NAME¶

std::ios_base::failure - std::ios_base::failure

Synopsis¶

Defined in header <ios>
class failure;

The class std::ios_base::failure defines an exception object that is thrown on
failure by the functions in the Input/Output library.

std::ios_base::failure may be defined either as a member class of
std::ios_base or as a synonym (typedef) for another class with (since C++17)
equivalent functionality.

std-ios base-failure-2003-inheritance.svg
(until C++11)
Inheritance diagram
std-ios base-failure-inheritance.svg
(since C++11)
Inheritance diagram

Member functions¶

constructor constructs a new failure object with the given message
(public member function)
operator= replaces the failure object
(public member function)
what returns the explanatory string
(public member function)

std::ios_base::failure::failure

explicit failure( const std::string& message ); (until C++11)
explicit failure( const std::string& message,
const std::error_code& ec = (since C++11)
std::io_errc::stream );
explicit failure( const char* message, (1)
const std::error_code& ec = (2) (since C++11)
std::io_errc::stream );
failure( const failure& other ); (3) (until C++11)
failure( const failure& other ) noexcept; (since C++11)

1,2) Constructs the exception object using message as explanation string which can
later be retrieved using what().
ec is used to identify the specific reason for the failure.
(since C++11)
3) Copy constructor. Initialize the contents with those of other.
If *this and other both have dynamic type std::ios_base::failure then
std::strcmp(what(), other.what()) == 0.
(since C++11)

Parameters¶

message - explanatory string
ec - error code to identify the specific reason for the failure
other - another failure to copy

Notes¶

Because copying std::ios_base::failure is not permitted to throw exceptions, this
message is typically stored internally as a separately-allocated reference-counted
string. This is also why there is no constructor taking std::string&&: it would have
to copy the content anyway.

std::ios_base::failure::operator=

failure& operator=( const failure& other ); (until C++11)
failure& operator=( const failure& other ) noexcept; (since C++11)

Assigns the contents with those of other.
If *this and other both have dynamic type std::ios_base::failure then
std::strcmp(what(), other.what()) == 0 after assignment.
(since C++11)

Parameters¶

other - another exception object to assign with

Return value¶

*this

std::ios_base::failure::what

virtual const char* what() const throw(); (until C++11)
virtual const char* what() const noexcept; (since C++11)

Returns the explanatory string.

Parameters¶

(none)

Return value¶

Pointer to a null-terminated string with explanatory information. The string is
suitable for conversion and display as a std::wstring. The pointer is guaranteed to
be valid at least until the exception object from which it is obtained is destroyed,
or until a non-const member function (e.g. copy assignment operator) on the
exception object is called.

Notes¶

Implementations are allowed but not required to override what().

Inherited from std::system_error

Member functions¶

code returns error code
(public member function of std::system_error)
what returns an explanatory string
[virtual] (virtual public member function of std::system_error)

Inherited from std::runtime_error

Inherited from std::exception

Member functions¶

destructor destroys the exception object
[virtual] (virtual public member function of std::exception)
what returns an explanatory string
[virtual] (virtual public member function of std::exception)

Notes¶

Before the resolution of LWG issue 331, std::ios_base::failure declared a destructor
without throw(), where std::exception::~exception() was declared with throw()^[1].
This means the std::ios_base::failure::~failure() had a weaker exception
specification. The resolution is to remove that declaration so that the non-throwing
exception specification is kept.

LWG issue 363 targets the same defect and its resolution is to add throw() to the
declaraion of std::ios_base::failure::~failure(). That resolution was not applied
due to the confict between the two resolutions.

1. ↑ The non-throwing exception specification is now applied globally across the
standard library, so the destructors of standard library classes are not
declared with throw() or noexcept.

Example¶

// Run this code

#include <fstream>
#include <iostream>

int main()
{
std::ifstream f("doesn't exist");

try
{
f.exceptions(f.failbit);
}
catch (const std::ios_base::failure& e)
{
std::cout << "Caught an ios_base::failure.\n"
<< "Explanatory string: " << e.what() << '\n'
<< "Error code: " << e.code() << '\n';
}
}

Possible output:¶

Caught an ios_base::failure.
Explanatory string: ios_base::clear: unspecified iostream_category error
Error code: iostream:1

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
the constructor overload (1) initialized
LWG 48 C++98 the base class std::exception corresponding
with msg, but the base class does not have description removed
a matching constructor
LWG 331 C++98 std::ios_base::failure declared a removed the destructor
destructor without throw() declaration

See also¶

io_errc the IO stream error codes
(C++11) (enum)