std::basic_ios<CharT,Traits>::operator bool - cppreference.com
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    std::basic_ios<CharT,Traits>::operator bool

    From cppreference.com
     
    C++
     
    Input/output library
     
    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: