Wierd template class

I am confused about the use of the template parameter "E" in the below
class. Since when is it allowed to use these parameters like "E(1)" and
what does it mean (where can I read more about this kind of use)?

template <typename E>
class Mytest {
    public:
    Mytest(int n) {
        s = E(0);
        a.resize(n,E(0));

    }

    void operator()() {
        std::cout << "wierd operator" << std::endl;

    }

    int primal() {

        return 34;
    }

    private:
       E s;
    std::vector<E> a;

};

The class above has a constructor "Mytest(int n)". But it also has the
freak of nature function: void operator()().

It seems that it is declaring a new operator "()" that in this case
prints "wierd operator" when called on an instance of the class, like:

Mytest<int> my(4);
my(); // executes cout in void operator block.

But if I change it to:

    void operator()@ {

        std::cout << "wierd operator" << std::endl;

    }

I get:

main.cpp:46: error: stray ?@? in program

So what does the "operator" keyword really do?