Why were parentheses disambiguated as a function declaration with std::istream_iterator?

Question

auto queue = [](string str) {
    istringstream ss(str);
    //std::copy(std::istream_iterator<string>(ss),
    //          std::istream_iterator<string>(),
    //          std::ostream_iterator<string>(std::cout, " "));

    //deque<string> q(std::istream_iterator<string>(ss), std::istream_iterator<string>{});
    deque<string> q(std::istream_iterator<string>(ss), std::istream_iterator<string>());
    return q;
};

Why would the compiler complain

parentheses were disambiguated as a function declaration [-Wvexing-parse]

if I construct a container with istream_iterator<string>().

Is there any difference with parameters in std::copy and container constructor?

Answer 1

This line

 deque<string> q(std::istream_iterator<string>(ss),
                 std::istream_iterator<string>());

is a function declaration with the return type deque<string> and two parameters of the type std::istream_iterator<string>. The first parameter has the name ss and the second parameter is unnamed.

To make this line a declaration of the variable q you should write either

 deque<string> q( ( std::istream_iterator<string>(ss) ),
                  (  std::istream_iterator<string>() ) );

or

 deque<string> q(std::istream_iterator<string>{ ss },
                 std::istream_iterator<string>{});

In this case there are used expressions instead of parameter declarations.

You may include declarators in parentheses. For example

int ( x );

When a declaration is a parameter declaration then you may omit a declarator like

int()

Here is an example of three declarations of the same function.

int f( int( x ) );
int f( int x );
int f( int( x ) )
{
    return 2 * x;
}