while(cin >> x), what is the boolean value here?

Question

What is checked in the following condition:

 while(cin >> x)

to have a while loop continue going? What's the boolean value of an expression assignment in the code above?

Answer 1

The expression result of cin >> x is a reference to the cin stream, a std::istream.

A std::istream effectively converts to boolean when used as an if or while condition expression.

The result is equivalent to writing

     !cin.fail().

---

In C++03 the conversion was technically an operator void*().

That, however, was not so nice with respect to overload resolution and some other issues.

So in C++11 it's an explicit operator bool(). The keyword explicit could only be used on constructors in C++03, but in C++11 it can also be applied to conversion operators. It prevents the usual inadvertent implicit conversions, but as a special case it is effectively ignored for conversion to boolean of an if or while condition expression.

Answer 2

cin is an instance of istream template class. operator >> acts on this istream instance to load input into data and returns a reference to this istream. Then in while condition it is tested by a call to cin::operator void*() const (explicit operator bool() const in C++11) which invokes fail() function to test if operation succeeded. This is why you can use this operation in while condition

while ( cin >> x)
{
   //...

Answer 3

The return type of operator >> for std::istream is std::istream again. It returns the left-hand argument (cin in your case). And std::istream has an operator bool() which enables using a stream in a conditional expression. These two things together enable testing of a stream the same way we test many other C++ objects for validity, such as pointers.