How Can `istream_iterator(cin)` Be Interpreted as a Type in the 'Most Vexing Parse'?

The Ambiguity of Parenthesization in Function Declarations

In the "Most Vexing Parse" phenomenon, ambiguities arise when using templates. One such instance occurs when parsing function declarations. Consider the following line:

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

This line can be interpreted as either a function declaration or a variable declaration. The question is, how can the first temporary iterator be interpreted as a type?

Contrary to intuition, istream_iterator(cin) is not a function declaration. Instead, it is an expression that creates a temporary instance of an istream_iterator initialized with the input stream cin. The superfluous parentheses around cin do not change this interpretation.

This peculiar syntax is inherited from C, where such parenthesization was also considered a mistake. In this case, the parentheses do not affect the meaning of the expression. It simply serves as an unnecessary noise, leading to confusion.

In conclusion, the first temporary iterator in the given line represents the type istream_iterator, and the parentheses around cin are merely superfluous. However, this ambiguous syntax should be used with caution to avoid potential misunderstandings.

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