Is `if (cin >> x)` a Valid Conditional Expression in C ?

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Assessing the Validity of cin as a Conditional Expression

Consider the following C code snippet:

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

Some programmers may find it confusing that the above condition is equivalent to:

cin >> x;
if (cin) {}

Understanding why this is the case requires a closer look at the nature of cin.

The enigmatic cin: Function or Object?

Despite the familiar notation that resembles a function call, cin is actually an object of the istream class. It represents the standard input stream and corresponds to the stdin stream in C.

The cin Overloaded Operator

The >> operator is overloaded for streams. When applied to a stream object, it returns a reference to the same stream.

Stream Conversion to Boolean

Streams can participate in boolean conditions through a conversion operator that evaluates them to true or false. This is particularly useful for detecting input errors.

If (cin >> x) - Breaking it Down

In if (cin >> x), the >> operator is used to extract a value from the input stream cin and store it in x. If the extraction is successful, the condition evaluates to true. On the other hand, if a non-numeric value is entered, the extraction fails, and the condition evaluates to false.

Conclusion

Therefore, if (cin >> x) is effectively checking whether cin is in a state that allows valid extraction. This condition allows programmers to conveniently handle input validation and respond accordingly.

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