Example Either type in C++

either.cpp

#include <stdio.h>

template <class T1, class T2> 
class Either
{
    bool isLeft;
    union
    {
        T1 left;
        T2 right;    
    };
    template<class T1_, class T2_> friend Either<T1_, T2_> Left(T1_ x);
    template<class T1_, class T2_> friend Either<T1_, T2_> Right(T1_ x);
public:
    bool matchLeft(T1& x)
    {
        if (isLeft)
            x = left;
        return isLeft;    
    }
    bool matchRight(T2& x)
    {
        if (!isLeft)
            x = right;
        return !isLeft;    
    }  
};

template <class T1, class T2> 
Either<T1, T2> Left(T1 x)
{
    Either<T1, T2> e;
    e.isLeft = true;
    e.left = x;
    return e;
}
template <class T1, class T2> 
Either<T1, T2> Right(T2 x)
{
    Either<T1, T2> e;
    e.isLeft = false;
    e.right = x;
    return e;
}

int main(int argc, char* argv[])
{
  Either<int, float> x = Left<int, float>(5);
  int c;
  if (x.matchLeft(c))
    printf("the number is %i", c);
  return 0;
}

Are you sure you meant
template<class T1_, class T2_> friend Either<T1_, T2_> Right(T1_ x);
not
template<class T1_, class T2_> friend Either<T1_, T2_> Right(T2_ x);
?