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

Checks whether the stream has no errors.

# Declarations

operator /* unspecified-boolean-type */() const;

(until C++11)

explicit operator bool() const;

(since C++11)

# 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

#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';
}

# Defect reports

# See also

• basic_ios
• good()
• fail()
• ios_base::iostate
• ios_base::iostate
• good()
• fail()
• bad()
• eof()
• operator!