std::ios_base::failure
| Defined in header <ios>
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class failure;
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The class std::ios_base::failure defines an exception object that is thrown on failure by the functions in the Input/Output library.
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(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 );
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(until C++11) | |
explicit failure( const std::string& message,
const std::error_code& ec = std::io_errc::stream );
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(since C++11) | |
explicit failure( const char* message,
const std::error_code& ec = std::io_errc::stream );
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(2) | (since C++11) |
| (3) | ||
failure( const failure& other );
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(until C++11) | |
failure( const failure& other ) noexcept;
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(since C++11) | |
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)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
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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 );
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(until C++11) | |
failure& operator=( const failure& other ) noexcept;
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(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();
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(until C++11) | |
virtual const char* what() const noexcept;
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(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.
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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)
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[virtual] |
returns an explanatory string (virtual public member function of std::system_error)
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Inherited from std::runtime_error
Inherited from std::exception
Member functions
[virtual] |
destroys the exception object (virtual public member function of std::exception)
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[virtual] |
returns an explanatory string (virtual public member function of std::exception)
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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.
- ↑ 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()ornoexcept.
Example
#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
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corresponding description removed |
| LWG 331 | C++98 | std::ios_base::failure declared a destructor without throw()
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removed the destructor declaration |
See also
(C++11) |
the IO stream error codes (enum) |
