std::basic_ios<CharT,Traits>::operator bool - cppreference.com (original) (raw)

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.

[edit] Parameters

(none)

[edit] 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.

[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

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.

[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: