General Maybe monad in C++

maybem.cpp

#include <iostream>
#include <cassert>
#include <cmath>

using namespace std;

template<typename a> class Maybe
{
        public:
                static Maybe<a> Just(a value) { return Maybe(value); }
                static Maybe<a> Nothing()     { return Maybe(); }

                bool isJust() { return m_valid; }
                a getJust() { assert(isJust()); return m_value; }

                template<typename b> Maybe<b> operator>>=(Maybe<b> (*f)(int)) {
                        return isJust() ? f(getJust()) : Maybe<b>::Nothing();
                }

                Maybe<a> unit(a value) { return Maybe(value); }

                void show() {
                        if(isJust())
                                cout << "Just " << getJust() << endl;
                        else
                                cout << "Nothing" << endl;
                }

        private:
                Maybe() : m_value(0), m_valid(false) {}
                Maybe(a value) : m_value(value), m_valid(true) {}
                a m_value;
                bool m_valid;
};

Maybe<int> IM_Square(int a) { return (abs(a) >= 0x8000) ? Maybe<int>::Nothing() : Maybe<int>::Just(a * a); }

int main(int argc, char** argv)
{
        Maybe<int> result = Maybe<int>::Nothing();

        result = Maybe<int>::Just(0x4000) >>= IM_Square;
        result.show();

        result = (Maybe<int>::Just(0x4000) >>= IM_Square) >>= IM_Square;
        result.show();

        return 0;
}