How does the compiler deal with the inputs following an invalid input exactly?

I know that, roughly speaking, when the user enters an invalid input, 0 will be assigned to that input (the failbit flag is set on the input stream) and all the subsequent inputs becomes unreliable.

In such cases, I wonder if some unreliable value (or some value with special meaning like 0 above) will be assigned to those subsequent inputs? Or if those those subsequent inputs will simply be escaped?


I actually made a toy program with multiple std::cin >> variables; statements to see if any value would be assigned to variables after the failbit flag has been set.

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It seems that, on my computer, all those subsequent cin are being escaped and the variables that would have been assigned a value by those subsequent cin remain to have the original values which were assigned to them during their initialisations (a is 0 while b and c are still 100 and 1000 respectively).

May I ask if such escapes happens all the time? Or if such behaviour differs from machine to machine?


Also, may I ask if the invalid input remain in the input buffer after the failbit flag is set unless we clear the input buffer?

Thanks in advanced!

1 answer

  • answered 2021-06-23 12:34 Caleth

    The various std::istream& operator>>(std::istream& is, arithmetic & num) are functions that an implementation (not just "the compiler") must provide. The standard defines what they do.

    The steps are laid out in the definition of FormattedInputFunction and std::num_get::get. Specifically:

    If the sentry returned false or sentry's constructor threw an exception, no input takes place

    If the conversion function fails to convert the entire field, the value ​0​ is stored in v.

    So the first input failure writes 0 to a number, and subsequent calls don't modify the right hand side argument.