std::ios_base::failure

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< cpp‎ | io‎ | ios base
 
C++
 
Input/output library
 
std::ios_base
 
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 equivalent functionality.

(since C++17)
cpp/error/exceptionstd-ios base-failure-2003-inheritance.svg

Inheritance diagram

(until C++11)
cpp/error/exceptioncpp/error/runtime errorcpp/error/system errorstd-ios base-failure-inheritance.svg

Inheritance diagram

(since C++11)

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

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

Return value

Pointer to an implementation-defined 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.

The returned string is encoded with the ordinary literal encoding during constant evaluation.

(since C++26)

Notes

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

Inherited from std::system_error

Member functions

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

Inherited from std::runtime_error


Inherited from std::exception

Member functions

[virtual]
 destroys the exception object
(virtual public member function of std::exception)
[virtual]
 returns an explanatory string
(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 declaration of std::ios_base::failure::~failure(). That resolution was not applied due to the conflict 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
LWG 48 C++98 the constructor overload (1) initialized the base class std::exception
with msg, but the base class does not have a matching constructor 		corresponding
description removed
LWG 331 C++98 std::ios_base::failure declared a destructor without throw() removed the destructor declaration

See also

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