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

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std::basic_ios
 
operator /* unspecified-boolean-type */() const;
 (1) (until C++11)
explicit operator bool() const;
 (2) (since C++11)

Checks whether the stream has no errors.

1) Returns a value that evaluates to false in a boolean context if fail() returns true, otherwise returns a value that evaluates to true in a boolean context.
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 (std::getline(stream, string)) {...}. Such loops execute the loop's body only if the input operation succeeded.

Parameters

(none)

Return value

1) A value that evaluates to true in a boolean context if the stream has no errors, a value that evaluates to false in a boolean context otherwise.
2) true if the stream has no errors, false otherwise.

Notes

This conversion can be used in contexts where a bool is expected (e.g. an if condition). However, implicit conversions (e.g. to int) that can occur with bool are not allowed.

In C++98, operator bool could not be provided directly due to the safe bool problem. The initial solution in C++98 is to provide operator void*, which returns a null pointer if fail() returns true or a non-null pointer otherwise. It is replaced by the resolution of LWG issue 468, which allows Safe Bool idiom to be applied.

Since C++11, conversion functions can be explicit. The resolution of LWG issue 1094 introduced the explicit operator bool and the boolean conversion is now safe.

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

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 468 C++98 operator void* was provided a conversion function to an unspecified boolean type is provided instead

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
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