std::basic_ios<CharT,Traits>::operator bool

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

strstreambuf (deprecated in C++98)

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

istrstream (deprecated in C++98)

ostrstream (deprecated in C++98)

strstream (deprecated in C++98)

Synchronized Output

basic_osyncstream (C++20)

Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

 

std::basic_ios

Member functions

basic_ios::basic_ios

basic_ios::~basic_ios

State functions

basic_ios::good

basic_ios::eof

basic_ios::fail

basic_ios::bad

basic_ios::operator!

basic_ios::operator bool

basic_ios::rdstate

basic_ios::setstate

basic_ios::clear

Formatting

basic_ios::copyfmt

basic_ios::fill

Miscellaneous

basic_ios::exceptions

basic_ios::imbue

basic_ios::rdbuf

basic_ios::tie

basic_ios::narrow

basic_ios::widen

Protected member functions

basic_ios::init

basic_ios::move

basic_ios::swap

basic_ios::set_rdbuf

 

operator void*() const;	(1)	(until C++11)
explicit operator bool() const;	(2)	(since C++11)

Checks whether the stream has no errors.

1) Returns a null pointer if fail() returns true, otherwise returns a non-null pointer. This pointer is implicitly convertible to bool and may be used in boolean contexts.

2) Returns true if the stream has no errors and is ready for I/O operations. Specifically, returns !fail().

This operator makes it possible to use streams and functions that return references to streams as loop conditions, resulting in the idiomatic C++ input loops such as while(stream >> value) {...} or while(getline(stream, string)){...}. Such loops execute the loop's body only if the input operation succeeded.

Parameters

(none)

Return value

true if the stream has no errors, false otherwise.

Example

Run this code

#include <iostream>
#include <sstream>
 
int main()
{
    std::istringstream s("1 2 3 error");
    int n;
    std::cout << std::boolalpha << "s is " << static_cast<bool>(s) << '\n';
    while (s >> n) {
        std::cout << n << '\n';
    }
    std::cout << "s is " << static_cast<bool>(s) << '\n';
}

Output:

s is true
1
2
3
s is false

See also

The following table shows the value of basic_ios accessors (good(), fail(), etc.) for all possible combinations of ios_base::iostate flags:

ios_base::iostate flags			basic_ios accessors
eofbit	failbit	badbit	good()	fail()	bad()	eof()	operator bool	operator!
false	false	false	true	false	false	false	true	false
false	false	true	false	true	true	false	false	true
false	true	false	false	true	false	false	false	true
false	true	true	false	true	true	false	false	true
true	false	false	false	false	false	true	true	false
true	false	true	false	true	true	true	false	true
true	true	false	false	true	false	true	false	true
true	true	true	false	true	true	true	false	true