std::ios_base::failure(3)

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)