std::basic_ios<CharT,Traits>::operator bool
Checks whether the stream has no errors.
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.
[edit] Parameters
(none)
[edit] Return value
[edit] 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.
[edit] 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'; }
Output:
s is true 1 2 3 s is false
[edit] 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 |
[edit] See also
The following table shows the value of basic_ios accessors (good(), fail(), etc.) for all possible combinations of ios_base::iostate flags:
basic_ios accessors