# std::ios_base::failure

From cppreference.com
< cpp‎ | io‎ | 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)

Inheritance diagram | (until C++11)
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.

### 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 [virtual] | returns an explanatory string (virtual public member function of `std::system_error`)

## Inherited from std::runtime_error

## Inherited from std::exception

### Member functions

(destructor) [virtual] | destroys the exception object (virtual public member function of `std::exception`)
what [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 desctructor 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 gloabally across the standard library, so the destructors of standard library classes are not declared with throw() or noexcept.

### Example

Run this code

    #include <iostream>
    #include <fstream>

    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

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

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/io/ios_base/failure&oldid=146797"