std::ios_base::failure(3)

💖 NAME 💖

std::ios_base::failure — yes bestie, the exception class itself. This page is std::ios_base::failure(3) from the C++ Standard Library, and we are absolutely gonna make it make sense.

✨ Synopsis ✨

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)

The page also drops these inheritance-diagram image filenames, so here they are exactly:

• std-ios base-failure-2003-inheritance.svg — Inheritance diagram (until C++11)
• std-ios base-failure-inheritance.svg — 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 🌸

The page’s signature block is a little chaotic, so here it is exactly as printed — do not let the line breaks gaslight you:

explicit failure( const std::string& message );          (until C++11)
explicit failure( const std::string& message,
                  const std::error_code& ec =            (since C++11)
std::io_errc::stream );
explicit failure( const char* message,           (1)
                  const std::error_code& ec =        (2) (since C++11)
std::io_errc::stream );
failure( const failure& other );                     (3)
(until C++11)
failure( const failure& other ) noexcept;
(since C++11)

• 1, 2) Constructs the exception object using message as the 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 — returns an explanatory string [virtual] (virtual public member function of std::system_error)

🍓 Inherited from std::runtime_error 🍓

That's the whole vibe here — the page just says it inherits from std::runtime_error, and no extra member-function list is printed under it.

🧚 Inherited from std::exception 🧚

Member functions

• destructor — destroys the exception object [virtual] (virtual public member function of std::exception)
• what — returns an explanatory string [virtual] (virtual public member function of std::exception)

💜 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.

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.

🔥 Example 🔥

// Run this code

#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: LWG 48 — applied to C++98 — behavior as published: the constructor overload (1) initialized the base class std::exception with msg, but the base class does not have a matching constructor — correct behavior: corresponding description removed.
• DR: LWG 331 — applied to C++98 — behavior as published: std::ios_base::failure declared a destructor without throw() — correct behavior: removed the destructor declaration.

🌟 See also 🌟

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

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Source/version: http://cppreference.com · date: 2024.06.10 · page: std::ios_base::failure(3). That's the whole manual page, bestie — go forth and catch those exceptions 💅✨🫶