std::ios_base::failure
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.
| (since C++17) |
 Inheritance diagram | (until C++11) |
 Inheritance diagram | (since C++11) |
Contents |
[edit]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(conststd::string& message ); | (until C++11) | |
explicit failure(conststd::string& message, conststd::error_code& ec =std::io_errc::stream); | (since C++11) | |
explicit failure(constchar* message, conststd::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) | |
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
virtualconstchar* what()constthrow(); | (until C++11) | |
virtualconstchar* what()constnoexcept; | (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) |
[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 |
[edit]See also
(C++11) | the IO stream error codes (enum) |
