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