std::iostream_category

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

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Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Input/output library

I/O manipulators

Print functions (C++23)

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

basic_spanbuf (C++23)

strstreambuf (C++98/26*)

basic_syncbuf (C++20)

Streams

Abstractions

ios_base

basic_ios

basic_istream

basic_ostream

basic_iostream

File I/O

basic_ifstream

basic_ofstream

basic_fstream

String I/O

basic_istringstream

basic_ostringstream

basic_stringstream

Array I/O

basic_ispanstream (C++23)

basic_ospanstream (C++23)

basic_spanstream (C++23)

istrstream (C++98/26*)

ostrstream (C++98/26*)

strstream (C++98/26*)

Synchronized Output

basic_osyncstream (C++20)

Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

 

Defined in header <ios>
const std::error_category& iostream_category() noexcept;		(since C++11)

Obtains a reference to the static error category object for iostream errors. The object is required to override the virtual function error_category::name() to return a pointer to the string "iostream". It is used to identify error codes provided in the exceptions of type std::ios_base::failure.

Parameters

(none)

Return value

A reference to the static object of unspecified runtime type, derived from std::error_category.

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"
                  << "Error code: " << e.code().value() 
                  << " (" << e.code().message() << ")\n"
                  << "Error category: " << e.code().category().name() << '\n';
 
    }
}

Possible output:

Caught an ios_base::failure.
Error code: 1 (unspecified iostream_category error)
Error category: iostream

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 2087	C++11	iostream_category was not declared noexcept	declared noexcept

See also

failure	stream exception (public member class of std::ios_base)
io_errc (C++11)	the IO stream error codes (enum)