Question

Possible Duplicate:
Why is it an error to use an empty set of brackets to call a constructor with no arguments?

The small piece code code can not be successfully compiled on
Microsoft Visual Studio 2005

#include <iterator>
#include <algorithm>
#include <vector>
#include <iostream>

int main()
{
    std::vector<int> a;
    std::istream_iterator<int> be(std::cin);
    std::istream_iterator<int> en();
    std::copy(be, en, std::back_inserter(a));       
}

But this one is ok

#include <iterator>
#include <algorithm>
#include <vector>
#include <iostream>

int main()
{
    std::vector<int> a;
    std::istream_iterator<int> be(std::cin);
    std::istream_iterator<int> en; //Same to upon, only here less '()'
    std::copy(be, en, std::back_inserter(a));       
}

Answer 1

In the first case en is being declared as a function, not a variable. This is one of the many traps present in C++ syntax that makes hard to parse a C++ program.

The rule applied is more or less “if it can be parsed both as a declaration or as a definition then it’s considered a declaration” and has been named “most vexing parse” by Scott Meyers. In your case the second line can be seen similar to

inf foo();

And is therefore considered a function declaration. Note that this very same trap can be even more subtle:

double x = 3.141592654;
int y(int(x));

here the second line is also a declaration for a function because language rules says here that the parenthesis around x can be ignored and therefore the meaning is int y(int x);.