Why Does `vector v(istream_iterator(cin), istream_iterator()());` Parse as a Function Declaration?

Unraveling a Puzzling Aspect of the Most Vexing Parse

The Most Vexing Parse is a notorious compiler quirk that can lead to unexpected interpretation of code. One perplexing case arises when a line resembling a function declaration actually represents a different construction.

Consider the following line:

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

Perplexingly, this line may be parsed as a function declaration. While the second temporary iterator can be logically interpreted as a type, the role of the first temporary iterator is less clear.

Clarifying the Nature of the First Temporary

Contrary to initial assumptions, istream_iterator(cin) is not a function type. Rather, it's the same as the alternative syntax istream_iterator cin.

In C, a similar declarator syntax exists where the name of an argument can be arbitrarily parenthesized. Although this practice is generally discouraged as a "mistake," it's still supported in C due to its legacy.

Therefore, istream_iterator(cin) declares a parameter of type istream_iterator named cin. By providing parentheses around the argument name, the syntax effectively separates the parameter name from the type specification.

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