c++ - if (cin >> x) - Why can you use that condition?

Question

I have been using "Accelerated C++" to learn C++ over the summer, and there's a concept which I don't seem to understand properly.

Why is

int x;
if (cin >> x){}

equivalent to

cin >> x;
if (cin){}

By looking at the code, it seems to me that we're using cin as a variable. But, I thought it was a function. Why can we use cin in this way when it is x that has whatever value we input into our keyboard?

Answer 1

cin is an object of class istream that represents the standard input stream. It corresponds to the cstdio stream stdin. The operator >>overload for streams return a reference to the same stream. The stream itself can be evaluated in a boolean condition to true or false through a conversion operator.

cin provides formatted stream extraction. The operation cin >> x;

where "x" is an int will fail if a non-numeric value is entered. So:

if(cin>>x)

will return false if you enter a letter rather than a digit.

This website on tips and tricks using C++ I/O will help you too.