What's the Deal with Parenthesizing Argument Names in the Most Vexing Parse?

Understanding the Ambiguity in the Most Vexing Parse: Parenthesizing Argument Names

The Most Vexing Parse refers to a grammatical ambiguity in C that can lead to unexpected results when declaring functions. One confusing detail that arises is the ability to parse certain lines as function declarations, even when they do not appear to conform to expected syntax.

Consider the following line:

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

This line can be interpreted as a function declaration due to the Most Vexing Parse. The first temporary iterator, istream_iterator(cin), is parsed as a type, while the second temporary iterator, istream_iterator(), is interpreted as a function returning an iterator with no arguments.

However, it is not immediately clear what type the first temporary iterator represents. The key to understanding this is realizing that istream_iterator(cin) is equivalent to istream_iterator cin. The parentheses are superfluous and do not affect the meaning of the expression. This means that the first temporary iterator is an istream_iterator of type int named cin.

This raises the question of whether it is possible to arbitrarily parenthesize argument names in function calls. The answer is yes, you can parenthesize argument names if you want to highlight them or clarify their purpose. However, it is generally discouraged as it can lead to confusing or unexpected code. The main purpose of this syntax is to support a legacy syntax inherited from C and is not considered best practice in modern C code.

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