Cin object return value c++ [duplicate]

问题

This question already has answers here:

What is cin doing inside argument of if? [duplicate] (2 answers)

Closed 6 years ago.

I woud like to ask what is return value of cin? I know it is istream object and when it is used in expression like if(!cin) there is actually called some function and I woud like to know what function it actually is. cin.fail() or cin.good() or.. Is if(!cin) same as if(cin.fail())?

回答1:

Yes.

cin overloads casting operators, and they return flag status fail().

A possible implementation:

operator void*() const {
    return !fail();
}

explicit operator bool(){
   return !fail();
}

bool operator!() const {
   return fail();
}

Look at here and here.

来源：https://stackoverflow.com/questions/19385141/cin-object-return-value-c