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 aswhile(stream>> value){...} orwhile(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 abool is expected (e.g. anif condition). However,implicit conversions (e.g. toint) that can occur withbool are not allowed.

In C++98,operatorbool could not be provided directly due tothe safe bool problem. The initial solution in C++98 is to provideoperatorvoid*, which returns a null pointer iffail() returnstrue or a non-null pointer otherwise. It is replaced by the resolution ofLWG issue 468, which allowsSafe Bool idiom to be applied.

Since C++11, conversion functions can beexplicit. The resolution ofLWG issue 1094 introduced the explicitoperatorbool 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';}

s is true123s is false

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

The following table shows the value ofbasic_ios accessors (good(),fail(), etc.) for all possible combinations ofios_base::iostate flags:

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