std::ios_base::failure
<ios>
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.
Inheritance diagram
Inheritance diagram
Contents
[edit] Member functions
failure object with the given message (public member function)
failure object (public member function)
(public member function)
std::ios_base::failure::failure
const std::error_code & ec = std::io_errc::stream );
const std::error_code & ec = std::io_errc::stream );
std::ios_base::failure then std::strcmp (what(), other.what()) == 0.(since C++11)Parameters
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=
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
Return value
*this
std::ios_base::failure::what
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.
Notes
Implementations are allowed but not required to override what().
Inherited from std::system_error
Member functions
Inherited from std::runtime_error
Inherited from std::exception
Member functions
[edit] 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() or noexcept.
[edit] 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
[edit] 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 |