Question

We can initialize a container deque by using standard input like this:

deque<int> c((istream_iterator<int>(cin)),(istream_iterator<int>()));

As 《The C++ Standard Library》 describes,the extra parentheses around the initializer arguments are necessary.Without them ,c declares a function with a return type that is deque.Its first parameter is of type istream_iterator with the name cin,and its second unnamed parameter is of type “function taking no arguments returning istream_iterator”.Look the following code

deque<int> c(istream_iterator<int>(cin),istream_iterator<int>());

But teh extra parentheses force the initializer not to match the syntax of a declaration.
I don’t understand why the extra parentheses can make the initializer not to match syntax of a declaration.Thanks a lot.

Answer 1

Because the grammar defined in the C++ standard does not accept this form as a function declaration:

T name((U), (V));