How Can `vector v(istream_iterator(cin), istream_iterator());` Be Interpreted as a Function Declaration in C ?

The Most Vexing Parse: Clarifying a Confusing Detail

In programming, the "Most Vexing Parse" refers to a notorious ambiguity in C syntax that can lead to unexpected function declarations. This article aims to unravel one such confusing aspect of the Most Vexing Parse.

Consider the following line of code:

vector<int> v(istream_iterator<int>(cin), istream_iterator<int>());

How can this line be parsed as a function declaration?

Understanding the Syntax

Normally, the Most Vexing Parse problem arises when a second temporary object is interpreted as a function returning an iterator and taking no arguments. However, in this case, it is the first temporary object, istream_iterator(cin), that poses the dilemma.

The first temporary object, istream_iterator(cin), is identical to istream_iterator cin with superfluous parentheses. This syntax was borrowed from C and has been acknowledged as a mistake.

The Interpretation

The compiler interprets istream_iterator(cin) as a declaration of a parameter named cin, which is of type istream_iterator. The parentheses do not affect the meaning of the statement.

Therefore, the line of code is interpreted as a function declaration with one parameter, cin, which is of type istream_iterator. The function returns a vector of integers.

Arbitrary Parentheses in Argument Lists

The parentheses around cin in istream_iterator(cin) do not indicate that it is an argument to a function within parentheses. Instead, they simply enclose an expression that evaluates to a value of type istream_iterator.

Conclusion

The key to understanding this case is to recognize that the superfluous parentheses do not alter the meaning of the statement. The interpretation of istream_iterator(cin) as a parameter type, rather than a function call within parentheses, eliminates the confusion in this particular example of the Most Vexing Parse.

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