(A) > 0`.

```
template<class... A>
  constexpr result( in_place_value_t, A&&... a );
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the value `T( std::forward<A>(a)... )`.
Remarks: ::
  This constructor is only enabled when `std::is_constructible<T, A...>::value` is `true`.

```
template<class... A>
  constexpr result( in_place_error_t, A&&... a );
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the error `E( std::forward<A>(a)... )`.
Remarks: ::
  This constructor is only enabled when `std::is_constructible<E, A...>::value` is `true`.

```
template<class T2, class E2>
  constexpr result( result<T2, E2> const& r2 );
```
[none]
* {blank}
+
Ensures: ::
  If `r2.has_value()` is `true`, `*this` holds the value `T( *r2 )`, otherwise `*this` holds the error `E( r2.error() )`.
Remarks: ::
  This constructor is only enabled when `std::is_convertible<T2, T>::value && std::is_convertible<E2, E>::value` is `true`.

```
template<class T2, class E2>
  constexpr result( result<T2, E2>&& r2 );
```
[none]
* {blank}
+
Ensures: ::
  If `r2.has_value()` is `true`, `*this` holds the value `T( std::move( *r2 ) )`, otherwise `*this` holds the error `E( r2.error() )`.
Remarks: ::
  This constructor is only enabled when `std::is_convertible<T2, T>::value && std::is_convertible<E2, E>::value` is `true`.

#### Queries

```
constexpr bool has_value() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `true` when `*this` holds a value, `false` otherwise.

```
constexpr bool has_error() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `!has_value()`.

```
constexpr explicit operator bool() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `has_value()`.

#### Checked Value Access

```
constexpr T& value(
  boost::source_location const & loc = BOOST_CURRENT_LOCATION ) & ;

constexpr T const& value(
  boost::source_location const & loc = BOOST_CURRENT_LOCATION ) const& ;

constexpr T&& value(
  boost::source_location const & loc = BOOST_CURRENT_LOCATION ) && ;

constexpr T const&& value(
  boost::source_location const & loc = BOOST_CURRENT_LOCATION ) const&& ;
```
[none]
* {blank}
+
Effects: ::
  If `*this` holds a value, returns a reference to it. Otherwise,
  calls `throw_exception_from_error`, passing it a reference to
  the held error, and `loc`.

#### Unchecked Value Access

```
constexpr T* operator->() noexcept;
constexpr T const* operator->() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  If `*this` holds a value, a pointer to it. Otherwise, `nullptr`.

```
constexpr T& operator*() & noexcept;
constexpr T const& operator*() const & noexcept;
```
[none]
* {blank}
+
Requires: :: `*this` holds a value.
Returns: ::
  `*operator\->()`.

```
constexpr T&& operator*() && noexcept;
constexpr T const&& operator*() const && noexcept;
```
[none]
* {blank}
+
Requires: :: `*this` holds a value.
Returns: ::
  `std::move( *operator\->() )`.

#### Error Access

```
constexpr E error() const &;
constexpr E error() &&;
```
[none]
* {blank}
+
Effects: ::
  If `*this` holds an error, returns it. Otherwise, returns `E()`.

#### emplace

```
template<class... A>
  constexpr T& emplace( A&&... a );
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the value `T( std::forward<A>(a)... )`.
Returns: ::
  A reference to the contained value.

#### swap

```
constexpr void swap( result& r );
```
[none]
* {blank}
+
Effects: ::
  Exchanges the contents of `*this` and `r`.

```
friend constexpr void swap( result & r1, result & r2 );
```
[none]
* {blank}
+
Effects: ::
  Exchanges the contents of `r1` and `r2`.

#### Equality

```
friend constexpr bool operator==( result const & r1, result const & r2 );
```
[none]
* {blank}
+
Effects: ::
  - If `r1` holds a value `t1` and `r2` holds a value `t2`, returns `t1 == t2`.
  - If `r1` holds an error `e1` and `r2` holds an error `e2`, returns `e1 == e2`.
  - Otherwise, returns `false`.

```
friend constexpr bool operator!=( result const & r1, result const & r2 );
```
[none]
* {blank}
+
Returns: ::
  `!( r1 == r2 )`.

#### Stream Insertion

```
template<class Ch, class Tr, class T, class E>
  std::basic_ostream<Ch, Tr>&
    operator<<( std::basic_ostream<Ch, Tr>& os, result<T, E> const & r );
```
[none]
* {blank}
+
Effects: ::
  - If `*this` holds a value `t`, `os << "value:" << t`.
  - If `*this` holds an error `e`, `os << "error:" << e`.
Returns: ::
  `os`.

### result<void, E>

```
namespace boost {
namespace system {

template<class E> class result<void, E>
{
public:

    using value_type = void;
    using error_type = E;

    static constexpr in_place_value_t in_place_value{};
    static constexpr in_place_error_t in_place_error{};

    // constructors

    constexpr result() noexcept;

    template<class... A>
      constexpr result( A&&... a );

    constexpr result( in_place_value_t ) noexcept;

    template<class... A>
      constexpr result( in_place_error_t, A&&... a );

    template<class E2>
      constexpr result( result<void, E2> const& r2 );

    // queries

    constexpr bool has_value() const noexcept;
    constexpr bool has_error() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // checked value access

    constexpr void value( boost::source_location const & loc =
      BOOST_CURRENT_LOCATION ) const;

    // unchecked value access

    constexpr void* operator->() noexcept;
    constexpr void const* operator->() const noexcept;

    constexpr void operator*() const noexcept;

    // error access

    constexpr E error() const &;
    constexpr E error() &&;

    // emplace

    constexpr void emplace();

    // swap

    constexpr void swap( result& r );
    friend constexpr void swap( result & r1, result & r2 );

    // equality

    friend constexpr bool operator==( result const & r1, result const & r2 );
    friend constexpr bool operator!=( result const & r1, result const & r2 );
};

// stream insertion

template<class Ch, class Tr, class E>
  std::basic_ostream<Ch, Tr>&
    operator<<( std::basic_ostream<Ch, Tr>& os, result<void, E> const & r );

} // namespace system
} // namespace boost
```

#### Constructors

```
constexpr result() noexcept;
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds an unspecified value.

```
template<class... A>
  constexpr result( A&&... a );
```
[none]
* {blank}
+
Effects: ::
  - If `std::is_constructible<E, A...>::value`,
    ensures that `*this` holds the error `E( std::forward<A>(a)... )`.
  - Otherwise, this constructor does not participate in overload resolution.
Remarks: ::
  This constructor is only enabled when `sizeof...(A) > 0`.

```
template<class... A>
  constexpr result( in_place_value_t ) noexcept;
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds an unspecified value.

```
template<class... A>
  constexpr result( in_place_error_t, A&&... a );
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the error `E( std::forward<A>(a)... )`.
Remarks: ::
  This constructor is only enabled when `std::is_constructible<E, A...>::value` is `true`.

```
template<class E2>
  constexpr result( result<void, E2> const& r2 );
```
[none]
* {blank}
+
Ensures: ::
  If `r2.has_value()` is `true`, `*this` holds an unspecified value, otherwise `*this` holds the error `E( r2.error() )`.
Remarks: ::
  This constructor is only enabled when `std::is_convertible<E2, E>::value` is `true`.

#### Queries

```
constexpr bool has_value() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `true` when `*this` holds a value, `false` otherwise.

```
constexpr bool has_error() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `!has_value()`.

```
constexpr explicit operator bool() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `has_value()`.

#### Checked Value Access

```
constexpr void value(
  boost::source_location const & loc = BOOST_CURRENT_LOCATION ) const;
```
[none]
* {blank}
+
Effects: ::
  If `*this` doesn't hold a value, calls `throw_exception_from_error`,
  passing it a reference to the held error, and `loc`.

#### Unchecked Value Access

```
constexpr void* operator->() noexcept;
constexpr void const* operator->() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  If `*this` holds a value, a pointer to it. Otherwise, `nullptr`.

```
constexpr void operator*() const noexcept;
```
[none]
* {blank}
+
Requires: :: `*this` holds a value.
Effects: ::
  none.

#### Error Access

```
constexpr E error() const &;
constexpr E error() &&;
```
[none]
* {blank}
+
Effects: ::
  If `*this` holds an error, returns it. Otherwise, returns `E()`.

#### emplace

```
constexpr void emplace();
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds an unspecified value.

#### swap

```
constexpr void swap( result& r );
```
[none]
* {blank}
+
Effects: ::
  Exchanges the contents of `*this` and `r`.

```
friend constexpr void swap( result & r1, result & r2 );
```
[none]
* {blank}
+
Effects: ::
  Exchanges the contents of `r1` and `r2`.

#### Equality

```
friend constexpr bool operator==( result const & r1, result const & r2 );
```
[none]
* {blank}
+
Effects: ::
  - If `r1` and `r2` hold values, returns `true`.
  - If `r1` holds an error `e1` and `r2` holds an error `e2`, returns `e1 == e2`.
  - Otherwise, returns `false`.

```
friend constexpr bool operator!=( result const & r1, result const & r2 );
```
[none]
* {blank}
+
Returns: ::
  `!( r1 == r2 )`.

#### Stream Insertion

```
template<class Ch, class Tr, class E>
  std::basic_ostream<Ch, Tr>&
    operator<<( std::basic_ostream<Ch, Tr>& os, result<void, E> const & r );
```
[none]
* {blank}
+
Effects: ::
  - If `*this` holds a value, `os << "value:void"`.
  - If `*this` holds an error `e`, `os << "error:" << e`.
Returns: ::
  `os`.

### result<U&, E>

```
namespace boost {
namespace system {

template<class U, class E> class result<U&, E>
{
public:

    using value_type = U&;
    using error_type = E;

    static constexpr in_place_value_t in_place_value{};
    static constexpr in_place_error_t in_place_error{};

    // constructors

    template<class A>
      constexpr result( A&& a ) noexcept;

    template<class... A>
      constexpr result( A&&... a );

    template<class A>
      constexpr result( in_place_value_t, A&& a ) noexcept;

    template<class... A>
      constexpr result( in_place_error_t, A&&... a );

    template<class U2, class E2>
      constexpr result( result<U2&, E2> const& r2 );

    // queries

    constexpr bool has_value() const noexcept;
    constexpr bool has_error() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // checked value access

    constexpr U& value( boost::source_location const & loc =
      BOOST_CURRENT_LOCATION ) const;

    // unchecked value access

    constexpr U* operator->() const noexcept;
    constexpr U& operator*() const noexcept;

    // error access

    constexpr E error() const &;
    constexpr E error() &&;

    // emplace

    template<class A>
      constexpr U& emplace( A&& a ) noexcept;

    // swap

    constexpr void swap( result& r );
    friend constexpr void swap( result & r1, result & r2 );

    // equality

    friend constexpr bool operator==( result const & r1, result const & r2 );
    friend constexpr bool operator!=( result const & r1, result const & r2 );
};

} // namespace system
} // namespace boost
```

#### Constructors

```
template<class A>
  constexpr result( A&& a ) noexcept;
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the reference `static_cast<U&>( std::forward<A>(a) )`.
Remarks: ::
  This constructor is only enabled when `A` is `B&` and `std::is_convertible<B*, U*>::value` is `true`.

```
template<class... A>
  constexpr result( A&&... a );
```
[none]
* {blank}
+
Effects: ::
  - If `std::is_constructible<E, A...>::value && !std::is_constructible<U&, A...>::value`,
    ensures that `*this` holds the error `E( std::forward<A>(a)... )`.
  - Otherwise, this constructor does not participate in overload resolution.
Remarks: ::
  This constructor is only enabled when `sizeof...(A) > 0`.

```
template<class A>
  constexpr result( in_place_value_t, A&& a ) noexcept;
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the reference `static_cast<U&>( std::forward<A>(a) )`.
Remarks: ::
  This constructor is only enabled when `A` is `B&` and `std::is_convertible<B*, U*>::value` is `true`.

```
template<class... A>
  constexpr result( in_place_error_t, A&&... a );
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the error `E( std::forward<A>(a)... )`.
Remarks: ::
  This constructor is only enabled when `std::is_constructible<E, A...>::value` is `true`.

```
template<class U2, class E2>
  constexpr result( result<U2&, E2> const& r2 );
```
[none]
* {blank}
+
Ensures: ::
  If `r2.has_value()` is `true`, `*this` holds the reference `static_cast<U&>( *r2 )`, otherwise `*this` holds the error `E( r2.error() )`.
Remarks: ::
  This constructor is only enabled when `std::is_convertible<U2*, U*>::value && std::is_convertible<E2, E>::value` is `true`.

#### Queries

```
constexpr bool has_value() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `true` when `*this` holds a value, `false` otherwise.

```
constexpr bool has_error() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `!has_value()`.

```
constexpr explicit operator bool() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  `has_value()`.

#### Checked Value Access

```
constexpr U& value(
  boost::source_location const & loc = BOOST_CURRENT_LOCATION ) const;
```
[none]
* {blank}
+
Effects: ::
  If `*this` holds a reference, returns it. Otherwise,
  calls `throw_exception_from_error`, passing it a reference to
  the held error, and `loc`.

#### Unchecked Value Access

```
constexpr U* operator->() const noexcept;
```
[none]
* {blank}
+
Returns: ::
  If `*this` holds a reference, a pointer to its referent. Otherwise, `nullptr`.

```
constexpr U& operator*() const noexcept;
```
[none]
* {blank}
+
Requires: :: `*this` holds a reference.
Returns: ::
  `*operator\->()`.

#### Error Access

```
constexpr E error() const &;
constexpr E error() &&;
```
[none]
* {blank}
+
Effects: ::
  If `*this` holds an error, returns it. Otherwise, returns `E()`.

#### emplace

```
template<class A>
  constexpr U& emplace( A&& a ) noexcept;
```
[none]
* {blank}
+
Ensures: ::
  `*this` holds the reference `static_cast<U&>( std::forward<A>(a)... )`.
Returns: ::
  The contained reference.
Remarks: ::
  This function is only enabled when `A` is `B&` and `std::is_convertible<B*, U*>::value` is `true`.

#### swap

```
constexpr void swap( result& r );
```
[none]
* {blank}
+
Effects: ::
  Exchanges the contents of `*this` and `r`.

```
friend constexpr void swap( result & r1, result & r2 );
```
[none]
* {blank}
+
Effects: ::
  Exchanges the contents of `r1` and `r2`.

#### Equality

```
friend constexpr bool operator==( result const & r1, result const & r2 );
```
[none]
* {blank}
+
Effects: ::
  - If `r1` holds a reference `t1` and `r2` holds a reference `t2`, returns `t1 == t2`.
  - If `r1` holds an error `e1` and `r2` holds an error `e2`, returns `e1 == e2`.
  - Otherwise, returns `false`.

```
friend constexpr bool operator!=( result const & r1, result const & r2 );
```
[none]
* {blank}
+
Returns: ::
  `!( r1 == r2 )`.

### Chaining

#### operator|

```
template<class T, class E, class U> T operator|( result<T, E> const& r, U&& u );
template<class T, class E, class U> T operator|( result<T, E>&& r, U&& u );
```
[none]
* {blank}
+
Returns the value in `r`, or if `r` contains an error, a default value `u`.
+
Effects: ::
  - If `r.has_value()` is `true`, returns `*r`.
  - Otherwise, returns `u`.
Remarks: ::
  Only enabled when `U` is convertible to `T`.
Example: ::
+
```
result<int> get_server_port(); // can fail

int get_port()
{
    return get_server_port() | 443;
}
```

```
template<class T, class E, class F> T operator|( result<T, E> const& r, F&& f );
template<class T, class E, class F> T operator|( result<T, E>&& r, F&& f );
```
[none]
* {blank}
+
Returns the value in `r`, or if `r` contains an error, a default value obtained
by invoking the function `f`.
+
Effects: ::
  - If `r.has_value()` is `true`, returns `*r`.
  - Otherwise, returns `f()`.
Remarks: ::
  Only enabled when `f()` is convertible to `T`.
Example: ::
+
```
result<int> get_server_port(); // can fail
int get_default_port();

int get_port()
{
    return get_server_port() | get_default_port;
}
```
+
Note that the right hand side is `get_default_port` and not `get_default_port()`.
This is important; spelled this way, the function `get_default_port` is called
only when `get_server_port_impl()` fails. If it were
+
```
    return get_server_port() | get_default_port();
```
+
the function would have been called unconditionally.
+
Another, equivalent, way is to use a lambda
+
```
    return get_server_port() | []{ return get_default_port(); };
```
+
which isn't necessary here, but would be needed for more complex expressions.

```
template<class T, class E, class F, class R = ...> R operator|( result<T, E> const& r, F&& f );
template<class T, class E, class F, class R = ...> R operator|( result<T, E>&& r, F&& f );
template<class E, class F, class R = ...> R operator|( result<void, E> const& r, F&& f );
template<class E, class F, class R = ...> R operator|( result<void, E>&& r, F&& f );
```
[none]
* {blank}
+
Returns the value in `r`, or if `r` contains an error, another `result` obtained
by invoking the function `f`.
+
Let `R` be the type of `f()`.
+
Effects: ::
  - If `r.has_value()` is `true`, returns `*r`.
  - Otherwise, returns `f()`.
Remarks: ::
  Only enabled when `R` is an instance of `result` and `T` is convertible to `R::value_type`.
Example: ::
+
```
result<int> get_server_port(); // can fail
result<int> get_default_port(); // can fail

int get_port()
{
    return get_server_port() | get_default_port | 443;
}
```

#### operator|=

```
template<class T, class E, class U> result<T, E>& operator|=( result<T, E>& r, U&& u );
```
[none]
* {blank}
+
If `r` contains an error, assigns a value to it, constructed from `u`.
+
Effects: ::
  If `r.has_value()` is `false`, assigns `u` to `r`.
Returns: ::
  `r`.
Remarks: ::
  Only enabled when `U` is convertible to `T`.

```
template<class T, class E, class F> result<T, E>& operator|=( result<T, E>& r, F&& f );
```
[none]
* {blank}
+
If `r` contains an error, assigns `f()` to it.
+
Effects: ::
  If `r.has_value()` is `false`, assigns `f()` to `r`.
Returns: ::
  `r`.
Remarks: ::
  Only enabled when the type of `f()` is either
  - convertible to `T` or
  - an instance of `result` convertible to `result<T, E>`.
Example: ::
+
```
result<response> query_impl( std::string_view server, request const& req );

result<response> query( std::vector<std::string_view> const& servers, request const& req )
{
    result<response> r = make_error_code( errc::invalid_argument );

    for( auto const& server: servers )
    {
        r |= std::bind( query_impl, server, std::ref( req ) );
    }

    return r;
}
```


#### operator&

```
template<class T, class E, class F, class U = ...>
  result<U, E> operator&( result<T, E> const& r, F&& f );

template<class T, class E, class F, class U = ...>
  result<U, E> operator&( result<T, E>&& r, F&& f );
```
[none]
* {blank}
+
Returns the error in `r`, or if `r` contains a value, transforms the value by calling `f` on it.
+
Let `U` be the type of `f(*r)`.
+
Effects: ::
  - If `r.has_value()` is `true`, returns `f(*r)`.
  - Otherwise, returns `r.error()`.
Remarks: ::
  Only enabled when `U` is not an instance of `result`.
Example: ::
+
```
struct currency_type
{
    char code_[ 4 ] = {};
};

result<double> get_exchange_rate( currency_type from, currency_type to );

result<double> convert( double amount, currency_type from, currency_type to )
{
    return get_exchange_rate( from, to ) & [&](double rate){ return rate * amount; };
}
```

```
template<class E, class F, class U = ...>
  result<U, E> operator&( result<void, E> const& r, F&& f );
```
[none]
* {blank}
+
Returns the error in `r`, or if `r` contains a value, replaces it with the result of calling `f`.
+
Let `U` be the type of `f()`.
+
Effects: ::
  - If `r.has_value()` is `true`, returns `f()`.
  - Otherwise, returns `r.error()`.
Remarks: ::
  Only enabled when `U` is not an instance of `result`.


```
template<class T, class E, class F, class R = ...> R operator&( result<T, E> const& r, F&& f );
template<class T, class E, class F, class R = ...> R operator&( result<T, E>&& r, F&& f );
```
[none]
* {blank}
+
Returns the error in `r`, or if `r` contains a value, another `result` obtained
by invoking the function `f` on the value in `r`.
+
Let `R` be the type of `f(*r)`.
+
Effects: ::
  - If `r.has_value()` is `true`, returns `f(*r)`.
  - Otherwise, returns `r.error()`.
Remarks: ::
  Only enabled when `R` is an instance of `result` and `E` is convertible to `R::error_type`.
Example: ::
+
```
struct JsonValue
{
    result<JsonValue const&> at( std::size_t i ) const noexcept;
    result<JsonValue const&> at( std::string_view key ) const noexcept;
    template<class T> result<T> to_number() const noexcept;
};

namespace helpers
{

inline auto at( std::size_t i ) {
    return [=](JsonValue const& jv){ return jv.at( i ); }; }

inline auto at( std::string_view key ) {
    return [=](JsonValue const& jv){ return jv.at( key ); }; }

template<class T> inline auto to_number() {
    return [](JsonValue const& jv){ return jv.to_number<T>(); }; }

} // namespace helpers

int get_port( JsonValue const& config, int def )
{
    using namespace helpers;
    return config.at( "servers" ) & at( 0 ) & at( "port" ) & to_number<int>() | def;
}
```

```
template<class E, class F, class R = ...> R operator&( result<void, E> const& r, F&& f );
```
[none]
* {blank}
+
Returns the error in `r`, or if `r` contains a value, another `result` obtained
by invoking the function `f`.
+
Let `R` be the type of `f()`.
+
Effects: ::
  - If `r.has_value()` is `true`, returns `f()`.
  - Otherwise, returns `r.error()`.
Remarks: ::
  Only enabled when `R` is an instance of `result` and `E` is convertible to `R::error_type`.


#### operator&=

```
template<class T, class E, class F, class U = ...>
  result<T, E>& operator&=( result<T, E>& r, F&& f );
```
[none]
* {blank}
+
If `r` contains a value, replaces it with the result of invoking the function `f` on the value in `r`.
+
Let `U` be the type of `f(*r)`.
+
Effects: :: If `r.has_value()` is `true`, assigns `f(*std::move(r))` to `r`.
Returns: :: `r`.
Remarks: ::
  Only enabled when `U` is not an instance of `result` and is convertible to `T`.

```
template<class T, class E, class F, class R = ...>
  result<T, E>& operator&=( result<T, E>& r, F&& f );
```
[none]
* {blank}
+
If `r` contains a value, replaces `r` with the result of invoking the function `f` on the value in `r`.
+
Let `R` be the type of `f(*r)`.
+
Effects: :: If `r.has_value()` is `true`, assigns `f(*std::move(r))` to `r`.
Returns: :: `r`.
Remarks: ::
  Only enabled when `R` is an instance of `result` and is convertible to `result<T, E>`.
Example: ::
+
```
struct JsonValue
{
    result<JsonValue const&> at( std::string_view key ) const noexcept;
};

namespace helpers
{

inline auto at( std::string_view key ) {
    return [=](JsonValue const& jv){ return jv.at( key ); }; }

} // namespace helpers

result<JsonValue const&> at_path( JsonValue const& jv,
    std::initializer_list<std::string_view> path )
{
    result<JsonValue const&> r( jv );

    using namespace helpers;

    for( auto key: path )
    {
        r &= at( key );
    }

    return r;
}
```

```
template<class E, class F, class R = ...>
  result<void, E>& operator&=( result<void, E>& r, F&& f );
```
[none]
* {blank}
+
If `r` contains a value, replaces `r` with the result of invoking the function `f`.
+
Let `R` be the type of `f()`.
+
Effects: :: If `r.has_value()` is `true`, assigns `f()` to `r`.
Returns: :: `r`.
Remarks: ::
  Only enabled when `R` is an instance of `result<void, E2>` and `E2` is convertible to `E`.

## <boost/system.hpp>

This convenience header includes all the headers previously described.
