How does the “return value” of operator >> of istream class work?

Question

I tried to understand the statement:

    int main() {
        fstream inf( \"ex.txt\", ios::in );
        char c;
        while( inf >> c ) {
                  


        

Answer 1

What does ( inf >> c ) return in the while loop above?

void*. The loop test resolves to while (inf.operator void*() != NULL).

How does a reference can be evaluated as true or false?

But supporting conversion to bool or something convertible to bool.

How does the internal implementation of istream actually work?

It just returns a reference to itself (return *this) so it can support chaining.

Answer 2

To return a self ref you just return *this. You don't actually declare an internal self reference. You can have reference members, but you must initialize them in the constructor initializer list: class Foo { Foo(int& some_int_ref) :my_ref_member(some_int_ref)

istream has another overload to determine its boolean value.

Example of a chainable member (like operator >>, but in this case just an ordinary function) and giving a class a boolean value (although the latter is a complex subject worthy of its own subject):

#include <iostream>

class X {
    bool B;
public:
    X() :B(false) { }
    X& toggle() { B = !B; return *this; }
    operator void*() { return B ? this : 0; }
};

int main()
{
    X x;

    x.toggle().toggle().toggle();
    if (x)
        std::cout << "true!" << std::endl;
}

EDIT: I didn't want to delve into operator bool vs operator void * in this answer, but this old stackoverflow question should give you good references: Why is "operator bool()" invoked when I cast to "long"?