Introduction
Errors originating from the operating system or other low-level application
program interfaces (APIs) are typically reported via an integer representing
an error code, either by returning the code directly from the function (e.g.
pthread_mutex_init
) or by using a side channel such as the errno
pseudo-variable under POSIX or GetLastError()
under Windows.
However, these integer error values can only be interpreted when their source
is known. The value 5 under Windows means ERROR_ACCESS_DENIED
when returned
by GetLastError()
, but EIO
when retrieved from errno
. And conversely,
the same error condition "access denied" is represented by the value 5 when
returned by GetLastError()
and 13 (EACCES
) when retrieved from errno
.
This means that in order for code to be able to handle errors from both sources (to retrieve a text message describing the error, or to check whether the error means "access denied"), it needs to know where the integer error value originated. For this to be possible, the integer error value needs to be accompanied by a piece of information identifying the source.
Boost.System provides a framework in which this is possible. Errors are
represented by a class error_code
which contains both the error value and
a pointer to their source (called "category"), represented as a class derived
from error_category
.
The category provides member functions such as message
, which returns a text
message for a specific error value, and equivalent
, which can be used to test
whether a specific error value correspond to an error condition such as "access
denied". error_code
uses these category-provided functions in the
implementation of its message
and operator==
member functions.
Boost.System contains two predefined category classes, the generic category
(a reference to which is returned by generic_category()
) and the system
category (system_category()
). The generic category represents the error
values of the portable subset of errno
values defined by the POSIX standard,
whereas the system category is OS dependent. Under POSIX, the system category
represents the errno
values returned by the OS APIs (a superset of those in
the generic category), whereas under Windows, the system category represents
the error values returned by GetLastError()
.
The framework is extensible. Users can define their own categories by
deriving a class from error_category
and implementing a function that
returns a reference to an instance of it. This capability is useful both for
describing library-defined error values, and for adapting existing C API
libraries that return integer error values.
For those who prefer error reporting via exceptions, Boost.System provides
a standard exception class system_error
that stores an error_code
.
Boost.System was standardized in C++11 as <system_error>
. For a while,
the two were equivalent, but Boost.System has evolved since then and now
contains a number of extensions over its standard sibling:
-
A non-allocating overload of
message
; -
Support for nonzero error codes meaning success, via the
failed
member functions; -
Support for 64 bit category identifiers, as a solution to the problem that sometimes it’s not possible to ensure that only one instance of a category exists in the program;
-
Support for attaching source locations (file/line/function) to error codes;
-
A class
result<T>
that can be used to return either a value or an error code from a function; -
Various other minor improvements.
boost::system::error_code
can be converted to, and constructed from,
std::error_code
.
Usage
All of the following code snippets assume that these lines
#include <boost/system.hpp>
namespace sys = boost::system;
are in effect.
Returning Errors from OS APIs under POSIX
Let’s suppose that we’re implementing a portable file
wrapper
over the OS file APIs. Its general outline is shown below:
class file
{
private:
int fd_;
public:
// ...
std::size_t read( void * buffer, std::size_t size, sys::error_code& ec );
std::size_t write( void const * buffer, std::size_t size, sys::error_code& ec );
};
Since we’re implementing the POSIX version of file
, its
data member is a POSIX file descriptor int fd_;
, although other
implementations will differ.
Our read
and write
functions return the number of bytes transferred, and signal
errors via the output parameter ec
, of type boost::system::error_code
.
An implementation of file::read
might look like this:
std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
{
ssize_t r = ::read( fd_, buffer, size );
if( r < 0 )
{
ec.assign( errno, sys::system_category() );
return 0;
}
ec = {}; // ec.clear(); under C++03
return r;
}
We first call the POSIX API read
; if it returns an error, we store the errno
value in ec
, using the system category, and return 0 as bytes transferred.
Otherwise, we clear ec
to signal success, and return the result of ::read
.
Note
|
Clearing ec on successful returns is an important step; do not omit it.
|
Under POSIX, the system category corresponds to POSIX errno
values, which is
why we use it.
In principle, since the generic category also corresponds to errno
values
under all platforms, we could have used it here; however, by convention under
POSIX, if the errno
value comes from the OS (the "system"), we use the system
category for it. That’s because the system category values may be a
platform-specific superset of the generic (platform-independent) values.
The implementation of file::write
is basically the same. We show it here for
completeness:
std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
{
ssize_t r = ::write( fd_, buffer, size );
if( r < 0 )
{
ec.assign( errno, sys::system_category() );
return 0;
}
ec = {}; // ec.clear(); under C++03
return r;
}
Returning Errors from OS APIs under Windows
Under Windows, our file
object will store a HANDLE
instead of an int
:
class file
{
private:
HANDLE fh_;
public:
// as before
};
and the implementation of file::read
will look like this:
std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
{
DWORD r = 0;
if( ::ReadFile( fh_, buffer, size, &r, 0 ) )
{
// success
ec = {}; // ec.clear(); under C++03
}
else
{
// failure
ec.assign( ::GetLastError(), sys::system_category() );
}
// In both cases, r is bytes transferred
return r;
}
Here, the system category corresponds to the values defined in the system
header <winerror.h>
and returned by GetLastError()
. Since we use the
Win32 API ReadFile
to implement file::read
, and it returns the error
code via GetLastError()
, we again store that value in ec
as belonging
to the system category.
The implementation of file::write
is, again, the same.
std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
{
DWORD r = 0;
if( ::WriteFile( fh_, buffer, size, &r, 0 ) )
{
ec = {}; // ec.clear(); under C++03
}
else
{
ec.assign( ::GetLastError(), sys::system_category() );
}
return r;
}
Returning Specific Errors under POSIX
Our implementation of file::read
has a problem; it accepts std::size_t
values for size
, but the behavior of ::read
is unspecified when the
requested value does not fit in ssize_t
. To avoid reliance on unspecified
behavior, let’s add a check for this condition and return an error:
std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
{
if( size > SSIZE_MAX )
{
ec.assign( EINVAL, sys::generic_category() );
return 0;
}
ssize_t r = ::read( fd_, buffer, size );
if( r < 0 )
{
ec.assign( errno, sys::system_category() );
return 0;
}
ec = {}; // ec.clear(); under C++03
return r;
}
In this case, since we’re returning the fixed errno
value EINVAL
, which
is part of the portable subset defined by the generic category, we mark the
error value in ec
as belonging to the generic category.
It’s possible to use system as well, as EINVAL
is also a system category
value under POSIX; however, using the generic category for values belonging
to the portable errno
subset is slightly preferrable.
Our implementation of file::write
needs to undergo a similar treatment.
There, however, we’ll apply another change. When there’s no space left on
the disk, ::write
returns a number of bytes written that is lower than
what we requested with size
, but our function signals no error. We’ll make
it return ENOSPC
in this case.
std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
{
if( size > SSIZE_MAX )
{
ec.assign( EINVAL, sys::generic_category() );
return 0;
}
ssize_t r = ::write( fd_, buffer, size );
if( r < 0 )
{
ec.assign( errno, sys::system_category() );
return 0;
}
if( r < size )
{
ec.assign( ENOSPC, sys::system_category() );
}
else
{
ec = {}; // ec.clear(); under C++03
}
return r;
}
We’ve used the system category to make it appear that the ENOSPC
value
has come from the ::write
API, mostly to illustrate that this is also a
possible approach. Using a generic value would have worked just as well.
Returning Specific Errors under Windows
Not much to say; the situation under Windows is exactly the same. The only
difference is that we must use the generic category for returning errno
values. The system category does not work; the integer values in the system
category are entirely different from those in the generic category.
std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
{
DWORD r = 0;
if( size > MAXDWORD )
{
ec.assign( EINVAL, sys::generic_category() );
}
else if( ::ReadFile( fh_, buffer, size, &r, 0 ) )
{
ec = {}; // ec.clear(); under C++03
}
else
{
ec.assign( ::GetLastError(), sys::system_category() );
}
return r;
}
std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
{
DWORD r = 0;
if( size > MAXDWORD )
{
ec.assign( EINVAL, sys::generic_category() );
}
else if( ::WriteFile( fh_, buffer, size, &r, 0 ) )
{
if( r < size )
{
ec.assign( ENOSPC, sys::generic_category() );
}
else
{
ec = {}; // ec.clear(); under C++03
}
}
else
{
ec.assign( ::GetLastError(), sys::system_category() );
}
return r;
}
Attaching a Source Location to Error Codes
Unlike the standard <system_error>
, Boost.System allows source locations
(file/line/function) to be stored in error_code
, so that functions handling
the error can display or log the source code location where the error occurred.
To take advantage of this functionality, our POSIX file::read
function needs
to be augmented as follows:
std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
{
if( size > SSIZE_MAX )
{
static constexpr boost::source_location loc = BOOST_CURRENT_LOCATION;
ec.assign( EINVAL, sys::generic_category(), &loc );
return 0;
}
ssize_t r = ::read( fd_, buffer, size );
if( r < 0 )
{
static constexpr boost::source_location loc = BOOST_CURRENT_LOCATION;
ec.assign( errno, sys::system_category(), &loc );
return 0;
}
ec = {}; // ec.clear(); under C++03
return r;
}
That is, before every ec.assign
statement, we need to declare a
static constexpr
variable holding the current source location, then pass
a pointer to it to assign
. Since error_code
is small and there’s no space
in it for more than a pointer, we can’t just store the source_location
in it
by value.
BOOST_CURRENT_LOCATION
is a macro expanding to the current source location
(a combination of __FILE__
, __LINE__
, and BOOST_CURRENT_FUNCTION
.)
It’s defined and documented in Boost.Assert.
Under C++03, instead of static constexpr
, one needs to use static const
.
Another option is BOOST_STATIC_CONSTEXPR
, a
Boost.Config macro that expands to either
static constexpr
or static const
, as appropriate.
To avoid repeating this boilerplate each time we do ec.assign
, we can define
a macro:
#define ASSIGN(ec, ...) { \
BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION; \
(ec).assign(__VA_ARGS__, &loc); }
which we can now use to augment, for example, the POSIX implementation of file::write
:
std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
{
if( size > SSIZE_MAX )
{
ASSIGN( ec, EINVAL, sys::generic_category() );
return 0;
}
ssize_t r = ::write( fd_, buffer, size );
if( r < 0 )
{
ASSIGN( ec, errno, sys::system_category() );
return 0;
}
if( r < size )
{
ASSIGN( ec, ENOSPC, sys::generic_category() );
}
else
{
ec = {}; // ec.clear(); under C++03
}
return r;
}
Obtaining Textual Representations of Error Codes for Logging and Display
Assuming that we have an error_code
instance ec
, returned to us by some
function, we have a variety of means to obtain textual representations of the
error code represented therein.
ec.to_string()
gives us the result of streaming ec
into a std::ostream
,
e.g. if std::cout << ec << std::endl;
outputs system:6
, this is what
ec.to_string()
will return. (system:6
under Windows is ERROR_INVALID_HANDLE
from <winerror.h>
.)
To obtain a human-readable error message corresponding to this code, we can
use ec.message()
. For ERROR_INVALID_HANDLE
, it would give us "The handle is
invalid" - possibly localized.
If ec
contains a source location, we can obtain its textual representation
via ec.location().to_string()
. This will give us something like
C:\Projects\testbed2019\testbed2019.cpp:98 in function 'unsigned __int64 __cdecl file::read(void *,unsigned __int64,class boost::system::error_code &)'
if there is a location in ec
, and
(unknown source location)
if there isn’t. (ec.has_location()
is true
when ec
contains a location.)
Finally, ec.what()
will give us a string that contains all of the above,
something like
The handle is invalid [system:6 at C:\Projects\testbed2019\testbed2019.cpp:98 in function 'unsigned __int64 __cdecl file::read(void *,unsigned __int64,class boost::system::error_code &)']
Most logging and diagnostic output that is not intended for the end user would
probably end up using what()
. (ec.what()
, augmented with the prefix
supplied at construction, is also what boost::system::system_error::what()
would return.)
Composing Functions Returning Error Codes
Let’s suppose that we need to implement a file copy function, with the following interface:
std::size_t file_copy( file& src, file& dest, sys::error_code& ec );
file_copy
uses src.read
to read bytes from src
, then writes these bytes
to dest
using dest.write
. This continues until one of these operations signals
an error, or until end of file is reached. It returns the number of bytes written,
and uses ec
to signal an error.
Here is one possible implementation:
std::size_t file_copy( file& src, file& dest, sys::error_code& ec )
{
std::size_t r = 0;
for( ;; )
{
unsigned char buffer[ 1024 ];
std::size_t n = src.read( buffer, sizeof( buffer ), ec );
// read failed, leave the error in ec and return
if( ec.failed() ) return r;
// end of file has been reached, exit loop
if( n == 0 ) return r;
r += dest.write( buffer, n, ec );
// write failed, leave the error in ec and return
if( ec.failed() ) return r;
}
}
Note that there is no longer any difference between POSIX and Windows
implementations; their differences are contained in file::read
and
file::write
. file_copy
is portable and works under any platform.
The general pattern in writing such higher-level functions is that
they pass the output error_code
parameter ec
they received from
the caller directly as the output parameter to the lower-level functions
they are built upon. This way, when they detect a failure in an intermediate
operation (by testing ec.failed()
), they can immediately return to the
caller, because the error code is already in its proper place.
Note that file_copy
doesn’t even need to clear ec
on success, by
using ec = {};
. Since we’ve already tested ec.failed()
, we know that
ec
contains a value that means success.
Providing Dual (Throwing and Nonthrowing) Overloads
Functions that signal errors via an output error_code& ec
parameter
require that the caller check ec
after calling them, and take appropriate
action (such as return immediately, as above.) Forgetting to check ec
results in logic errors.
While this is a preferred coding style for some, others prefer exceptions, which one cannot forget to check.
An approach that has been introduced by
Boost.Filesystem (which later turned
into std::filesystem
) is to provide both alternatives: a nonthrowing
function taking error_code& ec
, as file_copy
above, and a throwing
function that does not take an error_code
output parameter, and throws
exceptions on failure.
This is how this second throwing function is typically implemented:
std::size_t file_copy( file& src, file& dest )
{
sys::error_code ec;
std::size_t r = file_copy( src, dest, ec );
if( ec.failed() ) throw sys::system_error( ec, __func__ );
return r;
}
That is, we simply call the nonthrowing overload of file_copy
, and if
it signals failure in ec
, throw a system_error
exception.
We use our function name __func__
("file_copy"
) as the prefix,
although that’s a matter of taste.
Note that typically under this style the overloads taking error_code& ec
are decorated with noexcept
, so that it’s clear that they don’t throw
exceptions (although we haven’t done so in the preceding examples in order
to keep the code C++03-friendly.)
result<T> as an Alternative to Dual APIs
Instead of providing two functions for every operation, an alternative
approach is to make the function return sys::result<T>
instead of T
.
result<T>
is a class holding either T
or error_code
, similar to
variant<T, error_code>
.
Clients that prefer to check for errors and not rely on exceptions can
test whether a result<T> r
contains a value via if( r )
or its more
verbose equivalent if( r.has_value() )
, then obtain the value via
*r
or r.value()
. If r
doesn’t contain a value, the error_code
it holds can be obtained with r.error()
.
Those who prefer exceptions just call r.value()
directly, without
checking. In the no-value case, this will automatically throw a
system_error
corresponding to the error_code
in r
.
Assuming our base file
API is unchanged, this variation of file_copy
would look like this:
sys::result<std::size_t> file_copy( file& src, file& dest )
{
std::size_t r = 0;
sys::error_code ec;
for( ;; )
{
unsigned char buffer[ 1024 ];
std::size_t n = src.read( buffer, sizeof( buffer ), ec );
if( ec.failed() ) return ec;
if( n == 0 ) return r;
r += dest.write( buffer, n, ec );
if( ec.failed() ) return ec;
}
}
The only difference here is that we return ec
on error, instead of
r
.
Note, however, that we can no longer return both an error code and a
number of transferred bytes; that is, we can no longer signal partial
success. This is often not an issue at higher levels, but lower-level
primitives such as file::read
and file::write
might be better off
written using the old style.
Nevertheless, to demonstrate how result
returning APIs are composed,
we’ll show how file_copy
would look if file::read
and file::write
returned result<size_t>
:
class file
{
public:
// ...
sys::result<std::size_t> read( void * buffer, std::size_t size );
sys::result<std::size_t> write( void const * buffer, std::size_t size );
};
sys::result<std::size_t> file_copy( file& src, file& dest )
{
std::size_t m = 0;
for( ;; )
{
unsigned char buffer[ 1024 ];
auto r = src.read( buffer, sizeof( buffer ) );
if( !r ) return r;
std::size_t n = *r;
if( n == 0 ) return m;
auto r2 = dest.write( buffer, n );
if( !r2 ) return r2;
std::size_t n2 = *r2;
m += n2;
}
}
Testing for Specific Error Conditions
Let’s suppose that we have called a function that signals failure
using error_code
, we have passed it an error_code
variable ec
,
and now for some reason want to check whether the function has
failed with an error code of EINVAL
, "invalid argument".
Since error_code
can be compared for equality, our first instict
might be if( ec == error_code( EINVAL, generic_category() )
.
This is wrong, and we should never do it.
First, under POSIX, the function might have returned EINVAL
from
the system category (because the error might have been returned by
an OS API, and not by the function itself, as was the case in our
read
and write
implementations.)
Since error_code
comparisons are exact, EINVAL
from the generic
category does not compare equal to EINVAL
from the system category.
(And before you start thinking about just comparing ec.value()
to
EINVAL
, read on.)
Second, under Windows, the function might have returned error_code(
ERROR_INVALID_PARAMETER, system_category() )
. As we have already
mentioned, the integer error values in the system category under
Windows are completely unrelated to the integer errno
values.
The correct approach is to compare ec
not to specific error codes,
but to error_condition( EINVAL, generic_category() )
. Error
conditions are a platform-independent way to represent the meaning
of the concrete error codes. In our case, all error codes, under
both POSIX and Windows, that represent EINVAL
will compare equal
to error_condition( EINVAL, generic_category() )
.
In short, you should never compare error codes to error codes, and
should compare them to error conditions instead. This is the purpose
of the error_condition
class, which is very frequently misunderstood.
Since
if( ec == sys::error_condition( EINVAL, sys::generic_category() ) )
{
// handle EINVAL
}
is a bit verbose, Boost.System provides enumerator values for the
errno
values against which an error code can be compared directly.
These enumerators are defined in <boost/system/errc.hpp>
,
and enable the above test to be written
if( ec == sys::errc::invalid_argument )
{
// handle EINVAL
}
which is what one should generally use for testing for a specific error condition, as a best practice.
Adapting Existing Integer Error Values
Libraries with C (or extern "C"
) APIs often signal failure by returning
a library-specific integer error code (with zero typically being reserved
for "no error".) When writing portable C++ wrappers, we need to decide
how to expose these error codes, and using error_code
is a good way to
do it.
Because the integer error codes are library specific, and in general match
neither errno
values or system category values, we need to define a
library-specific error category.
Adapting SQLite Errors
We’ll take SQLite as an example. The general outline of a custom error category is as follows:
class sqlite3_category_impl: public sys::error_category
{
// TODO add whatever's needed here
};
sys::error_category const& sqlite3_category()
{
static const sqlite3_category_impl instance;
return instance;
}
which can then be used similarly to the predefined generic and system categories:
int r = some_sqlite3_function( ... );
ec.assign( r, sqlite3_category() );
If we try to compile the above category definition as-is, it will complain
about our not implementing two pure virtual member functions, name
and
message
, so at minimum, we’ll need to add these. In addition, we’ll also
implement the non-allocating overload of message
. It’s not pure virtual,
but its default implementation calls the std::string
-returning overload,
and that’s almost never what one wants. (This default implementation is only
provided for backward compatibility, in order to not break existing
user-defined categories that were written before this overload was added.)
So, the minimum we need to implement is this:
class sqlite3_category_impl: public sys::error_category
{
public:
const char * name() const noexcept;
std::string message( int ev ) const;
char const * message( int ev, char * buffer, std::size_t len ) const noexcept;
};
name
is easy, it just returns the category name:
const char * sqlite3_category_impl::name() const noexcept
{
return "sqlite3";
}
message
is used to obtain an error message given an integer error code.
SQLite provides the function sqlite3_errstr
for this, so we don’t need
to do any work:
std::string sqlite3_category_impl::message( int ev ) const
{
return sqlite3_errstr( ev );
}
char const * sqlite3_category_impl::message( int ev, char * buffer, std::size_t len ) const noexcept
{
std::snprintf( buffer, len, "%s", sqlite3_errstr( ev ) );
return buffer;
}
and we’re done. sqlite3_category()
can now be used like the predefined
categories, and we can put an SQLite error code int r
into a Boost.System
error_code ec
by means of ec.assign( r, sqlite3_category() )
.
Adapting ZLib Errors
Another widely used C library is ZLib, and the portion of zlib.h
that
defines its error codes is shown below:
#define Z_OK 0
#define Z_STREAM_END 1
#define Z_NEED_DICT 2
#define Z_ERRNO (-1)
#define Z_STREAM_ERROR (-2)
#define Z_DATA_ERROR (-3)
#define Z_MEM_ERROR (-4)
#define Z_BUF_ERROR (-5)
#define Z_VERSION_ERROR (-6)
/* Return codes for the compression/decompression functions. Negative values
* are errors, positive values are used for special but normal events.
*/
There are three relevant differences with the previous case of SQLite:
-
While for SQLite all non-zero values were errors, as is the typical case, here negative values are errors, but positive values are "special but normal", that is, they represent success, not failure;
-
ZLib does not provide a function that returns the error message corresponding to a specific error code;
-
When
Z_ERRNO
is returned, the error code should be retrieved fromerrno
.
Our category implementation will look like this:
class zlib_category_impl: public sys::error_category
{
public:
const char * name() const noexcept;
std::string message( int ev ) const;
char const * message( int ev, char * buffer, std::size_t len ) const noexcept;
bool failed( int ev ) const noexcept;
};
sys::error_category const& zlib_category()
{
static const zlib_category_impl instance;
return instance;
}
As usual, the implementation of name
is trivial:
const char * zlib_category_impl::name() const noexcept
{
return "zlib";
}
We’ll need to work a bit harder to implement message
this time, as there’s
no preexisting function to lean on:
char const * zlib_category_impl::message( int ev, char * buffer, std::size_t len ) const noexcept
{
switch( ev )
{
case Z_OK: return "No error";
case Z_STREAM_END: return "End of stream";
case Z_NEED_DICT: return "A dictionary is needed";
case Z_ERRNO: return "OS API error";
case Z_STREAM_ERROR: return "Inconsistent stream state or invalid argument";
case Z_DATA_ERROR: return "Data error";
case Z_MEM_ERROR: return "Out of memory";
case Z_BUF_ERROR: return "Insufficient buffer space";
case Z_VERSION_ERROR: return "Library version mismatch";
}
std::snprintf( buffer, len, "Unknown zlib error %d", ev );
return buffer;
}
This is a typical implementation of the non-throwing message
overload. Note
that message
is allowed to return something different from buffer
, which
means that we can return character literals directly, without copying them
into the supplied buffer first. This allows our function to return the correct
message text even when the buffer is too small.
The std::string
overload of message
is now trivial:
std::string zlib_category_impl::message( int ev ) const
{
char buffer[ 64 ];
return this->message( ev, buffer, sizeof( buffer ) );
}
Finally, we need to implement failed
, in order to override its default
behavior of returning true
for all nonzero values:
bool zlib_category_impl::failed( int ev ) const noexcept
{
return ev < 0;
}
This completes the implementation of zlib_category()
and takes care of the
first two bullets above, but we still haven’t addressed the third one; namely,
that we need to retrieve the error from errno
in the Z_ERRNO
case.
To do that, we’ll define a helper function that would be used to assign a ZLib
error code to an error_code
:
void assign_zlib_error( sys::error_code & ec, int r )
{
if( r != Z_ERRNO )
{
ec.assign( r, zlib_category() );
}
else
{
ec.assign( errno, sys::generic_category() );
}
}
so that, instead of using ec.assign( r, zlib_category() )
directly, code
would do
int r = some_zlib_function( ... );
assign_zlib_error( ec, r );
We can stop here, as this covers everything we set out to do, but we can take
an extra step and enable source locations for our error codes. For that, we’ll
need to change assign_zlib_error
to take a source_location
:
void assign_zlib_error( sys::error_code & ec, int r, boost::source_location const* loc )
{
if( r != Z_ERRNO )
{
ec.assign( r, zlib_category(), loc );
}
else
{
ec.assign( errno, sys::generic_category(), loc );
}
}
Define a helper macro to avoid the boilerplate of defining the static
constexpr
source location object each time:
#define ASSIGN_ZLIB_ERROR(ec, r) { \
BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION; \
assign_zlib_error( ec, r, &loc ); }
And then use the macro instead of the function:
int r = some_zlib_function( ... );
ASSIGN_ZLIB_ERROR( ec, r );
Supporting Comparisons against Conditions
We notice that some of the ZLib error codes correspond to portable errno
conditions. Z_STREAM_ERROR
, for instance, is returned in cases where
POSIX functions would have returned EINVAL
; Z_MEM_ERROR
is ENOMEM
;
and Z_BUF_ERROR
, insufficient space in the output buffer to store the
result, roughly corresponds to ERANGE
, result out of range.
To encode this relationship, we need to implement either
default_error_condition
or equivalent
in our category. Since we have
a simple one to one mapping, the former will suffice:
class zlib_category_impl: public sys::error_category
{
public:
const char * name() const noexcept;
std::string message( int ev ) const;
char const * message( int ev, char * buffer, std::size_t len ) const noexcept;
bool failed( int ev ) const noexcept;
sys::error_condition default_error_condition( int ev ) const noexcept;
};
The implementation is straightforward:
sys::error_condition zlib_category_impl::default_error_condition( int ev ) const noexcept
{
switch( ev )
{
case Z_OK: return sys::error_condition();
case Z_STREAM_ERROR: return sys::errc::invalid_argument;
case Z_MEM_ERROR: return sys::errc::not_enough_memory;
case Z_BUF_ERROR: return sys::errc::result_out_of_range;
}
return sys::error_condition( ev, *this );
}
Once this is added, we will be able to compare a ZLib error_code ec
against
errc
enumerators:
if( ec == sys::errc::not_enough_memory )
{
// Z_MEM_ERROR, or ENOMEM
}
Defining Library-Specific Error Codes
Let’s suppose that we are writing a library libmyimg
for reading some
hypothetical image format, and that we have defined the following API
function for that:
namespace libmyimg
{
struct image;
void load_image( file& f, image& im, sys::error_code& ec );
} // namespace libmyimg
(using our portable file
class from the preceding examples.)
Our hypothetical image format is simple, consisting of a fixed header,
followed by the image data, so an implementation of load_image
might
have the following structure:
namespace libmyimg
{
struct image_header
{
uint32_t signature;
uint32_t width;
uint32_t height;
uint32_t bits_per_pixel;
uint32_t channels;
};
void load_image_header( file& f, image_header& im, sys::error_code& ec );
struct image;
void load_image( file& f, image& im, sys::error_code& ec )
{
image_header ih = {};
load_image_header( f, ih, ec );
if( ec.failed() ) return;
if( ih.signature != 0xFF0AD71A )
{
// return an "invalid signature" error
}
if( ih.width == 0 )
{
// return an "invalid width" error
}
if( ih.height == 0 )
{
// return an "invalid height" error
}
if( ih.bits_per_pixel != 8 )
{
// return an "unsupported bit depth" error
}
if( ih.channels != 1 && ih.channels != 3 && ih.channels != 4 )
{
// return an "unsupported channel count" error
}
// initialize `im` and read image data
// ...
}
} // namespace libmyimg
We can see that we need to define five error codes of our own. (Our function
can also return other kinds of failures in ec
— those will come from
file::read
which load_image_header
will use to read the header.)
To define these errors, we’ll use a scoped enumeration type. (This example will take advantage of C++11 features.)
namespace libmyimg
{
enum class error
{
success = 0,
invalid_signature,
invalid_width,
invalid_height,
unsupported_bit_depth,
unsupported_channel_count
};
} // namespace libmyimg
Boost.System supports being told that an enumeration type represents an error
code, which enables implicit conversions between the enumeration type and
error_code
. It’s done by specializing the is_error_code_enum
type trait,
which resides in namespace boost::system
like the rest of the library:
namespace boost
{
namespace system
{
template<> struct is_error_code_enum< ::libmyimg::error >: std::true_type
{
};
} // namespace system
} // namespace boost
Once this is in place, we can now assign values of libmyimg::error
to
sys::error_code
, which enables the implementation of load_image
to be
written as follows:
void load_image( file& f, image& im, sys::error_code& ec )
{
image_header ih = {};
load_image_header( f, ih, ec );
if( ec.failed() ) return;
if( ih.signature != 0xFF0AD71A )
{
ec = error::invalid_signature;
return;
}
if( ih.width == 0 )
{
ec = error::invalid_width;
return;
}
if( ih.height == 0 )
{
ec = error::invalid_height;
return;
}
if( ih.bits_per_pixel != 8 )
{
ec = error::unsupported_bit_depth;
return;
}
if( ih.channels != 1 && ih.channels != 3 && ih.channels != 4 )
{
ec = error::unsupported_channel_count;
return;
}
// initialize `image` and read image data
// ...
}
This is however not enough; we still need to define the error category for our enumerators, and associate them with it.
The first step follows our previous two examples very closely:
namespace libmyimg
{
class myimg_category_impl: public sys::error_category
{
public:
const char * name() const noexcept;
std::string message( int ev ) const;
char const * message( int ev, char * buffer, std::size_t len ) const noexcept;
};
const char * myimg_category_impl::name() const noexcept
{
return "libmyimg";
}
std::string myimg_category_impl::message( int ev ) const
{
char buffer[ 64 ];
return this->message( ev, buffer, sizeof( buffer ) );
}
char const * myimg_category_impl::message( int ev, char * buffer, std::size_t len ) const noexcept
{
switch( static_cast<error>( ev ) )
{
case error::success: return "No error";
case error::invalid_signature: return "Invalid image signature";
case error::invalid_width: return "Invalid image width";
case error::invalid_height: return "Invalid image height";
case error::unsupported_bit_depth: return "Unsupported bit depth";
case error::unsupported_channel_count: return "Unsupported number of channels";
}
std::snprintf( buffer, len, "Unknown libmyimg error %d", ev );
return buffer;
}
sys::error_category const& myimg_category()
{
static const myimg_category_impl instance;
return instance;
}
} // namespace libmyimg
The second step involves implementing a function make_error_code
in
the namespace of our enumeration type error
that takes error
and
returns boost::system::error_code
:
namespace libmyimg
{
sys::error_code make_error_code( error e )
{
return sys::error_code( static_cast<int>( e ), myimg_category() );
}
} // namespace libmyimg
Now load_image
will compile, and we just need to fill the rest of its
implementation with code that uses file::read
to read the image data.
There’s one additional embellishment we can make. As we know, Boost.System
was proposed for, and accepted into, the C++11 standard, and now there’s
a standard implementation of it in <system_error>
. We can make our
error enumeration type compatible with std::error_code
as well, by
specializing the standard type trait std::is_error_code_enum
:
namespace std
{
template<> struct is_error_code_enum< ::libmyimg::error >: std::true_type
{
};
} // namespace std
This makes our enumerators convertible to std::error_code
.
(The reason this works is that boost::system::error_code
is convertible
to std::error_code
, so the return value of our make_error_code
overload
can be used to initialize a std::error_code
.)
Defining Library-Specific Error Conditions
All of the libmyimg::error
error codes we have so far represent the same
error condition - invalid or unsupported image format. It might make sense to
enable testing for this condition without the need to enumerate all five
specific codes. To do this, we can define an error condition enumeration type:
namespace libmyimg
{
enum class condition
{
invalid_format = 1
};
} // namespace libmyimg
which we can tag as representing an error condition by specializing
is_error_condition_enum
:
namespace boost
{
namespace system
{
template<> struct is_error_condition_enum< ::libmyimg::condition >: std::true_type
{
};
} // namespace system
} // namespace boost
namespace std
{
template<> struct is_error_condition_enum< ::libmyimg::condition >: std::true_type
{
};
} // namespace std
Similarly to the error code enumeration type, which needed a make_error_code
overload, this one will need to have a make_error_condition
overload, and a
category.
It’s in principle possible to reuse the category we already defined for our error codes, by making the condition values start from, say, 10000 instead of 1. This saves some typing, but a better practice is to use a separate category for the error conditions. So that’s what we’ll do:
namespace libmyimg
{
class myimg_condition_category_impl: public sys::error_category
{
public:
const char * name() const noexcept;
std::string message( int ev ) const;
char const * message( int ev, char * buffer, std::size_t len ) const noexcept;
};
const char * myimg_condition_category_impl::name() const noexcept
{
return "libmyimg_condition";
}
std::string myimg_condition_category_impl::message( int ev ) const
{
char buffer[ 64 ];
return this->message( ev, buffer, sizeof( buffer ) );
}
char const * myimg_condition_category_impl::message( int ev, char * buffer, std::size_t len ) const noexcept
{
switch( static_cast<condition>( ev ) )
{
case condition::invalid_format: return "Invalid or unsupported image format";
}
std::snprintf( buffer, len, "Unknown libmyimg condition %d", ev );
return buffer;
}
sys::error_category const& myimg_condition_category()
{
static const myimg_condition_category_impl instance;
return instance;
}
sys::error_condition make_error_condition( condition e )
{
return sys::error_condition( static_cast<int>( e ), myimg_condition_category() );
}
} // namespace libmyimg
We have our condition, but it doesn’t do anything yet. To enable
libmyimg::condition::invalid_format
to compare equal to our error codes,
we need to implement default_error_condition
in the error code category:
namespace libmyimg
{
class myimg_category_impl: public sys::error_category
{
public:
const char * name() const noexcept;
std::string message( int ev ) const;
char const * message( int ev, char * buffer, std::size_t len ) const noexcept;
sys::error_condition default_error_condition( int ev ) const noexcept;
};
sys::error_condition myimg_category_impl::default_error_condition( int ev ) const noexcept
{
switch( static_cast<error>( ev ) )
{
case error::success:
return {};
case error::invalid_signature:
case error::invalid_width:
case error::invalid_height:
case error::unsupported_bit_depth:
case error::unsupported_channel_count:
return condition::invalid_format;
}
return sys::error_condition( ev, *this );
}
} // namespace libmyimg
That’s it; now ec == libmyimg::condition::invalid_format
can be used to test
whether ec
contains one of our error codes corresponding to the "invalid
image format" condition.
Revision History
Changes in Boost 1.81
-
The macro
BOOST_SYSTEM_DISABLE_THREADS
can now be defined to disable the use of<mutex>
(e.g. on single-threaded libstdc++). -
Added
value_type
,error_type
,in_place_value
,in_place_error
toresult<>
. -
Added
emplace
toresult<>
.
Changes in Boost 1.80
-
When an
error_code
is converted tostd::error_code
and then back toerror_code
, the original is now restored, if possible. -
Reworked the conversion from
error_category
tostd::error_category
to avoid the one-time allocation that shows up on leak checkers. -
Added a constructor that allows replacing the source location of an
error_code
, and a correspondingassign
. -
Added a converting constructor to
result
.
Changes in Boost 1.79
-
Added a
boost::source_location
parameter tothrow_exception_from_error
. -
Added
throw_exception_from_error
overloads forerrc::errc_t
,std::error_code
,std::errc
,std::exception_ptr
. -
result<T>::value
now automatically suppliesBOOST_CURRENT_LOCATION
tothrow_exception_from_error
via a default argument. -
Added an
errc::make_error_code
overload taking a source location.
Changes in Boost 1.78
-
Added support for source locations to
error_code
. -
Added
error_code::to_string
,error_condition::to_string
,error_code::what
. -
system_error::what()
now contains the source location, if present. -
Added
result<T, E = error_code>
, a class holding either a value or an error, defined in<boost/system/result.hpp>
.
Changes in Boost 1.77
-
The conversion operator from
error_category
tostd::error_category
has been improved and no longer requires<map>
or<mutex>
. -
The comparison operators of
error_category
are now inline friends instead of member functions (a side effect of the previous change.) -
error_condition
now defers callinggeneric_category()
to avoid instantiating the object until it’s actually needed. -
error_condition::failed
anderror_condition::message
have been undeprecated, andoperator bool()
now once again returnsfailed()
. -
The system category now doesn’t call
generic_category()
, to avoid instantiating the object. -
The return value of
default_error_condition
changes in some cases into anerror_condition
from the generic category, instead of from the system category. This happens on POSIX when the inputerror_code
is from the system category and does not correspond to anyerrc_t
value. -
The interoperability of
error_code
andstd::error_code
has been improved substantially. It is now possible to constructboost::system::error_code
fromstd::error_code
, and it’s possible to passboost::system::error_code
to functions takingstd::error_code&
. -
A stream insertion operator for
error_condition
has been added.
Changes in Boost 1.76
-
windows_error.hpp
is no longer deprecated.
Changes in Boost 1.75
-
The platform-specific headers
windows_error.hpp
,linux_error.hpp
, andcygwin_error.hpp
emit deprecation messages and are slated for removal. -
The old names for
generic_category()
andsystem_category()
emit deprecation messages and are slated for removal. -
error_condition::failed
is deprecated and is slated for removal.operator bool()
forerror_condition
has been reverted to its old meaning ofvalue() != 0
. This is done for compatibility withstd::error_condition
as the next release is expected to improve interoperability with<system_error>
even further. Note that this does not affecterror_code::failed
, which is still alive and well. -
The overload of
error_condition::message
that takes a buffer is deprecated and is slated for removal, for the same reasons. Note that this does not affecterror_code::message
.
Changes in Boost 1.74
-
operator bool()
now returnsfailed()
instead ofvalue() != 0
.
Changes in Boost 1.69
-
Boost.System is now header-only. A stub library is still built for compatibility, but linking to it is no longer necessary.
-
Even more functions have been marked
constexpr
. -
The destructor of
error_category
is now protected and no longer virtual. This is a potentially breaking change but its impact is expected to be limited. -
error_category
now has a constructor that accepts a 64 bit identifier, enabling distinct category objects to compare equal. -
The constructors of
error_category
are now protected. -
A non-allocating, nonthrowing overload of
message
has been added. -
A virtual function
failed
has been added, allowing categories for which success is not synonymous with 0. -
The deprecated
boost::system::throws
object has been removed. -
boost::throws()
is now deprecated and its use is discouraged. -
The constructor of
system_error
taking a singleerror_code
argument is now explicit. -
system_error::code()
now returns by value.
Changes in Boost 1.68
On a C++14 compiler, many Boost.System functions and member functions
are now constexpr
, and error_code
and error_condition
are literal
classes.
In addition to enabling use in constant expressions (and constexpr
functions), this significantly improves the quality of the generated code.
As a result of this change, however, now using Boost.System from C++14
or C++17 code requires that the library be also built with C++14 or
above. This is the default on GCC 6 and newer, but not on GCC 5 or Clang.
One can build Boost for C++14 by passing the cxxstd=14
option to b2
.
(Previous versions allowed code built against any C++ standard to link with Boost.System built against any C++ standard. In 1.68, code using any C++ standard can link with Boost.System built with C++14 or above, but if Boost.System is built with C++11 or below, only code also built with C++11 and below can link to it successfully.)
Changes in Boost 1.65
On a C++11 compiler, Boost.System now provides implicit conversions
from boost::system::error_category
, error_code
, and error_condition
to their standard equivalents from <system_error>
.
This allows libraries to expose a C++11 interface and report errors
via std::error_code
even when using Boost.System, directly or through a
dependency such as Boost.ASIO.
Design Rationale
error_code
and error_condition
are designed as value types so
they can be copied without slicing and do not require heap allocation, but
still have polymorphic behavior based on the error category. This is achieved
by abstract base class error_category
supplying the polymorphic behavior,
and error_code
and error_condition
containing a pointer to an object of a
type derived from error_category
.
Many of the detailed design decisions were driven by the requirements that users to be able to add additional error categories, and that it be no more difficult to write portable code than system-specific code.
The operator<<
overload for error_code
eliminates a misleading conversion to
bool
in code like cout << ec
, where ec
is of type error_code
. It is also
useful in its own right.
Reference
Use of C++11 and C++14 Features
The library is documented to use several C++11 and C++14 features,
including noexcept
, explicit conversion operators and constexpr
. The
actual implementation uses C++11 and C++14 features only when they are
available, and otherwise falls back on C++03 features.
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 C++11 standard library, the
C++ 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 |
---|---|
|
|
|
|
|
|
|
|
|
|
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|
|
|
<boost/system/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/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/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;
-
- Returns:
-
error_condition( e, generic_category() )
.
Since
errc::errc_t
provides a specialization ofis_error_condition_enum
and an overload ofmake_error_condition
, it can be converted implicitly to anerror_condition
. This is typically useful when comparingerror_code
values returned from APIs to a portable condition, as in the below example: -
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;
-
- Returns:
-
error_code( e, generic_category() )
.
In addition to
make_error_condition
,errc::errc_t
provides an overload ofmake_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: -
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;
-
- Returns:
-
error_code( e, generic_category(), loc )
.
Same as the above overload, but takes a source location.
-
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/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;
-
- 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 typeC
exists in the program.
explicit constexpr error_category( unsigned long long id ) noexcept;
-
- Effects:
-
Initializes
id_
toid
. - 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;
-
- Returns:
-
In derived classes, a character literal naming the error category.
virtual error_condition default_error_condition( int ev ) const noexcept;
-
- 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;
-
- Returns:
-
-
In derived classes,
true
whenerror_code( code, *this )
is equivalent tocondition
. -
In the default implementation,
default_error_condition( code ) == condition
.
-
virtual bool equivalent( const error_code & code, int condition )
const noexcept;
-
- Returns:
-
-
In derived classes,
true
whencode
is equivalent toerror_condition( condition, *this )
. -
In the default implementation,
*this == code.category() && code.value() == condition
.
-
virtual std::string message( int ev ) const = 0;
-
- 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;
-
- 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 tolen-1
characters and storing a null terminator, and returnbuffer
. Iflen
is 0, nothing is copied, but the function still returnsbuffer
. Note that whenlen
is 0,buffer
may benullptr
.
-
-
The default implementation calls
message(ev)
and copies the result intobuffer
, truncating it tolen-1
characters and storing a null terminator. Iflen
is 0, copies nothing. Returnsbuffer
. Ifmessage(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;
-
- Returns:
-
-
In derived classes,
true
whenev
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;
-
- 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;
-
- Returns:
-
!( lhs == rhs )
.
constexpr bool operator< ( const error_category & lhs,
const error_category & rhs ) const noexcept;
-
- 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;
-
- Returns:
-
A reference to an
std::error_category
object corresponding to*this
.
<boost/system/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;
-
- Returns:
-
A reference to a predefined
error_category
object identifying errors originating from the operating system.
<boost/system/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;
-
- Returns:
-
A reference to a predefined
error_category
object identifying portable error codes and conditions.
<boost/system/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;
-
- Ensures:
-
value() == 0
;category() == system_category()
;!has_location()
.
constexpr error_code( int val, const error_category & cat ) noexcept;
-
- Ensures:
-
value() == val
;category() == cat
;!has_location()
.
error_code( int val, const error_category & cat,
boost::source_location const * loc ) noexcept;
-
- Requires:
-
loc
points to a validboost::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;
-
- Ensures:
-
*this == make_error_code( e )
. - Remarks:
-
This constructor is only enabled when
is_error_code_enum<ErrorCodeEnum>::value
istrue
.
error_code( error_code const& ec,
boost::source_location const * loc ) noexcept;
-
- Requires:
-
loc
points to a validboost::source_location
object with static storage duration, or isnullptr
. - Ensures:
-
*this == ec
. - Remarks:
-
When
ec
is a default-constructederror_code
or wraps astd::error_code
, or whenloc
isnullptr
,*this
stores no location (has_location()
isfalse
). Otherwise,*this
storesloc
(has_location()
istrue
and&location()
isloc
.)
error_code( std::error_code const & ec ) noexcept;
-
- Effects:
-
Construct an
error_code
that wrapsec
. - Remarks:
-
value()
andcategory()
are unspecified.has_location()
isfalse
.
Modifiers
constexpr void assign( int val, const error_category & cat ) noexcept;
-
- Effects:
-
*this = error_code( val, cat )
.
void assign( int val, const error_category & cat,
boost::source_location const * loc ) noexcept;
-
- Effects:
-
*this = error_code( val, cat, loc )
.
template<class ErrorCodeEnum>
constexpr error_code & operator=( ErrorCodeEnum e ) noexcept;
-
- Ensures:
-
*this == make_error_code( e )
. - Remarks:
-
This operator is only enabled when
is_error_code_enum<ErrorCodeEnum>::value
istrue
.
void assign( error_code const& ec,
boost::source_location const * loc ) noexcept;
-
- Effects:
-
*this = error_code( ec, loc )
.
constexpr void clear() noexcept;
-
- Effects:
-
*this = error_code()
.
Observers
constexpr int value() const noexcept;
-
- Returns:
-
the error value.
constexpr const error_category & category() const noexcept;
-
- Returns:
-
the error category.
error_condition default_error_condition() const noexcept;
-
- Returns:
-
category().default_error_condition( value() )
.
std::string message() const;
-
- Returns:
-
If
*this
wraps astd::error_code
objectec
,ec.message()
. Otherwise,category().message( value() )
.
char const * message( char * buffer, std::size_t len ) const noexcept;
-
- Effects:
-
If
*this
wraps astd::error_code
objectec
, copies the string returned fromec.message()
intobuffer
and returnsbuffer
. Otherwise, returnscategory().message( value(), buffer, len )
.
constexpr bool failed() const noexcept;
-
- Returns:
-
If
*this
wraps astd::error_code
objectec
,ec.value() != 0
. Otherwise,category().failed( value() )
.
constexpr explicit operator bool() const noexcept;
-
- Returns:
-
failed()
.
bool has_location() const noexcept;
-
- Returns:
-
true
if*this
has been constructed with a pointer to a source location,false
otherwise.
boost::source_location const & location() const noexcept;
-
- Returns:
-
*loc
if*this
has been constructed with a pointer to a source locationloc
, a reference to a default-constructedboost::source_location
otherwise.
Comparisons
friend constexpr bool operator==( const error_code & lhs,
const error_code & rhs ) noexcept;
-
- Returns:
-
If both
lhs
andrhs
wrapstd::error_code
objectse1
ande2
,e1 == e2
. Otherwise,lhs.value() == rhs.value() && lhs.category() == rhs.category()
.
friend constexpr bool operator!=( const error_code & lhs,
const error_code & rhs ) noexcept;
-
- Returns:
-
!( lhs == rhs )
.
friend constexpr bool operator<( const error_code & lhs,
const error_code & rhs ) noexcept;
-
- Returns:
-
If both
lhs
andrhs
wrapstd::error_code
objectse1
ande2
,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;
-
- Returns:
-
If
code
wraps astd::error_code
objectec
,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;
-
- Returns:
-
!( lhs == rhs )
.
friend bool operator==( const error_code & lhs,
const std::error_code & rhs ) noexcept;
-
- Returns:
-
static_cast<std::error_code>(lhs) == rhs
.
friend bool operator==( const std::error_code & lhs,
const error_code & rhs ) noexcept;
-
- 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;
-
- Returns:
-
!( lhs == rhs )
.
template<class E>
friend constexpr bool operator==( const error_code & lhs, E rhs ) noexcept;
-
- Effects:
-
When
is_error_code_enum<E>::value
istrue
, returnslhs == make_error_code(rhs)
; -
When
is_error_condition_enum<E>::value
istrue
, returnslhs == make_error_condition(rhs)
; -
Otherwise, this overload is disabled.
template<class E>
friend constexpr bool operator==( E lhs, const error_code & rhs ) noexcept;
-
- Effects:
-
When
is_error_code_enum<E>::value
istrue
, returnsmake_error_code(lhs) == rhs
; -
When
is_error_condition_enum<E>::value
istrue
, returnsmake_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;
-
- Returns:
-
!( lhs == rhs )
. - Remarks:
-
These overloads are only enabled when
is_error_code_enum<E>::value
istrue
oris_error_condition_enum<E>::value
istrue
.
Conversions
operator std::error_code() const;
operator std::error_code();
-
- Returns:
-
If
*this
wraps astd::error_code
objectec
,ec
. Otherwise,std::error_code( value(), category() )
.
template<class T> operator T&();
-
- Effects:
-
If
*this
wraps astd::error_code
objectec
, returns a reference toec
. Otherwise, makes*this
wrapstd::error_code( *this )
, then returns a reference to it. - Remarks:
-
This operator is only enabled when
T
isstd::error_code
.
to_string
std::string to_string() const;
-
- Returns:
-
If
*this
wraps astd::error_code
objecte2
, a string that is the concatenation of"std:"
,e2.category().name()
,':'
, and the string representation ofe2.value()
. Otherwise, the concatenation ofcategory().name()
,':'
, and the string representation ofvalue()
.
Stream Insertion
template <class charT, class traits>
std::basic_ostream<charT, traits>&
operator<<( basic_ostream<charT, traits>& os, const error_code & ec );
-
- Effects:
-
os << to_string()
. - Returns:
-
os
.
what
std::string what() const;
-
- Returns:
-
A string representation of
*this
, suitable for logging and diagnostic output. Typically incorporatesmessage()
,to_string()
, andlocation().to_string()
(if available.)
Nonmembers
std::size_t hash_value( const error_code & ec );
-
- Returns:
-
If
ec
wraps astd::error_code
objecte2
,std::hash<std::error_code>()(e2)
. Otherwise, a hash value representingec
.
<boost/system/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;
-
- Ensures:
-
value() == 0
;category() == generic_category()
.
constexpr error_condition( int val, const error_category & cat ) noexcept;
-
- Ensures:
-
value() == val
;category() == cat
.
template <class ErrorConditionEnum>
constexpr error_condition( ErrorConditionEnum e ) noexcept;
-
- Ensures:
-
*this == make_error_condition( e )
. - Remarks:
-
This constructor is only enabled when
is_error_condition_enum<ErrorConditionEnum>::value
istrue
.
Modifiers
constexpr void assign( int val, const error_category & cat ) noexcept;
-
- Ensures:
-
value() == val
;category() == cat
.
template <class ErrorConditionEnum>
constexpr error_condition & operator=( ErrorConditionEnum e ) noexcept;
-
- Ensures:
-
*this == make_error_condition( e )
. - Remarks:
-
This operator is only enabled when
is_error_condition_enum<ErrorConditionEnum>::value
istrue
.
constexpr void clear() noexcept;
-
- Ensures:
-
value() == 0
;category() == generic_category()
.
Observers
constexpr int value() const noexcept;
-
- Returns:
-
the error value.
constexpr const error_category & category() const noexcept;
-
- Returns:
-
the error category.
std::string message() const;
-
- Returns:
-
category().message( value() )
.
char const * message( char * buffer, std::size_t len ) const noexcept;
-
- Returns:
-
category().message( value(), buffer, len )
.
constexpr bool failed() const noexcept;
-
- Returns:
-
category().failed( value() )
.
constexpr explicit operator bool() const noexcept;
-
- Returns:
-
failed()
.
Comparisons
friend constexpr bool operator==( const error_condition & lhs,
const error_condition & rhs ) noexcept;
-
- Returns:
-
lhs.value() == rhs.value() && lhs.category() == rhs.category()
.
friend constexpr bool operator!=( const error_condition & lhs,
const error_condition & rhs ) noexcept;
-
- Returns:
-
!( lhs == rhs )
.
friend constexpr bool operator<( const error_condition & lhs,
const error_condition & rhs ) noexcept;
-
- 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;
-
- 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;
-
- Returns:
-
!( lhs == rhs )
.
Conversions
operator std::error_condition() const;
-
- Returns:
-
std::error_condition( value(), category() )
.
to_string
std::string to_string() const;
-
- Returns:
-
The concatenation of
"cond:"
,category().name()
,':'
, and the string representation ofvalue()
.
Stream Insertion
template <class charT, class traits>
std::basic_ostream<charT, traits>&
operator<<( basic_ostream<charT, traits>& os, const error_condition & en );
-
- Effects:
-
os << en.to_string()
. - Returns:
-
os
.
<boost/system/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 );
-
- 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 );
-
- Ensures:
-
code() == error_code( ev, ecat )
.
Observers
error_code code() const noexcept;
-
- Returns:
-
ec
orerror_code( ev, ecat )
, from the constructor, as appropriate.
const char * what() const noexcept;
-
- 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 C++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>;
} // 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 );
-
- 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 );
-
- 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 );
-
- 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 );
-
- 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 );
-
- Effects:
-
-
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();
-
- Ensures:
-
*this
holds the valueT()
. - Remarks:
-
This constructor is only enabled when
std::is_default_constructible<T>::value
istrue
.
template<class... A>
constexpr result( A&&... a );
-
- Effects:
-
-
If
std::is_constructible<T, A…>::value && !std::is_constructible<E, A…>::value
, ensures that*this
holds the valueT( std::forward<A>(a)… )
. -
If
std::is_constructible<E, A…>::value && !std::is_constructible<T, A…>::value
, ensures that*this
holds the errorE( std::forward<A>(a)… )
. -
Otherwise, this constructor does not participate in overload resolution.
-
- Remarks:
-
This constructor is only enabled when
sizeof…(T) > 0
.
template<class... A>
constexpr result( in_place_value_t, A&&... a );
-
- Ensures:
-
*this
holds the valueT( std::forward<A>(a)… )
. - Remarks:
-
This constructor is only enabled when
std::is_constructible<T, A…>::value
istrue
.
template<class... A>
constexpr result( in_place_error_t, A&&... a );
-
- Ensures:
-
*this
holds the errorE( std::forward<A>(a)… )
. - Remarks:
-
This constructor is only enabled when
std::is_constructible<E, A…>::value
istrue
.
template<class T2, class E2>
constexpr result( result<T2, E2> const& r2 );
-
- Ensures:
-
If
r2.has_value()
istrue
,*this
holds the valueT( *r2 )
, otherwise*this
holds the valueE( r2.error() )
. - Remarks:
-
This constructor is only enabled when
std::is_convertible<T2, T>::value && std::is_convertible<E2, E>::value
istrue
.
template<class T2, class E2>
constexpr result( result<T2, E2>&& r2 );
-
- Ensures:
-
If
r2.has_value()
istrue
,*this
holds the valueT( std::move( *r2 ) )
, otherwise*this
holds the valueE( r2.error() )
. - Remarks:
-
This constructor is only enabled when
std::is_convertible<T2, T>::value && std::is_convertible<E2, E>::value
istrue
.
Queries
constexpr bool has_value() const noexcept;
-
- Returns:
-
true
when*this
holds a value,false
otherwise.
constexpr bool has_error() const noexcept;
-
- Returns:
-
!has_value()
.
constexpr explicit operator bool() const noexcept;
-
- 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&& ;
-
- Effects:
-
If
*this
holds a value, returns a reference to it. Otherwise, callsthrow_exception_from_error
, passing it a reference to the held error, andloc
.
Unchecked Value Access
constexpr T* operator->() noexcept;
constexpr T const* operator->() const noexcept;
-
- Returns:
-
If
*this
holds a value, a pointer to it. Otherwise,nullptr
.
constexpr T& operator*() & noexcept;
constexpr T const& operator*() const & noexcept;
-
- Requires:
-
*this
holds a value. - Returns:
-
*operator->()
.
constexpr T&& operator*() && noexcept;
constexpr T const&& operator*() const && noexcept;
-
- Requires:
-
*this
holds a value. - Returns:
-
std::move( *operator->() )
.
Error Access
constexpr E error() const &;
constexpr E error() &&;
-
- Effects:
-
If
*this
holds an error, returns it. Otherwise, returnsE()
.
emplace
template<class... A>
constexpr T& emplace( A&&... a );
-
- Ensures:
-
*this
holds the valueT( std::forward<A>(a)… )
. - Returns:
-
A reference to the contained value.
swap
constexpr void swap( result& r );
-
- Effects:
-
Exchanges the contents of
*this
andr
.
friend constexpr void swap( result & r1, result & r2 );
-
- Effects:
-
Exchanges the contents of
r1
andr2
.
Equality
friend constexpr bool operator==( result const & r1, result const & r2 );
-
- Effects:
-
-
If
r1
holds a valuet1
andr2
holds a valuet2
, returnst1 == t2
. -
If
r1
holds an errore1
andr2
holds an errore2
, returnse1 == e2
. -
Otherwise, returns
false
.
-
friend constexpr bool operator!=( result const & r1, result const & r2 );
-
- 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 );
-
- Effects:
-
-
If
*this
holds a valuet
,os << "value:" << t
. -
If
*this
holds an errore
,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 );
// 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;
-
- Ensures:
-
*this
holds an unspecified value.
template<class... A>
constexpr result( A&&... a );
-
- Effects:
-
-
If
std::is_constructible<E, A…>::value
, ensures that*this
holds the errorE( std::forward<A>(a)… )
. -
Otherwise, this constructor does not participate in overload resolution.
-
- Remarks:
-
This constructor is only enabled when
sizeof…(T) > 0
.
template<class... A>
constexpr result( in_place_value_t ) noexcept;
-
- Ensures:
-
*this
holds an unspecified value.
template<class... A>
constexpr result( in_place_error_t, A&&... a );
-
- Ensures:
-
*this
holds the errorE( std::forward<A>(a)… )
. - Remarks:
-
This constructor is only enabled when
std::is_constructible<E, A…>::value
istrue
.
Queries
constexpr bool has_value() const noexcept;
-
- Returns:
-
true
when*this
holds a value,false
otherwise.
constexpr bool has_error() const noexcept;
-
- Returns:
-
!has_value()
.
constexpr explicit operator bool() const noexcept;
-
- Returns:
-
has_value()
.
Checked Value Access
constexpr void value(
boost::source_location const & loc = BOOST_CURRENT_LOCATION ) const;
-
- Effects:
-
If
*this
doesn’t hold a value, callsthrow_exception_from_error
, passing it a reference to the held error, andloc
.
Unchecked Value Access
constexpr void* operator->() noexcept;
constexpr void const* operator->() const noexcept;
-
- Returns:
-
If
*this
holds a value, a pointer to it. Otherwise,nullptr
.
constexpr void operator*() const noexcept;
-
- Requires:
-
*this
holds a value. - Effects:
-
none.
Error Access
constexpr E error() const &;
constexpr E error() &&;
-
- Effects:
-
If
*this
holds an error, returns it. Otherwise, returnsE()
.
emplace
constexpr void emplace();
-
- Ensures:
-
*this
holds an unspecified value.
swap
constexpr void swap( result& r );
-
- Effects:
-
Exchanges the contents of
*this
andr
.
friend constexpr void swap( result & r1, result & r2 );
-
- Effects:
-
Exchanges the contents of
r1
andr2
.
Equality
friend constexpr bool operator==( result const & r1, result const & r2 );
-
- Effects:
-
-
If
r1
andr2
hold values, returnstrue
. -
If
r1
holds an errore1
andr2
holds an errore2
, returnse1 == e2
. -
Otherwise, returns
false
.
-
friend constexpr bool operator!=( result const & r1, result const & r2 );
-
- 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 );
-
- Effects:
-
-
If
*this
holds a value,os << "value:void"
. -
If
*this
holds an errore
,os << "error:" << e
.
-
- Returns:
-
os
.
<boost/system.hpp>
This convenience header includes all the headers previously described.
History
N1975, Filesystem Library Proposal for TR2, accepted for Library Technical Report 2 (TR2) at the Berlin meeting, included additional components to supplement the Standard Library’s Diagnostics clause. Since then, these error reporting components have received wider public scrutiny and enhancements have been made to the design. The enhanced version has been used by N2054, Networking Library Proposal for TR2, demonstrating that these error reporting components are useful beyond the original Filesystem Library.
The original proposal viewed error categories as a binary choice between
errno
(i.e. POSIX-style) and the native operating system’s error
codes. The proposed components now allow as many additional error categories as
are needed by either implementations or by users. The need to support additional
error categories, for example, occurs in some networking library implementations
because they are built on top of the POSIX getaddrinfo
API that
uses error codes not based on errno
.
Acknowledgments
Christopher Kohlhoff and Peter Dimov made important contributions to the design. Comments and suggestions were also received from Pavel Vozenilek, Gennaro Prota, Dave Abrahams, Jeff Garland, Iain Hanson, Oliver Kowalke, and Oleg Abrosimov. Christopher Kohlhoff suggested several improvements to the N2066 paper. Johan Nilsson’s comments led to several of the refinements in N2066.
Copyright and License
This documentation is
-
Copyright 2003-2017 Beman Dawes
-
Copyright 2018-2022 Peter Dimov
and is distributed under the Boost Software License, Version 1.0.