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std::basic_...peratorbool(3) C++ Standard Libary std::basic_...peratorbool(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 op- erations. 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 oc- cur 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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct be- havior 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 http://cppreference.com 2024.06.10 std::basic_...peratorbool(3)
NAME | Synopsis | Parameters | Return value | Notes | Example | Output: | See also
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