std::basic_ios::operatorbool(3)

NAME
std::basic_ios::operatorbool - std::basic_ios::operatorbool

Synopsis
   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 a conversion function to an unspecified
                      provided 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