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std::ios_base::failure - cppreference.com

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

Constructs the exception object using

as explanation string which can later be retrieved using

.

Copy constructor. Initialize the contents with those of

.

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.

failure& operator=( const failure& other );

failure& operator=( const failure& other ) noexcept;

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)

*this

virtual const char* what() const throw();

virtual const char* what() const noexcept;

Returns the explanatory string.

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.

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

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.

#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

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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