Adding parentheses to object declaration causes constructor not to run

Question

Say I have an extremely simple Foo class like so:

#include <iostream>

class Foo
{
    public:
        Foo() {std::cout << "bar" << std::endl;}
};

I have always thought that both Foo foo and Foo foo() would output bar, as they both call the constructor. But when I try:

int main()
{
    Foo foo();
    return 0;
}

Nothing gets outputted. Is there a reason for this? How can I make sure that the constructor gets called in both instances?

Ideone test